THE ANTIBODY

One of the major functions of the immune system is the production of soluble proteins that circulate freely and exhibit properties that contribute specifically to immunity and protection against foreign material. These soluble proteins are the antibodies, which belong to the class of proteins called globulins because of their globular structure. Today, they are collectively known as immunoglobulins.

As a general rule, all immunoglobulins are antibodies but not all antibodies are immunoglobulin.

Serum is the antibody–containing component of blood left when it has clotted. When subjected to electrophoresis, five major components can be seen visualized: albumin, α1, α2, ß, ϒ globulin.

Forms of immunoglobulins:

1. Secreted antibodies are produced by plasma cells (the terminally differentiated B cells that serve as antibody factories housed largely within the bone marrow).

2. Membrane bound antibody is present on the surface of B cells where it serves as the antigen–specific receptor. It is associated with a heterodimer called Igα/Igß to form the B cell receptor (BCR).

Characteristic features of immunoglobulins:

1. Specificity

This is the ability of the antibody to combine only with those substances that contain one particular antigenic structure and are attributed to a defined region of the antibody molecule containing the hypervariable or complementary–determining region (CDR).

2. Biologic activity

This is attributed to the isotypic (class) structure of the antibody. While one part of the antibody molecule must be adaptable to allow the accommodation of a large number of epitopes, another part of the antibody molecule must be adaptable to allow the antibody molecule to participate in biologic activities common to many antibodies.

Some of the biologic activities of antibodies are:

a. Neutralization of toxins

b. Immobilization of microorganisms or of antigenic particles

c. Binding with soluble antigen leading to the formation of precipitates

d. Activating serum complement to facilitate the lysis of microorganisms or their phagocytosis and destruction either by phagocytic cells or by killer lymphocytes.

e. Ability to cross the placenta from the mother to fetus

Structure of Antibody

1. Fab (fragment antigen binding) – consists of two fragments that are considered univalent, possessing one binding site each and identical to each other. This is a result of papain digestion that cleaves the N–terminal of the disulfide bridge of the heavy chain in the hinge region.

F(ab’)2 – a divalent fragment that is a result of cleavage of C–terminal of the disulfide bridge from pepsin digestion.

2. Fc (fragment crystallizable) – the fragment that can be crystallized out of solution, a property indicative of its apparent homogeneity. It cannot bind antigen but responsible for the biologic functions of the antibody molecule after antigen has been bound to the Fab part of the intact molecule.

3. Two identical Light chain (L chain)

Any one individual of specie produces both types of L chain, but the ratio of κ chains to λ chains varies with the species. However, in any one immunoglobulin molecule, the L chain are always either κ chain or both λ, never one of each.

Two major classes of L chains

a.      κ chain (kappa chain)

b.      λ chain (lambda chain)

4. Two identical Heavy chain

While there are two types of L chains, the immunoglobulin of virtually all species have been shown to consist of five different classes (isotypes) that differ in the structure of their H chains. These H chains differ as antigens, in carbohydrate content, and in size. They also confer different biologic functions on each isotype. The H chains, whose constant regions are derived from Ig heavy chain genes are designated with Greek letters as shown below:

Immunoglobulin class (isotype)	Heavy chain
IgM	µ
IgD	δ
IgG	ϒ
IgA	α
IgE	ε

Any individual of a species makes all H chains, in proportions characteristic of the species, but in any one antibody molecule both H chains are identical (i.e., 2ϒ or 2ε). Thus, an antibody molecule of the IgG class could have the structure κ2ϒ2 or λ2ϒ2, while an antibody of the IgE class could have the structure κ2ε2 or λ2ε2. In each case, it is the nature of the H chains that confers on the molecule its unique biologic properties, such as half–life in the circulation, its ability to bind to certain receptors, and its ability to activate enzymes on combination with antigens.

5. Domains

A compact segment of an immunoglobulin or TCR chain, made up of amino acids around an S–S bond.

Each domain is designated by a letter that indicates its position. The first domain on L and H chains is highly variable, in terms of amino acid sequence, from one antibody to the next, and it is designated V_L or V_H accordingly. The second and subsequent domains on both chains are much more constant in amino acid sequence and are designated CL or C_H1, C_H2, and C_H3. In addition to their interchain disulfide bonding, the globular domains bind to each other in homologous pairs, largely by hydrophobic interactions, as follows: V_HV_L, C_H1C_L, C_H2C_H2 and C_H3C_H3.

6. Hinge Region

This is the flexible and open segment of an antibody molecule that allows bending of the molecule. The hinge region is located between Fab and Fc and is susceptible to enzymatic cleavage.

The hinge region is composed of a short segment of amino acids and is found between the C_H1 and C_H2 regions of the H chains. This segment is made up predominantly of cysteine and proline residues. The cysteines are involved in formation of interchain disulfide bonds, and the proline residues prevent folding in a globular structure.

7. Variable Region

The N–terminal portion of an Ig or TCR which contains the antigen–binding region of the molecule; V regions are formed by the recombination of V (D) and J gene segment.

It is the variable region that constitutes the part of the molecule that binds to the epitope.

Hypervariable regions are portions of the light and heavy immunoglobulin chains that are highly variable in amino acid sequence from one immunoglobulin molecule to another, and that together constitute the antigen – binding site of antibody molecule. The less variable stretches, which occur between these hypervariable regions are called framework regions.

Complemetarity–determining regions (CDRs) are parts of the immunoglobulins and T–cell receptors that determine their specificity and make contact with specific ligand. The CDRs are the most variable part of the molecule and contribute to the diversity of the molecules. There are three such regions (CDR1, CDR2 and CDR3) in each V domain.  Thus they are also classified as hypervariable regions.

The variability in these CDRs provides the diversity in the shape of the combining site that is required for the function of antibodies of different specificities. All the known forces involved in antigen–antibody interactions are weak, noncovalent interactions (e.g., ionic, hydrogen–bonding, and hydrophobic interactions). It is therefore necessary that there be a close fit between antigen and antibody over a sufficiently large region to allow a total binding force that is adequate for stable interaction. Contributions to this binding interaction by both H and L chains are involved in the overall association between epitope and antibody.

It should now be apparent that two antibody molecules with different antigenic specificities must have different amino acid sequences in their hypervariable regions and that those with similar sequences will generally have similar specificities. However, it is possible for two antibodies with different amino acid sequences to have specificity to the same epitope. In this case, the binding affinities of the antibodies with the epitope will probably be different because there be differences in the number and types of binding forces available to bind identical antigens to the different binding sites of the two antibodies.

An additional source of variability involve the size of the combining site on the antibody, which is usually (but not always) considered to take the form of a depression of cleft. In some instances, especially when small, hydrophobic haptens are involved, the epitopes do not occupy the entire combining site, yet they achieve sufficient affinity of binding. It has been shown that antibodies specific for such a small hapten may, in fact, react with other antigens that have no obvious similarity to the hapten. These large, dissimilar antigens bind either to a larger area or to a different area of the combining site may have the ability to combine with two (or more) apparently diverse epitopes, a property called redundancy. The ability of a single antibody molecule to cross – react with an unknown number of epitopes may reduce the number of different antibodies needed to defend an individual against the range of antigenic challenges.

Immunoglobulin variants

1. Isotypes

This is also known as antibody classes. Antibodies that differ in the heavy chain constant regions: IgM, IgG, IgD, IgA and IgE. These differences result in distinct biological activities of the antibodies; distinguishable also on the basis of reaction with antisera raised in another species.

2. Allotypes

Antigenic determinants that is present in allelic (alternate) forms. When used in association with immunoglobulin, allotypes describe allelic variants of immunoglobulins detected by antibodies raised between members of the same species.

As a result of allotypy, a heavy chain or light chain constituent of any immunoglobulin can be present in some members of a species and absent in others. This situation contrasts with that of immunoglobulin classes or subclasses, which are present in all members of a species.

Allotypic differences at known loci usually involve changes in only one or two amino acids in the constant region of a chain. With a few exceptions, the presence of allotypic differences in two identical immunoglobulin molecules does not generally affect binding with antigen, but it serves as an important marker for analysis of Mendelian inheritance.

3. Idiotypes

This is the combined antigenic determinant (idiotopes) expressed in the variable region of antibodies of an individual that are directed at a particular antigen.

In some cases, anti–idiotypic sera prevent binding of the antibody with its antigen, in which event the idiotypic determinant is considered to be in or very near the combining site itself. Anti–idiotypic sera, which do not block binding of antibody with antigen, are probably directed against variable determinants of the framework area, outside the combining site. The anti–idiotype may represent a facsimile or an internal image of the nominal epitope.

Public or cross–reacting idiotypes are anti–idiotypic antibodies that react with several different antibodies that are directed against the same epitope and share idiotypes.

Private idiotype are sera that react with only one particular antibody molecule.

Features of Immunoglobulin Isotypes

	IgG	IgA	IgM	IgD	IgE
Molecular Weight	150,000	160,000 (monomer)	900,000	180,000	200,000
Additional Subunits	–	J and S	J	–	–
Serum Concentration (mg/ml)	12	1.8	1	0 – 0.4	0.00002
% of total IgG	80	13	6	0.2	0.002
Distribution	IV & EV	IV and secretions	Mostly IV	Present on lymphocyte surface	On basophils and mast cells present in saliva and nasal secretions
Half–life (days)	23	5.5	5	2.8	2.0
Placental Passage	++	–	–	–	–
Presence in Secretions	–	++	–	–	–
Presence in Milk	+	+	0 to trace	–	–
Activation of complement	+	–	+++	–	–
Agglutinating Capacity	+	++	+++	–	–
Antiviral Activity	+++	+++	+	–	–
Antibacterial Activity	+++	++ (with lysozyme)	+++ (with complement)	–	–
Antitoxin Activity	+++	–	–	–	–
Allergic Activity	–	–	–	–	++
IV – intravascular ; EV – extravascular

The IgG

IgG is the predominant immunoglobulin in blood, lymph fluid, cerebrospinal fluid, and peritoneal fluid. The IgG molecule consists of two ϒ H chains of molecular weight of approximately 50,000 Da each and two L chains (either λ or κ) of molecular weight approximately 25,000 Da each, held together by disulfide bonds. Thus, the IgG molecule has a molecular weight of approximately 150,000 Da and a sedimentation coefficient of 7S. Electrophoretically, the IgG molecule is the least anodic of all serum proteins, and it migrates to the ϒ range of serum globulins; hence it’s earlier designation as ϒ–globulin or 7S immunoglobulin.

IgG present in the serum of human adults represents about 15% of the total protein. IgG is distributed approximately equally between the intravascular and extravascular spaces.

The prolonged survival of IgG in blood is attributed to the presence of saturable IgG protection receptor (FcRp, also called Brambell receptor). This receptor is found in cellular endosomes and selectively recycles endocytized IgG (e.g., following endocytosis of antigen–antibody immune complexes) back to the circulation. This mechanism operates to “cleanse” IgG antibody of antigen and harvest antigen for presentation without destruction. Conditions associated with high IgG levels saturate the FcRp receptors rendering the catabolism of excess IgG indistinguishable from albumin or other Ig isotypes.  

Differences between Human IgG subclasses

	IgG1	IgG2	IgG3	IgG4
% of total IgG	70	20	7	3
Half–life (days)	23	23	7	23
Complement binding	+	+	+++	–
Placental Passage	++	±	++	++
Binding of monocytes	+++	+	+++	±

Biologic properties of IgG:

1. Agglutination and formation of precipitate

IgG molecules can cause agglutination or clumping of particulate with “insoluble antigens” such as microorganisms. The reaction of IgG with “soluble antigen” will then produce precipitate.

2. Passage through the placenta and absorption in neonates

Except for IgG₂, most IgG isotype can pass through the placenta enabling the mother to transfer her immunity to the fetus. Placental transfer is facilitated by expression of an IgG protection receptor (FcRn) expressed on placental cells. FcRn was recently shown to be identical to IgG protection receptor (FcRp) found in the cellular endosomes.

The IgG protection receptor (FcRn) expressed on placental cells is transiently superexpressed in the intestinal tissue of neonates. Absorption of maternal IgG contained the colostrums of nursing mothers is achieved by its binding to these high density receptors in intestinal tissue.

While passage of IgG molecules across the placenta confers immunity to infection on the fetus, it may also be responsible for hemolytic disease of the newborn (erythroblastosis fetalis). This is caused by maternal antibodies to fetal red blood cells.

3. Opsonization

Many phagocytic cells, including macrophages and polymorphonuclear phagocytes, bear receptors for the Fc portion of the IgG molecule. These cells adhere to the antibody–coated bacteria by virtue of their receptors for Fc. The net effect is a zipperlike closure of the surface membrane of the phagocytic cell around the organism, as receptor for Fc and the Fc regions on the antibodies continue to combine, leading to the final engulfing and destruction of the microorganism.

4. Antibody–dependent, Cell–mediated Cytotoxicity (ADCC)

In this form of cytotoxicity, the Fab portion binds with the target cell, whether it is a microorganism or a tumor cell, and the Fc portion binds with specific receptors for Fc that is found on certain large granular lymphocytic cells called natural killer (NK) cells. By this mechanism, the IgG molecule focuses the killer cells on their target, and the killer cells destroy the target, not by phagocytosis but with various substances that they release.

5. Activation of Complement

Activation of complement results in the release of several important biologically active molecules and leads to lysis if the antibody is bound to antigen on the surface of a cell. Some of the complement components are also opsonins; they bind to the target antigen ad thereby direct phagocytes, which carry receptors specific for these opsonins, to focus their phagocytic activity on the target antigen. Other components from the activation of complement are chemotactic; specifically, they attract phagocytic cells.

6. Neutralization of Toxin

The IgG molecule is an excellent antibody for the neutralization of such toxins as tetanus and botulinus, or for the activation of, for example, snake and scorpion venoms. Because of its ability to neutralize such poisons (mostly by blocking their active sites) and because of its long half–life, compared to that of other isotypes, the IgG molecule is the isotype of choice for passive immunization (i.e., the transfer of antibodies) against toxins and venoms.

7. Immobilization of Bacteria

IgG molecules are efficient in immobilizing various mobile bacteria. Reaction of antibodies specific for the flagella and cilia of certain microorganisms causes them to clump, thereby arresting their movement and preventing their ability to spread or invade tissue.

8. Neutralization of Viruses

One mechanism of neutralization is that in which the antibody binds with antigenic determinants present on various portions of the virus coat, among which is the region used by the virus for attachment to the target cell. Inhibition of viral attachment effectively arrests infection. Other antibodies are thought to inhibit viral penetration or shedding of the viral coat required for release of the viral DNA or RNA needed to induce infection.

The IgM

IgM is the first immunoglobulin produced following immunization. Its name derives from its initial description from as a macroglobulin (M) of high molecular weight (900,000 Da). It has a sedimentation coefficient 0f 19S, and it has an extra C_H domain. In comparison to the IgG molecule, which consists of one four–chain structure, IgM is a pentameric molecule composed of five such units, each of which consists of two L and two H chains, all joined together by additional disulfide bonds between their Fc portions and by a polypeptide chain termed the J chain. The J chain, which, like L and H chains, is synthesized in the B cell or plasma cell, has a molecular weight of 15,000 Da. This pentameric ensemble of IgM, which is held together by disulfide bonds, comes apart after mild treatment with reducing agents such as mercaptoethanol.

Surprisingly, each pentameric IgM molecule appears to have a valence of 5 (i.e., five antigen combining sites), instead of the expected valence of 10 predicted by the 10 Fab segments contained in the pentamer. This apparent reduction in valence is probably the result of conformational constraints imposed by the polymerization. It is known that pentameric IgM has a planar configuration, such that each of its 10 Fab portions cannot open fully with respect to the adjacent Fab, when it combines with antigen, as is possible in the case of IgG. Thus, any large antigen bound to one Fab may block a neighboring site from binding with antigen, making the molecule appear pentavalent (or of even lesser valence).

Aside from intravascular spaces, IgM is also found on the surface of mature B cells together with IgD, where it serves as an antigen–specific B cell receptor (BCR). Once the B cell is activated by antigen following litigation of the BCR, it may undergo class switching and begin to secrete and express other membrane Ig isotypes.

IgM antibodies do not pass through the placenta, however, since this is the only class of immunoglobulin that is synthesized by the fetus beginning at approximately 5 months of gestation, elevated levels of IgM in the fetus are indicative of congenital or perinatal infection.

IgM is the isotype synthesized by children and adults in appreciable amounts after immunization or exposure to T–independent antigens, and it is the first isotype that is synthesized after immunization with T–dependent antigens. Thus, elevated levels of IgM usually indicate either recent infection or recent exposure to antigen.

Biologic properties of IgM

1. Agglutination

Because of their pentameric form, IgM antibodies can form macromolecular bridges between epitopes on molecules that may be too distant from each other to be bridged by the smaller IgG antibodies. Furthermore, because of their pentameric form and multiple valence, IgM antibodies are particularly well suited to combine with antigens that contain repeated patterns of the same antigenic determinant, as in the case of polysaccharide antigens or cellular antigens, which are multiply expressed on cell surfaces.

2. Isohemagglutinins

The IgM antibodies include the so–called natural isohemagglutinins – the naturally occurring antibodies against the red blood cell antigens of the ABO blood groups. The antibodies are presumed to arise as a result of immunization by bacteria in the gastrointestinal and respiratory tracts, which bear determinants similar to the oligosaccharides of the ABO blood groups.

IgM isohemagglutinins do not pass through the placenta, so incompatibility of the ABO groups between mother and fetus poses no danger to the fetus. However, transfusion reactions, which arise as a result of ABO incompatibility, and in which the recipient’s isohemagglutinins react with the donor’s red blood cells, may have disastrous consequences.

3. Activation of complement

Because of its pentameric form, IgM is an excellent complement–fixing or complement–activating antibody. Unlike other classes of immunoglobulins, a single molecule of IgM, on binding to antigen with at least two of its Fab arms, can initiate the complement sequence, making it the most efficient immunoglobulin as an initiator of the complement–mediated lysis of microorganisms and other cells. This ability, taken together with the appearance of IgM as the first class of antibodies generated after immunization or infection, makes IgM antibodies very important as providers of an early line of immunologic defense against bacterial infections.

In contrast to IgG, the IgM antibodies are not very versatile; they are poor toxin – neutralizing antibodies, and they are not efficient in the neutralization of viruses.

The IgA

IgA is the major immunoglobulin in external secretions such as saliva, mucus, sweat, gastric fluid, and tears. It is moreover, the major immunoglobulin of colostrums and milk, and it may provide the neonate with a major source of intestinal protection against pathogens. The IgA molecule consists of either two κ chains or two λ chains and two H α chains. The α chain is somewhat larger than ϒ chain.

The IgA class of immunoglobulins contains two subclasses: IgA₁ (93%) and IgA₂ (7%). It is interesting to note that if all production of IgA on mucosal surfaces (respiratory, gastrointestinal and urinary tracts) is taken into account, IgA would be the major immunoglobulin in terms of quantity.

Serum IgA has no known biologic activity. The IgA present in serum is predominantly monomeric (one four–chain unit) and has presumably been released before dimerization so that it fails to bind to the secretory component. Dimeric IgA has a molecular weight of 400,000 Da.

In secretions such as tears, saliva and mucus, it serves a specific biologic function as part of the mucosa–associated lymphoid tissue (MALT). Within mucous secretions, IgA exists as a dimer consisting of two four–chain units linked by the same joining (J) chain found in IgM molecules. Plasma cells synthesize only the basic IgA molecules and the J chains, which form the dimers. Such IgA–secreting plasma cells are located predominantly in the connective tissue called lamina propria that lies immediately below the basement membrane of many surface epithelia (e.g., parotid gland, along the gastrointestinal tract in the intestinal villi, in tear glands, in the lactating breast, or beneath bronchial mucosa). When these dimeric molecules are released from plasma cells, they bind to the poly–IgG receptor expressed on the basal membranes of adjacent epithelial cells. This receptor transports the molecules through the epithelial cells and releases them into extracellular fluids (e.g., in the gut or bronchi). Release is facilitated by enzymatic cleavage of the poly–IgG receptor, leaving a large 70,000 Da fragment (i.e., the secretory component) of the receptor still attached to the Fc piece of the dimeric IgA molecule. The secretory component may help to protect the dimeric IgA from proteolytic cleavage. It should also be noted that the secretory component also binds and transports pentameric IgM to mucosal surfaces in small amounts.

Biologic properties of IgA

1. Role in Mucosal infections

Because of its presence in secretions, such as saliva, urine, and gastric fluid, secretory IgA is of importance in the primary immunologic defense against local infections in such areas as the respiratory or gastrointestinal tract. Its protective effect is thought to be due to its ability to prevent the invading organism from attaching to and penetrating the epithelial surface.

Thus, for protection against local infections, routes of immunization that result in local production of IgA are much more effective than routes that primarily produce antibodies in serum.

2. Bactericidal activity

The IgA molecule does not contain receptors for complement and, thus, IgA is not a complement–activating or complement–fixing immunoglobulin. Consequently, it does not induce complement–mediated bacterial lysis. However, IgA has been shown to possess bactericidal activity against gram–negative organisms, but only in the presence of lysozyme, which is also present in the same secretions that contain secretory IgA.

3. Antiviral activity

Secretory IgA is an efficient antiviral antibody, preventing the viruses from entering host cells. In addition, secretory IgA is an efficient agglutinating antibody.

The IgD

The IgD molecule consists of either two κ or two λ L chains and two H δ–chains. IgD is present as a monomer with a molecular weight of 180,000 Da, it has a sedimentation coefficient of 7S, and it migrates to the fast ϒ–region of serum globulins. No H–chain allotypes or subclasses have been reported for the IgD molecule.

IgD is present in serum in very low and variable amounts, probably because it is not secreted by plasma cells and because, among immunoglobulins, it is uniquely susceptible to proteolytic degradation. In addition, following B–cell activation, transcription of the δ heavy chain protein is rapidly downregulated – a phenomenon that also helps to explain the low serum IgD levels.

IgD is coexpressed with IgM on the surface of mature B cells and, like IgM, functions as an antigen–specific BCR. Its presence there serves as a marker of the differentiation of B cells to a more mature form. Thus, during ontogeny of B cells, expression of IgD lags behind that of IgM.

While the function of IgD has not been fully elucidated, expression of membrane IgD appears to correlate with the elimination of B cells with the capacity to generate self– reactive antibodies. Furthermore, in mature B cells, binding autoreactive of IgM– and IgD– positive cells to self–antigens results in the loss of surface expression of IgM but not of IgD. Such cells are unable to enter the primary follicles in lymphoid tissues where appropriate T cell help would facilitate the development of autoreactive responses. Therefore, these cells are rendered anergic and are rapidly lost. The removal of self–antigens, however, allows these cells to enter the follicles. This indicates that neither the mere loss of IgM surface expression not the exclusive expression of IgD (without Ig M surface expression nor the exclusive expression of IgD (without IgM) can explain the failure of such cells to enter the lymphoid follicles.

The IgE

The IgE molecule consists of two L chains (κ or λ) and two H ε–chains. Like the IgM molecule, IgE has an extra C_H domain. IgE has a molecular weight of approximately 200,000 Da, its sedimentation coefficient is 8S, and it migrates electrophoretically to the fast ϒ–region of serum globulins.

IgE, also termed reaginic antibody, has a half–life in serum of 2 days, the shortest half–life of all classes of immunoglobulins. It is present in serum in the lowest concentration of all immunoglobulins. These low levels are due in part to a low rate of synthesis and to the unique ability of the Fc portion of IgE containing the extra C_H domain to bind with very high affinity to receptors (Fcε receptors) found on mast cells and basophils. Once bound to these high–affinity receptors, IgE may be retained by these cells for weeks or months. When antigen reappears, it combines with the Fab portion of the IgE attached to these cells causing it to be cross–linked. The cells become activated and release the contents of their granules: histamine, heparin, leukotrienes, and other pharmacologically active compounds that trigger the immediate hypersensitivity reactions. These reactions may be mild, as in the case of a mosquito bite, or severe, as in the case of bronchial asthma; they may even result in systemic anaphylaxis, which can cause death within minutes.

IgE is not an agglutinating or complement–activating antibody; nevertheless, it has a role in protection against certain parasites, such as helminthes, a protection achieved by activation of the same acute inflammatory response seen in a more pathologic form of immediate hypersensitivity responses. Elevated levels of IgE in serum have been shown to occur during infections with ascaris (a roundworm). In fact, immunization with ascaris antigen induces the formation of IgE.

Effects of antibody response following immunization

1. Primary response

The first exposure of an individual to a particular immunogen is referred to as the primary immunization and the measurable response that ensues is called primary response. The primary antibody response may be divided into several phases, as follows:

a. Latent or lag phase

After initial injection of antigen, a significant amount of time elapses before antibody is detectable in the serum. The length of this period is generally 12 weeks, depending on the species immunized, the antigen and other factors that will become apparent in subsequent chapters. The length of the latent period is also greatly dependent on the sensitivity of the assay used to measure the product of the response. As we shall see in more detail in subsequent chapters, the latent period includes the time taken for T and B cells to make contact with the antigen, to proliferate, and to differentiate. B cells must also secrete antibody in sufficient quantity so that it can be detected in the serum.

b.  Exponential production phase

During this phase, the concentration of antibody in the serum increases exponentially.

c. Steady state

During this period, production and degradation of antibody are balanced.

d. Declining phase

Finally, the immune response begins to shut down, and the concentration of antibody in serum declines rapidly.

In the primary response, the immune response detected is generally IgM, which in some instances may be the only class of immunoglobulin that is made. If production of IgG antibody ensues, its appearance is generally accompanied by a rapid cessation of production of IgM.

2. Secondary response

Although production of antibody after a priming contact with antigen may cease entirely within a few weeks, the immunized individual is left with a cellular memory of this contact. This memory becomes apparent when a response is triggered by a second injection of the same antigen. After the second injection, the lag phase is considerably shorter and antibody may appear in less than half the time required for the primary response. The production of antibody is much greater, and higher concentrations of antibody are detectable in the serum. The production of antibody may also continue for a longer period, with persistent levels remaining in serum months, or even years later.

There is a marked change in the type and quality of antibody produced in the secondary response. There is a shift in class response known as class switching, with IgG antibodies appearing in higher concentrations, and with greater persistence, than accompanied by the appearance of IgA and IgE. In addition, affinity maturation occurs, such that the average affinity (binding constant) of the antibodies for the antigen increases as the secondary response develops. The driving force for this increase in affinity may be a selection process during which B cells compete with free antibody to capture a decreasing amount of antigen. Thus, only those B–cell clones with high affinity Ig receptors on their surfaces will bind enough antigens to ensure that the B cells are triggered to differentiate into plasma cells. These plasma cells, which arise from preferentially selected B cells, synthesize this antibody with high affinity for antigen.

The capacity to make a secondary or anamnestic (memory) response may persist for a long time (years in humans), and it provides an obvious selective advantage for an individual that survives the first contact with an invading pathogen. Establishment of this memory for generating a specific response is, of course, the purpose of public health immunization program.

The Immunoglobulin Superfamily

The shared structural features of immunoglobulin heavy and light chains which include the immunoglobulin–fold domains are also seen in a large number of proteins. Most of these have been found to be membrane–bound glycoproteins. Because of this structural similarity, these proteins are classified as members of the immunoglobulin superfamily. The redundant structural characteristic seen in these proteins suggests that the genes that encode them arose from a common primordial gene – on that generated the basic domain structure. Duplication and subsequent divergence of this primordial gene would explain the existence of the large number of membrane proteins that possess one or more regions homologous to the immunoglobulin–fold domain. Genetic and functional analyses of these immunoglobulin superfamily proteins have indicated that these genes have evolved independently, since they do not share genetic linkage or function.