Thursday, July 26, 2007


P. Sanjana DBZ 8

Immunoglobulins (Ig):These are the molecules that are produced by plasma cells in response to an immunogen and which function as antibodies. The immunoglobulins derive their name from the finding that they migrate with globular proteins when antibody-containing serum is placed in an electrical field.

A. Antigen binding
Immunoglobulins bind specifically to one or a few closely related antigens. Each immunoglobulin actually binds to a specific antigenic determinant. Antigen binding by antibodies is the primary function of antibodies and can result in protection of the host. The valency of antibody refers to the number of antigenic determinants that an individual antibody molecule can bind. The valency of all antibodies is at least two and in some instances more.
B. Effector Functions
Frequently the binding of an antibody to an antigen has no direct biological effect. Rather, the significant biological effects are a consequence of secondary "effector functions" of antibodies. The immunoglobulins mediate a variety of these effector functions. Usually the ability to carry out a particular effector function requires that the antibody bind to its antigen. Not every immunoglobulin will mediate all effector functions. Such effector functions include:
1. Fixation of complement - This result in lysis of cells and release of biologically active molecules.
2. Binding to various cell types - Phagocytic cells, lymphocytes, platelets, mast cells, and basophils have receptors that bind immunoglobulins. This binding can activate the cells to perform some function. Some immunoglobulins also bind to receptors on placental trophoblasts, which results in transfer of the immunoglobulin across the placenta. As a result, the transferred maternal antibodies provide immunity to the fetus and newborn.

The basic structure of the immunoglobulins is illustrated in the. Although different immunoglobulins can differ structurally they all are built from the same basic units.
A. Heavy and Light Chains
All immunoglobulins have a four chain structure as their basic unit. They are composed of two identical light chains (23kD) and two identical heavy chains (50-70kD)
B. Disulfide bonds
1. Inter-chain disulfide bonds - The heavy and light chains and the two heavy chains are held together by inter-chain disulfide bonds and by non-covalent interactions. The number of inter-chain disulfide bonds varies among different immunoglobulin molecules.
2. Intra-chain disulfide bnds - Within each of the polypeptide chains there are also intra-chain disulfide bonds.

C. Variable (V) and Constant (C) Regions
After the amino acid sequences of many different heavy chains and light chains were compared, it became clear that both the heavy and light chain could be divided into two regions based on variability in the amino acid sequences. These are the:
1. Light Chain - VL (110 amino acids) and CL (110 amino acids)
2. Heavy Chain - VH (110 amino acids) and CH (330-440 amino acids)
D. Hinge Region
This is the region at which the arms of the antibody molecule form a Y. It is called the hinge region because there is some flexibility in the molecule at this point.
E. Domains
Three dimensional images of the immunoglobulin molecule show that it is not straight.
Rather, it is folded into globular regions each of which contains an intra-chain disulfide bond. These regions are called domains.
1. Light Chain Domains - VL and CL
2. Heavy Chain Domains - VH, CH1 - CH3 (or CH4)
F. Oligosaccharides
Carbohydrates are attached to the CH2 domain in most immunoglobulins. However, in some cases carbohydrates may also be attached at other locations.

A. Hypervariable (HVR) or complementarity determining regions (CDR)
Comparisons of the amino acid sequences of the variable regions of immunoglobulins show that most of the variability resides in three regions called the hypervariable regions or the complementarity determining regions. Antibodies with different specificities (i.e. different combining sites) have different complementarity determining regions while antibodies of the exact same specificity have identical complementarity determining regions (i.e. CDR is the antibody combining site). Complementarity determining regions are found in both the H and the L chains.
B. Framework regions
The regions between the complementary determining regions in the variable region are called the framework regions. Based on similarities and differences in the framework regions the immunoglobulin heavy and light chain variable regions can be divided into groups and subgroups. These represent the products of different variable region genes.

Immunoglobulin fragments produced by proteolytic digestion have proven very useful in elucidating structure/function relationships in immunoglobulins.
A. Fab
Digestion with papain breaks the immunoglobulin molecule in the hinge region before the H-H inter-chain disulfide bond Figure 4. This results in the formation of two identical fragments that contain the light chain and the VH and CH1 domains of the heavy chain. Antigen binding - These fragments were called the Fab fragments because they contained the antigen binding sites of the antibody. Each Fab fragment is monovalent whereas the original molecule was divalent. The combining site of the antibody is created by both VH and VL. An antibody is able to bind a particular antigenic determinant because it has a particular combination of VH and VL. Different combinations of a VH and VL result in antibodies that can bind a different antigenic determinants.
B. Fc
Digestion with papain also produces a fragment that contains the remainder of the two heavy chains each containing a CH2 and CH3 domain. This fragment was called Fc because it was easily crystallized.
Effector functions - The effector functions of immunoglobulins are mediated by this part of the molecule. Different functions are mediated by the different domains in this fragment. Normally the ability of an antibody to carry out an effector function requires the prior binding of an antigen; however, there are exceptions to this rule.
C. F(ab')2
Treatment of immunoglobulins with pepsin results in cleavage of the heavy chain after the H-H inter-chain disulfide bonds resulting in a fragment that contains both antigen binding sites. This fragment was called F(ab')2 because it was divalent. The Fc region of the molecule is digested into small peptides by pepsin. The F(ab')2 binds antigen but it does not mediate the effector functions of antibodies.

A. Immunoglobulin classes
The immunoglobulins can be divided into five different classes, based on differences in the amino acid sequences in the constant region of the heavy chains. All immunoglobulins within a given class will have very similar heavy chain constant regions. These differences can be detected by sequence studies or more commonly by serological means (i.e. by the use of antibodies directed to these differences).
1. IgG - Gamma heavy chains
2. IgM - Mu heavy chains
3. IgA - Alpha heavy chains
4. IgD - Delta heavy chains
5. IgE - Epsilon heavy chains
B. Immunoglobulin Subclasses
The classes of immunoglobulins can de divided into subclasses based on small differences in the amino acid sequences in the constant region of the heavy chains. All immunoglobulins within a subclass will have very similar heavy chain constant region amino acid sequences. Again these differences are most commonly detected by serological means.
1. IgG Subclasses
a) IgG1 - Gamma 1 heavy chains
b) IgG2 - Gamma 2 heavy chains
c) IgG3 - Gamma 3 heavy chains
d) IgG4 - Gamma 4 heavy chains
2. IgA Subclasses
a) IgA1 - Alpha 1 heavy chains
b) IgA2 - Alpha 2 heavy chains
C. Immunoglobulin Types
Immunoglobulins can also be classified by the type of light chain that they have. Light chain types are based on differences in the amino acid sequence in the constant region of the light chain. These differences are detected by serological means.
1. Kappa light chains
2. Lambda light chains
D. Immunoglobulin Subtypes
The light chains can also be divided into subtypes based on differences in the amino acid sequences in the constant region of the light chain.
Lambda subtypes
a) Lambda 1
b) Lambda 2
c) Lambda 3
d) Lambda 4
E. Nomenclature
Immunoglobulins are named based on the class, or subclass of the heavy chain and type or subtype of light chain. Unless it is stated precisely you are to assume that all subclass, types and subtypes are present. IgG means that all subclasses and types are present.
F. Heterogeneity
Immunoglobulins considered as a population of molecules are normally very heterogeneous because they are composed of different classes and subclasses each of which has different types and subtypes of light chains. In addition, different immunoglobulin molecules can have different antigen binding properties because of different VH and VL regions.

A. IgG
1. Structure:
All IgG's are monomers (7S immunoglobulin). The subclasses differ in the number of disulfide bonds and length of the hinge region.
2. Properties
Most versatile immunoglobulin because it is capable of carrying out all of the functions of immunoglobulin molecules.
a) IgG is the major Ig in serum - 75% of serum Ig is IgG. b) IgG is the major Ig in extra vascular spaces
c) Placental transfer - IgG is the only class of Ig that crosses the placenta. Transfer is mediated by receptor on placental cells for the Fc region of IgG. Not all subclasses cross equally; IgG2 does not cross well.
d) Fixes complement - Not all subclasses fix equally well; IgG4 does not fix complement
e) Binding to cells - Macrophages, monocytes, PMN's and some lymphocytes have Fc receptors for the Fc region of IgG. Not all subclasses bind equally well; IgG2 and IgG4 do not bind to Fc receptors. A consequence of binding to the Fc receptors on PMN's, monocytes and macrophages is that the cell can now internalize the antigen better. The antibody has prepared the antigen for eating by the phagocytic cells. The term opsonin is used to describe substances that enhance phagocytosis. IgG is a good opsonin. Binding of IgG to Fc receptors on other types of cells result in the activation of other functions.
B. IgM
1. Structure:
IgM normally exists as a pentamer (19S immunoglobulin) but it can also exist as a monomer. In the pentameric form all heavy chains are identical and all light chains are identical. Thus, the valence is theoretically 10. IgM has an extra domain on the mu chain (CH4) and it has another protein covalently bound via a S-S bond called the J chain. This chain functions in polymerization of the molecule into a pentamer.
2. Properties
a) IgM is the third most common serum Ig.
b) IgM is the first Ig to be made by the fetus and the first Ig to be made by a virgin B cells when it is stimulated by antigen.
c) As a consequence of its pentameric structure, IgM is a good complement fixing Ig. Thus, IgM antibodies are very efficient in leading to the lysis of microorganisms.
d) As a consequence of its structure, IgM is also a good agglutinating Ig . Thus, IgM antibodies are very good in clumping microorganisms for eventual elimination from the body.
e) IgM binds to some cells via Fc receptors.
f) B cell surface Ig
Surface IgM exists as a monomer and lacks J chain but it has extra 20 amino acids at the C-terminus to anchor it into the membrane. Cell surface IgM functions as a receptor for antigen on B cells. Surface IgM is noncovalently associated with two additional proteins in the membrane of the B cell called Ig-alpha and Ig-beta as indicated in Figure 10. These additional proteins act as signal transducing molecules since the cytoplasmic tail of the Ig molecule itself is too short to transduce a signal. Contact between surface immunoglobulin and an antigen is required before a signal can be transduced by the Ig-alpha and Ig-beta chains. In the case of T-independent antigens, contact between the antigen and surface immunoglobulin is sufficient to activate B cells to differentiate into antibody secreting plasma cells. However, for T-dependent antigens, a second signal provided by helper T cells is required before B cells are activated.
C. IgA
1. Structure
Serum IgA is a monomer but IgA found in secretions is a dimmer. When IgA exits as a dimer, a J chain is associated with it.
When IgA is found in secretions is also has another protein associated with it called the secretory piece or T piece; sIgA is sometimes referred to as 11S immunoglobulin. Unlike the remainder of the IgA which is made in the plasma cell, the secretory piece is made in epithelial cells and is added to the IgA as it passes into the secretions. The secretory piece helps IgA to be transported across mucosa and also protects it from degradation in the secretions.
2. Properties
a) IgA is the 2nd most common serum Ig.
b) IgA is the major class of Ig in secretions - tears, saliva, colostrum, mucus. Since it is found in secretions secretory IgA is important in local (mucosal) immunity.
c) Normally IgA does not fix complement, unless aggregated.
d) IgA can binding to some cells - PMN's and some lymphocytes.
D. IgD
1. Structure:
IgD exists only as a monomer.
2. Properties
a) IgD is found in low levels in serum; its role in serum uncertain.
b) IgD is primarily found on B cell surfaces where it functions as a receptor for antigen. IgD on the surface of B cells has extra amino acids at C-terminal end for anchoring to the membrane. It also associates with the Ig-alpha and Ig-beta chains.
c) IgD does not bind complement.
E. IgE
1. Structure:
IgE exists as a monomer and has an extra domain in the constant region.
2. Properties
a) IgE is the least common serum Ig since it binds very tightly to Fc receptors on basophils and mast cells even before interacting with antigen.
b) Involved in allergic reactions - As a consequence of its binding to basophils an mast cells, IgE is involved in allergic reactions. Binding of the allergen to the IgE on the cells result in the release of various pharmacological mediators that result in allergic symptoms.
c) IgE also plays a role in parasitic helminth diseases. Since serum IgE levels rise in parasitic diseases, measuring IgE levels is helpful in diagnosing parasitic infections. Eosinophils have Fc receptors for IgE and binding of eosinophils to IgE-coated helminths results in killing of the parasite.
d) IgE does not fix complement.
- Mathew Srirangam
Lymphatic System
As blood circulates under pressure, the fluid component of the blood (plasma) seeps through the thin wall of the capillaries into the surrounding tissues. This fluid is called the interstitial fluid or extra cellular fluid. Much of this fluid returns to the blood through the capillary membranes. The reminder of the interstitial fluid now called lymph flows from the connective tissue spaces into a network of thin open lymphatic capillaries and then into a series of progressively larges collecting vessels called lymphatic vessels. The largest lymphatic vessel called the thoracic duct opens into the left subclavian vein near the heart. In this way lymphatic system functions to capture fluid lost from the blood and returns it to the blood, thus ensuring the fluid steady state of the blood.

As the lymph is draining the connective tissues of the body, when a foreign antigen gains entrance into the tissues, it is picked up by the lymphatic system, and carried to various organized lymphatic tissues, which trap the antigen. Various types of organized lymphoid tissues are located along the vessels of the lymphatic system.

Some lymphoid tissues in the lung and lamina propria of the intestinal wall consist of diffuse collections of lymphocytes and macrophages. Other lymphoid tissue is organized into structures called lymphoid follicles. In the absence of antigen stimulus, a lymphoid follicle called primary follicle, comprises a network of follicular dendritic cells and small resting B-cells. Following an antigenic challenge, a primary follicle becomes a large secondary follicle – a ring of concentrically packed B lymphocytes surrounding a center called the germinal center in which proliferating B lymphocytes, memory cells and plasma cells are interspersed with macrophages and follicular dendritic cells.

Dendritic cells in lymphoid organs are two types.
In T-cell areas they are
called interdigitating dendritic cells
and in B-cell areas they are
called follicular dendritic cells

The germinal center is a site of intense B-cell activation. Here the B-cells that interact with antigen displayed on the membrane of follicular dendritic cells are induced to proliferate and differentiate into plasma and memory cells. In the absence of antigen activation, the B-cells appear to undergo programmed cell death with in the germinal center.

In order to maintain steady state levels of cells, the cells undergo programmed
cell death. These cells exhibit morphological changes collectively called
apoptosis. Apoptosis includes, decrease in cell volume, modification of
the cytoskeleton resulting in pronounced membrane blebbing (pinching off small

Lymph Nodes:
Lymph nodes are encapsulated bean shaped structures containing a reticular network packed with lymphocytes, macrophages and dendritic cells. Clustered at junctions of the lymphatic vessels, lymph nodes are the first organized lymphoid structure to encounter the antigens that enter the tissue spaces. As lymph percolates through a node, the cellular network of phagocytic cells and dendritic cells will trap any particulate antigen that is brought in with the lymph.

Morphologically, a lymph node can be divided into three roughly concentric regions – the cortex, the paracortex and the medulla. The outermost layer, the cortex consists of lymphocytes (mostly B cells) macrophages, and follicular dendritic cells arranged in primary follicle. With antigenic challenge, the primary follicles enlarge into secondary follicles, each containing la germinal center. Intense B-cells activation and differentiation into plasma and memory cells occur in the germinal centers. Beneath the cortex is the paracortex, which is populated largely with T-lymphocytes and also contains interdigitating dendritic cells. These dendritic cells express high levels of Class II MHC molecules, which are necessary for antigen presentation to TH cells. The innermost layer of lymph node is the medulla, which is sparsely populated with lymphocytes, but many of these are plasma cells.

Afferent lymphatic vessels pierce the capsule of lymph node at numerous sites and empty lymph into the sub capsular sinus. Lymph coming from the tissues percolates slowly inward through the cortex, the paracortex and medulla; allowing phagocytic cells and dendritic cells to trap any bacteria or particulate material, (like antigen-antibody complexes) carried by the lymph.

The trapped antigen is processed and presented together with Class I MHC molecules by interdigitating dendritic cells in the paracortex resulting in TH cell activation. Followed by the activation of B cells. TH and B cells bring about their immunological responses respectively.


Spleen is a large, ovoid, secondary lymphoid organ situated high in the left abdominal cavity. Unlike lymph nodes, which are specialized to trap localized antigen from regional tissue spaces, the spleen is adapted to filtering blood and trapping blood-borne antigens, and thus can respond to systemic infections.

The spleen is surrounded by a capsule that sends a number of projections (trabeculae) into the interior to form a compartmentalized structure. The compartments are of two types, the red pulp and the white pulp, which are separated by a diffuse marginal zone. The splenic red pulp consists of a network of sinusoids populated with macrophages and numerous red blood cells (erythrocytes). It is the site where old and defective red blood cells are destroyed and removed. Many of the macrophages within the red pulp contain engulfed red blood cells or iron pigments from degraded hemoglobin. The splenic white pulp surrounds the arteries, forming a periarteriolar lymphoid sheath (PALS) populated mainly by T-lymphocytes. The marginal zone, located peripheral to the PALS, is rich in B cells organized into primary lymphoid follicles.
The main immunological function of the spleen is to filter the blood and trap blood borne microorganisms and producing an immune response to them.

Tuesday, July 10, 2007

e-Mail ID of the Department of Zoology for correspondence

The following is the e-mail ID of the Department of Zoology for all correspondence.

Wednesday, July 4, 2007

Attn: To all Immunology students

As agreed earlier, no one came out with their presentation. I request all those who have been informed earlier to come out with their presentations (oral) for the next class, i.e., on 6th July 2007.

Srirangam. G. M.

Monday, July 2, 2007

Intro' Ecology - Light

Introductory remarks on Ecology and Light
By G. Mathew Srirangam

origin of word: oikos = the family household logy = the study of
interesting parallel to economy = management of the household many principles in common – resources allocation, cost benefit ratios
definitions: Haeckel (German zoologist) 1870: “By ecology we mean the body of knowledge concerning the economy of Nature - the investigation of the total relations of the animal to its inorganic and organic environment.”
Burdon-Sanderson (1890s): Elevated Ecology to one of the three natural divisions of Biology: Physiology - Morphology – Ecology
Andrewartha (1961): “The scientific study of the distribution and abundance of organisms.”
Odum (1963): “The structure and function of Nature.”
Definition we will use (Krebs 1972):
“Ecology is the scientific study of the processes regulating the distribution andabundance of organisms and the interactions among them, and the study of how theseorganisms in turn mediate the transport and transformation of energy and matter in the biosphere (i.e., the study of the design of ecosystem structure and function).”
The goal of ecology is to understand the principles of operation of natural systems and to predict their responses to change.
Solar radiation is the primary determinant of global climate. The amount of energy reaching the outer atmosphere from the sun is known as the solar constant: 2.0 cal/cm2/minute. Most, but not all, of solar radiation is visible light.
Of this light:
21% is reflected by clouds
5% reflected by dust, aerosols, and soot.
6% reflected by earth
32% total reflected (due to albedo)
18% of radiation is absorbed by dust, water vapor, clouds, carbon dioxide, and soot.
Total radiation removed by atmosphere: 50%. However, different wavelengths are removed differently. Nearly all ultraviolet light is removed.
albedo: Reflection of solar radiation by the earth. Snow and ice have high albedos, forests have low albedos.
Solar radiation is lower at poles than at equator because of the curvature of the earth.


Introduction. Light from the sun supports photosynthesis, permits vision, and heats the earth.
Solar constant = 1.94 cal/cm2 striking upper atmosphere perpendicular to sun at mean distance from sun Fate of 100 units striking atmosphere
Reflected Absorbed
Atmosphere 25 25
Earth 5 *45

*29% = thermals & evaporation, 16% = long wave IR)
88% of IR light is reflected back to earth (12% passes through IR windows)
Low intensity. Low intensity of light reduces photosynthesis. Some animals are adapted to darkness.
Winter: angle of sun greater, more air absorption, shorter day length
In shade: ferns, mosses survive in low light
Under water
Absorption in water is logarithmic for a given wave length
Affected by water color, suspended materials, plankton
Photosynthesis down to 5-50 m in fresh water, 100 m in ocean
Compensation intensity is about 1% of surface intensity for terrestrial and aquatic plants
At night
Carnivores and ungulates have a reflector behind their retina that sends light back through it.
Insects, owls, see in low light
Pit viper snakes, boas detect infra red

High intensity. Plants must adapt to high amounts of light that can destroy their enzymes. When sun directly overhead, and toward equator
Photosynthesis decreases
Photo-oxidation of enzymes, increased respiration
Photosynthesis maximal at 10-20% of full sunlight for leaf perpendicular to sun
5% efficient at high light, 20% at low light
Move to low intensity areas (phytoflagellates)
Turn angle of leaves from sun
Sun vs. shade leaves

Wave length. Visible light is only a small portion of the electromagnetic spectrum that ranges from short wave length gamma radiation to long wave length radio waves.
Electromagnetic spectrum
Gamma: danger in space travel, bombs, radioisotopes, reactors
X-ray: danger from x-ray machines
Ultra violet: kills surface cells (germicidal lamps)
Visible: photosynthesis, vision
Infra red: heat, laser = burns
Microwave: radar, microwave ovens cellular phones TV
Radio: no known effects
Long wave (submarines)
Ecological considerations
Photosynthesis: chlorophyll mainly uses blue and red
Color vision in some arthropods, fishes, birds, mammals
Insects see UV to orange
Absorption by water
IR, UV, red, orange absorbed first
Yellow = intermediate
Blue, green, violet = penetrate deepest (red algae use these wavelengths)

Duration. Animals use variation in day length or phase of the moon coordinate their periodicity.
Daily variation: diurnal, crepuscular, nocturnal
Monthly: moon light used to coordinate reproductive periods
Seasonal: day length varies with seasons, used in timing of migration, reproductive cycles, etc.

Structure of Immunoglobulin-Fig.


G. Mathew Srirangam

Antibodies are glycoproteins that bind antigens with high specificity and affinity (they hold on tightly). They are molecules, originally identified in the serum, which are also referred to as ‘immunoglobulins’; a term often used interchangeably with antibodies. In humans there are five chemically and physically distinct classes of antibodies – IgG, IgA, IgM, IgD, IgE). Immunoglobulins are synthesized by plasma cells and to some extent by lymphocytes also. All antibodies are immunoglobulins but all immunoglobulins may not be antibodies. Immunoglobulins constitute 20-25 percent of the total serum proteins.

Basic Structure of Immunoglobulin:

Immunoglobulins when digested by papain in the presence of cysteine, will split into two fractions – an insoluble fraction which crystallized in the cold called – Fc (for crystallizable) fragment, and a soluble fragment which is unable to precipitate and is called – Fab (for antigen binding) fragment. Each molecule of immunoglobulin is split by papain into three parts, one Fc and two Fab pieces. When treated with pepsin, a 5S fragment is obtained, which is composed essentially of two Fab fragments held together in position. This fragment is called F(ab)2. The Fc portion is digested by pepsin into smaller fragments.

Immunoglobulins are glycoproteins, each molecule consisting of two pairs of polypeptide chains of different sizes. The smaller chains are called ‘light’ (L) chains and larger ones ‘heavy’ (H) chains. The L chain has a molecular weight of approximately 25000 and the H chain of 50,000. The L chain is attached to the H chain by a disulfide bond. The two-H chains are joined together by 1-5 S-S bonds, depending on the class of immunoglobulins.

The H chains are structurally and antigenically distinct for each class and are designated by the Greek letter corresponding to the immunoglobulin class, like –
IgG g(gamma)
IgA a(alpha)
IgM m(mu)
IgD d(delta)
IgE e(epsilon)
The L chains are similar in all classes of immunoglobulins. They occur in two varieties kappa ( k ) and lambda ( l ). A molecule of immunoglobulin may have either kappa or lambda chains, but never both together.

The antigen-combining site of the molecule is at its aminoterminus. It is composed of both L and H chains. Of the 214-aminoacid residues that make up the L chain, about 107 that constitute the carboxyterminal half occur only in a constant sequence. This part of the chain is therefore called the ‘constant region’. Only two sequence patterns are seen in the constant region- those determining the kappa and lambda specificities. On the other hand the aminoacid sequence in the aminoterminal half of the chain is highly variable, the variability determining the immunological specificity of the antibody molecule. It is therefore called the ‘variable region’. The H chain also has ‘constant’ and ‘variable’ regions. While in the L chain the two regions are of equal length, in the H chains the variable region constitutes approximately only a fifth of the chain and is located at its aminoterminus. The infinite range of the antibody specificity of immunoglobulins depends on the variability of the aminoacid sequences at the ‘variable regions’ of the H and L chains, which form the antigen combining sites.

The aminoacid sequences of the variable regions of the L and H chains are not uniformly variable along their length but consist of relatively invariable and some highly variable zones. The highly variable zones numbering three in the L and four in the H chains are called hypervariable regions or hot spots and are involved with the formation of the antigen binding sites.

The Fc fragment is composed of the carboxyterminal portion of the H chains. It does not possess antigen-combing activity but determines the biological properties of the immunoglobulin molecule. The portion of the H chain present in the Fab fragment is called the Fd piece.

Each immunoglobulin peptide chain has internal disulfide links in addition to interchain disulfide bonds, which bridge the H and L chains. These intrachain disulfide bonds form loops and each of the loop is compactly folded to form a globular domain, each domain having a separate function. The variable region domains, VL and VH are responsible for the formation of a specific antigen-binding site. The area of the H chain in the C region between the first and second C region domains (CH1 and CH2) is the hinge region. It is more flexible and is more exposed to enzymes and chemicals.

Organs of Immune system

G. Mathew Srirangam

The immune system consists of many structurally and functionally diverse organs and tissues that are widely dispersed throughout the body. These organs can be classified based on functional differences into two main groups –

1. The primary lymphoid organs or Central lymphoid organs
2. The secondary lymphoid organs or Peripheral lymphoid organs.

The primary lymphoid organs provide appropriate microenvironments in which the precursor lymphocytes proliferate, develop and acquire immunological capability. And the secondary lymphoid organs trap antigen form defined tissues or vascular spaces and provide sites where mature lymphocytes can interact effectively with that antigen. The thymus and bone marrow constitute the primary lymphoid organs. Spleen, Lymph nodes and various mucosal-associated tissues (MALT) compose the secondary lymphoid organs.

Thymus develops from the epithelium of the third and fourth pharyngeal pouches at about the sixth week of gestation and by the eight week, grows into a compact epithelial structure. Mesenchymal stem cells or precursors of lymphocytes from the yolk sac, foetal liver and bone marrow reach the thymus and differentiate into the thymic lymphoid cells (thymocytes). The thymus acquires its characteristic lymphoid appearance by the third month of gestation. It is thus the first organ to become predominantly lymphoid. In human beings, the thymus reaches its maximal relative size just prior to birth. It continues to grow till about the 12th year. After puberty, it undergoes spontaneous progressive involution, indicating that it functions best in early life.

The thymus is a flat, bilobed organ located behind the upper part of the sternum above the heart. Each lobe of thymus is surrounded by a capsule and is divided into lobules, which are separated from each other by strands of connective tissue called trabeculae. Each lobule is organized into two compartments; the outer compartment or cortex, which is densely packed with immature T cells, called thymocytes. The inner compartment or medulla is sparsely populated with thymocytes but have mature lymphocytes between are present Hassall’s corpuscles which are whorl-like aggregations of epithelial cells. Both the cortex and medulla of the thymus are criss-crossed by a three-dimensional stroma-cell network composed of epithelial cells, interdigitating dendric cells and macrophages. Some epithelial cells in the outer cortex, called nurse cells, have long membrane processes that surround as many as 50 thymocytes, forming large multicellular complexes.

The primary function of thymus is the production of thymic lymphocytes or T lymphocytes or Thymus dependent (T) lymphocytes. It is the major site for lymphocyte proliferation. The thymus confers immunological competence on the lymphocytes during their stay in the organ, so that they are capable of mounting cell-mediated immune response against appropriate antigens. Finally the T lymphocytes are selectively seeded into certain sites in the peripheral lymphatic tissues, being found in the white pulp of the spleen, around the central arterioles and in the paracortical areas of lymphoid nodes. A congenital birth defect in humans called DiGeorge’s syndrome and in certain mice (nude mice) involves the failure of the thymus to develop T-lymphocytes and absence of cell-mediated immunity.

Bone Marrow

In birds a lymphoid organ called the bursa of Fabricus is the primary site of B-cell maturation. There is no bursa in mammals and no single counterpart to it as a primary lymphoid organ. Instead, regions of the bone marrow serve as the ‘bursal equivalent’ where B cell maturation occurs. Immature B cells proliferate and differentiate within the microenvironment of the bone marrow. Bone marrow is composed of hematopoietic cells of various lineage and maturity packed between fat cells, thin bands of bony tissue (trabeculae), collagen fibers, fibroblasts and dendritic cells. All the hematopoietic cells are derived from multipotential stem cells which give rise not only to all of the lymphoid cells found in the lymphoid tissue, but also to all of the cells found in the blood. The bone marrow gives rise to all of the lymphoid cells that migrate to the thymus for T-cell maturation as well as to the major population of conventional B cells. B cells mature in the bone marrow and undergo selection for non-self before making their way to the peripheral lymphoid tissues. Stromal cells within the bone marrow interacts directly with the B cells and secretes various cytokines that are required before the B-cell developmental process.


Spleen is a large, ovoid, secondary lymphoid organ situated high in the left abdominal cavity. Unlike lymph nodes, which are specialized to trap localized antigen from regional tissue spaces, the spleen is adapted to filtering blood and trapping blood-borne antigens, and thus can respond to systemic infections.

The spleen is surrounded by a capsule that sends a number of projections (trabeculae) into the interior to form a compartmentalized structure. The compartments are of two types, the red pulp and the white pulp, which are separated by a diffuse marginal zone. The splenic red pulp consists of a network of sinusoids populated with macrophages and numerous red blood cells (erythrocytes). It is the site where old and defective red blood cells are destroyed and removed. Many of the macrophages within the red pulp contain engulfed red blood cells or iron pigments from degraded hemoglobin. The splenic white pulp surrounds the arteries, forming a periarteriolar lymphoid sheath (PALS) populated mainly by T-lymphocytes. The marginal zone, located peripheral to the PALS, is rich in B cells organized into primary lymphoid follicles.

The main immunological function of the spleen is to filter the blood and trap blood borne microorganisms and producing an immune response to them.

Acquired or specific immunity reflects the presence of a functional immune system that is capable of specifically recognizing and selectively eliminating foreign microorganisms and molecules (i.e., foreign antigens). Unlike innate immune responses, acquired immune responses are adaptive and display four characteristic attributes:
• Antigen specificity
• Diversity
• Immunological memory
• Self/nonself recognition
The antigen specificity of the immune system permits it to distinguish subtle (exact) differences among antigens. The immune system is capable of generating tremendous diversity in its recognition molecules, allowing it to specifically recognize billions of uniquely different structures on foreign antigens. Once the immune system has recognized and responded to an antigen, it exhibits immunologic memory; that is, a second encounter with the same antigen induces a heightened state of immune reactivity. Finally, the immune system normally responds only to foreign antigens indicating that it is capable of self/nonself recognition. The ability of the immune system to distinguish self from nonself and respond only to nonself-molecules is essential, for the outcome of an inappropriate response to self-molecules can be a fatal autoimmune disease.

Cells of Immune System:
Generation of an effective immune response involves two major groups of cells; lymphocytes and antigen presenting cells. Lymphocytes are one of many types of white blood cells produced in the bone marrow during the process of hematopoiesis. Lymphocytes leave the bone marrow circulate in the blood and lymph system, and reside in various lymphoid organs. Lymphocytes, which possess antigen-binding cell-surface receptors, possess the defining immunologic attributes of specificity, diversity, memory, and self/nonself recognition. The two major populations of lymphocytes are: B – lymphocytes (B cells) and T – Lymphocytes (T cells)

B Lymphocytes
B-lymphocytes mature within the bone marrow and leave the marrow expressing a unique antigen-binding receptor on their membrane. The B – Lymphocyte receptor is a membrane-bound antibody molecule. Antibodies are glycoproteins. The basic structure of the antibody molecule consists of two identical light polypeptide chains. The chains are held together by disulfide bonds. The amino-terminal ends of each pair of heavy and light chains form a cleft within which antigen binds. When a naïve B cell, which has not previously encountered antigen, first encounters the antigen for which its membrane bound antibody is specific, the cell begins to divide rapidly; its progeny differentiate into memory B cells and effector B cells called plasma cells.
Memory B cells have a longer life span and continue to express membrane-bound antibody with the same specificity as the original parent naïve B cell. Plasma cells do not express membrane-bound antibody; instead they produce the antibody in a form that can be secreted. Although plasma cells live for only a few days, they secrete enormous amounts of antibody during the time.
T Lymphocytes:
T lymphocytes also arise from hematopoietic stem cells in the bone marrow. Unlike B cells, which mature within the bone marrow, T cells migrate to the thymus gland to mature. During its maturation within the thymus, the T cell comes to express a unique antigen-binding receptor on its membrane, called the T-Cell receptor. Unlike membrane-bound antibodies on B cells, which can recognize antigen alone, T cell receptors can only recognize antigen that is associated with cell-membrane proteins known as major histocompatibility complex (MHC) molecules. When a naïve T cell encounters antigen associated with a MHC molecule on a cell, the T cell proliferates and differentiates into memory T cell and various effector T cells.
There are two well-defined subpopulation of T cells - T helper (TH) and T cytotoxic (TC) cells. After a TH cell recognizes and interacts with an antigen-MHC molecule complex, the cell is activated and becomes an effector cell that secretes various growth factors know collectively as cytokines. Under the influence of TH -derived cytokines, a TC cell that recognizes an antigen-MHC complex proliferates and differentiates into an effector cell called a cytotoxic T Lymphocyte.

Functions of Humoral and Cell-Mediated Immune Responses:

Immune responses can be divided into humoral and cell-mediated responses.
The term humoral is derived from the Latin humor, meaning “body fluid”; thus humoral immune system involves interaction of B cells with antigen and their subsequent proliferation and differentiation into antibodies. Antibody functions as the effector of the humoral response by binding to antigen and neutralizing it or facilitating its elimination. When an antigen is coated with antibody, it can be eliminated in several ways. For example, antibody can cross-link the antigen, forming clusters that are more readily ingested by phagocytic cells. Binding of antibody to antigen on a microorganism also can activate the complement system, resulting in lysis of the foreign organism. Antibody can also neutralize toxins or viral particles by coating them and preventing their subsequent binding to host cells.

Effector T cells generated in response to antigen are responsible for cell-mediated immunity. Activated TH cells serve as effector cells in cell-mediated immune reactions. Cytokines secreted by TH cells can activate various phagocytic cells, enabling them to phagocytose and kill microorganisms more effectively. This type of cell-mediated immune response is especially important in host defense against intracellular bacteria and protozoa. Cytotoxic T lymphocytes (CTLs) participate in the cell-mediated immune reactions by killing altered self-cells; they play an important role in the killing of virus-infected cells and tumor cells.

Innate Immunity

Innate immunity has four types of defensive barriers; anatomic, physiologic, endocytic and phagocytic, and inflammatory.

Anatomic barriers: Physical and anatomic barriers that tend to prevent the entry of pathogens are an organism’s first line of defence against infection.

The skin and the surface of mucous membranes are included in the category because they provide an effective barrier to the entry of most microorganisms. The skin consists of two distinct layers; a relatively thin outer layer- the epidermis and a thicker layer the dermis. The epidermis contains several layers of tightly packed epithelial cells. The outer layer consists of dead cells and is filled with a waterproofing protein called keratin. The dermis, is composed of connective tissue, contains blood vessels, hair follicles, sebaceous glands. The sebaceous glands secrete an oily substance called sebum. Sebum consists of lactic acid, which maintains the pH of the skin between 3 and 5. Thus intact skin not only prevents the penetration of most pathogens but also inhibits most bacterial growth due its low pH.

The conjunctiva and the alimentary, respiratory, and urinogenital tracts are lined by mucous membranes, not by the dry, protective skin covering the exterior of the body. These membranes consist of an outer epithelial layer and an underlying connective tissue layer. The secretions of these mucus membranes wash away potential invaders and also contain antibacterial or antiviral substances. The viscous fluid called mucus, which is secreted by epithelial cells of mucous membrane, entraps foreign microorganisms.

Physiologic barriers: The physiologic barriers that contribute to innate immunity include temperature, pH, oxygen tension and various soluble factors. Many species are not susceptible to certain diseases simply because their body temperature inhibits pathogen growth. Gastric acidity also provides an innate physiologic barrier to infection because very few ingested microorganisms can survive the low pH of the stomach. One reason newborns are susceptible to some diseases that do not afflict adults is that their stomach contents are less acid than that of adults. Among the soluble proteins are lysozyme, interferon, and complement. Lysozyme, a hydrolytic enzyme found in mucous secretions, is able to cleave the peptidoglycan layer of the proteins of the bacterial cell wall. Interferon comprises a group of proteins produced by virus-infected cells. Among the many functions of the interferons is the ability to bind to nearby cells and induce a generalized antiviral state. Complement is a group of serum proteins that circulate in an inactive proenzyme state. These proteins can be activated by a variety of specific and nonspecific immunological mechanisms that convert the inactive proenzymes into active enzymes. The activated complement components participate in a controlled enzymatic cascade that results in damage to the membranes of pathogenic organisms, either destroying the pathogens or facilitating their clearance.

Endocytic and Phagocytic Barriers: Another important innate defense mechanism is the ingestion of extracellular macromolecules via endocytosis and of particulate material via phagocytosis. These tow internalization processes not only bring different types of extracellular material into the cell, they also differ in several other ways.

In endocytosis, macromolecules within the extracellular tissue fluid are internalized by cells via the invagination (inward folding) and pinching off of small regions of the plasma membrane. Phagocytosis involves the ingestion of particulate material, including whole pathogenic microorganisms.

Barriers created by the Inflammatory Response:
Tissue damage caused by a wound or by invasion by a pathogenic microorganism induces a complex sequence of events collectively known as the inflammatory response. In the first century A.D. the Roman Physician Celsus described the four cardinal signs of inflammation are rubor (redness), tumor (swelling), calor (heat) and dolor (pain). Afterwards the fifth sign functio laesa (loss of function) was added in second century A.D. Inflammatory response includes three events:

1. Vasodilation: an increase in the diameter of blood vessels – occurs as the vessels that carry blood away from an affected area constrict, resulting in engorgement of the capillary network. The engorged capillaries are responsible for tissue redness (erythema) and an increase in tissue temperature.
2. An increase in capillary permeability facilitates an influx of fluid and cells form the engorged capillaries into the tissue. The fluid that accumulates (exudate) has a much higher protein content than fluid normally released from the vasculature. Accumulation of exudate contributes to tissue swelling (edema).
3. Influx of phagocytes from the capillaries into the tissues is facilitated by the increased capillary permeability. The emigration of phagocytes involves a complex series of events including adherence of the cells to the endothelial wall (margination) followed by their emigration between the capillary endothelial cells into the tissue (diapedesis or extravasation) and, finally, their migration through the tissue to the site of the inflammatory response (chemotaxis). As phagocytic cells accumulate at the site and begin to phagocytose bacteria, they release lytic enzymes, which can damage nearby healthy cells. The accumulation of dead cells, digested material, and fluid forms a substance called pus.

The events in the inflammatory response are initiated by a complex series of interactions involving a variety of chemical mediators. One of the principal mediators is histamine, a chemical released by a variety of cells in response to tissue injury. Histamine binds to receptors on nearby capillaries and venules, causing vasodialtion and increased permeability. Once the inflammatory response has subsided and phagocytic cells have cleared most of the debris away, tissue repair and regeneration of new tissue occur.

Immunology - An Introduction

The immune system is a remarkably adaptive defense system that has evolved in vertebrates to protect them from invading pathogenic microorganisms. It is able to generate an enormous variety of cells and molecules capable of specifically recognizing and eliminating an apparently limitless variety of foreign traders.

Functionally, an immune response can be divided into two interrelated activities – recognition and response. Immune recognition is remarkable for its specificity. The immune system is able to recognize subtle chemical differences that distinguish one foreign pathogen from another. At the same time, the system is able to discriminate between foreign molecules and the body’s own cells and proteins. Once a foreign organism is recognized the immune system enlists the participation of a variety of cells and molecules to mount an appropriate response, known as effector response, to eliminate or neutralize the organism. In this way the system is able to convert the initial recognition event into different effector responses, each uniquely suited to eliminate a particular type of pathogen. Later exposure to the same foreign organism includes a memory response, characterized by a heightened immune reactivity, which serves to eliminate the pathogen and prevent disease.

Variolation – dried crusts derived form smallpox pustules were either inhaled into the nostrils or inserted into small cuts in the skin. Edward Jenner in 1798 significantly improved this technique.
Next technique is Vaccination done by Louis Pasteur.
Immunity mediated by antibodies (serum / antigen) contained in body fluids (known at the time as humors) is called humoral immunity.
In 1883, even before the discovery of antibodies, Elie Metchinkoff demonstrated that cells also contribute to the immune state of an animal. He observed that certain white blood cells, which he termed phagocytes, were able to ingest microorganisms and other foreign material. This is called cell-mediated immunity. Both the humoral and cell mediated immunities are interrelated and both were necessary for the immune response. In 1950s the lymphocyte was identified as the cell responsible for both cellular and humoral immunity.

Components of Immunity:
Immunity – the state of protection from infectious disease- has both nonspecific and specific components. Innate or nonspecific immunity refers to the basic resistance to disease that an individual in born with. Acquired or specific immunity requires the activity of a functional immune system, involving cells called lymphocytes and their products. Innate defense mechanisms provide the first line of host defense against invading pathogens until an acquired immune response develops.