Antigen recognition molecule

In order for the immune system to respond to non-self, i.e. foreign antigen, a recognition system capable of precisely distinguishing self from non-self has to evolve. What are the molecules, which recognize and bind to an-tigen (foreign particles)?

1. Antibodies [immunoglobulins (Ig)], which include both:a. Membrane-bound receptor on the

surface of B cells [B cell antigen re-ceptor (BCR)], closely associated with secondary component such as Ig /Ig heterodimer.

b. Soluble molecules (secreted from plasma cells) present in serum and tissue fluids, which are structurally identical to the B cell antigen recep-tor, but lack transmembrane and in-tracytoplasmic portion.

2. T cell antigen-specific membrane-bound receptors (all the T cell popula-tion such as CD4, CD8, CD2, etc).

3. Products of major histocompatibility complex (MHC), includes both MHC I and MHC II. Class I MHC molecules bind to peptides, derived from cytosolic (intracellular protein), known as endog-enous antigens. Class II MHC molecules bind to peptides derived from extracel-lular proteins that have been brought in to the cell by phagocytosis or endocyto-sis (exogenous antigens).

All the above stated molecules have structural features in common. They have a domain structure built on three dimensional features known as immunoglobulin fold (Ig fold). Their structure and functions so pre-sumed to be members belonging to one gene family known as Ig supergene family (Fig. 5.1). Large numbers of other membrane pro-teins such as cell adhesion molecules [vas-cular cell adhesion molecule-1 (VCAM-1), intracellular adhesion molecules-2 (ICAM-2), leukocyte function antigen (LFA-3)], platelet-derived growth factor, 2-microglobulin, etc. are included in Ig supergene family.

ANTIBODIES— IMMUNOGLOBULINS

The humoral basis of immunity was estab-lished at the early 18th century. Following injection of antigen into the animal, certain substances appeared in the serum and the tissue fluid called antibody, which reacted with the antigen specifically in an observable manner. Depending on the type of reaction, the antibodies were known as agglutinin, precipitin and complement-fixing antibod-ies and so on. Sera having a higher antibody level are known as immune sera.

Fractionation of immune sera by half-saturation with ammonium sulfate separated serum protein into soluble albumins and in-soluble globulins. The insoluble globulin had

Antigen Recognition

Molecules 5

42 Textbook of Immunology

two fractions, water-soluble pseudoglobulin and another is the water insoluble, i.e. eu-globulin. Most antibodies were found to be euglobulins.

Tiselius, in 1937 separated serum pro-teins by electrophoresis into albumin, alpha- globulin, beta-globulin and gamma-glob-ulin. Antibody activity was associated with gamma-globulin.

Sedimentation studies using ultracentri-fuge disclosed the diversity of the antibody molecules. While most were sedimented at 7S, molecular weight 150,000 (S = Svedberg unit, i.e. a sedimentation constant of 1 10-13

second), some were heavier 19S (900,000).

Thus, indiscriminate use of various termi-nologies led to confusion. In 1964, a com-mon terminology was evolved called Ig and was accepted internationally.

Definition

Immunoglobulins are proteins of animal ori-gin, endowed with known antibody activity and for certain other proteins related to them by chemical structure. That means the Ig

include, besides antibody globulin, the ab-normal proteins found in myeloma, macro-globulinemia, cryoglobulinemia, etc. While Ig satisfies the structural and chemical con-cept, the antibody provides biological and functional concept. All antibodies are Ig, but all Ig are not antibodies.

Immunoglobulins constitute 20% to 25% of the serum protein. Based on the physi-cochemical, antigenic differences and the types of heavy chain Igs are classified into five types.

All Igs are made up of light (molecular weight 25,000) and heavy polypeptide chains (molecular weight 50,000). Light (L) chains are of one of the two, kappa (K) or lambda ( ). Both types can occur in all classes of Ig (IgG, IgM, IgA, IgE and IgD), but any one Ig contains only one type of L chain. Both the L chain of one Ig molecule cannot have both kappa and lambda chain. The amino-termi-nal portion of each L chain contains a part of antigen-binding site.

Heavy (H) chains are distinct for each of the five Ig classes and are designated (gamma),

Fig. 5.1: Some members of the immunoglobulin supergene family, a group of structurally related usually membrane-bound glycoproteins

Antigen Recognition Molecules 43

(mu), (alpha), (delta) and (epsilon). The amino-terminal portion of each H chain participates in the antigen-binding site. The carboxy-terminal portion forms the fraction crystallizable (Fc) fragment, which has vari-ous biologic activities (complement activa-tion, macrophage fixation, reactivity with rheumatoid factor and binding to cell-surface receptors). An individual antibody molecule consists of two H chains and L chains, cova-lently linked by disulfide bonds. Both the H chains and L chains are identical.

Proteolytic cleavage of IgG by Porter, Edelman and their colleagues led to a better understanding of the detailed structure of the Ig molecule. Pepsin treatment produces a di-meric F(ab)

2 fragment. Papain treatment pro-

duces monovalent antigen-binding fragment (Fab) and Fc fragments. The F(ab)

2 and Fab

fragments bind antigen, but lack a functional Fc region (Fig. 5.2).

Light and heavy chains are subdivided into variable regions and constant regions. A L chain consists of one variable domain (V

L)

and one constant domain (CL). Most H chains

consist of one variable domain (VH) and three

or more constant domains (CH). Each domain

is approximately 110 amino acids long. Vari-able regions are for antigen-binding and the constant regions are responsible for other biologic functions.

In the variable regions of both L and H chains, there are three extremely variable (hypervariable) amino acid sequences that form the antigen-binding site. The hyper-variable region (HVR) form the complemen-tary region of the antigenic determinant and therefore, known as complementary deter-mining regions (CDRs) (Fig. 5.3).

Classes of Immunoglobulin

There are five classes of immunoglobulins, according to their properties (Table 5.1). They are:

1. Immunoglobulin G (IgG). 2. Immunoglobulin A (IgA). 3. Immunoglobulin M (IgM). 4. Immunoglobulin D (IgD). 5. Immunoglobulin E (IgE).

Immunoglobulin G

Immunoglobulin G is the main class of immunoglobulin in serum. It exists as a

Fig. 5.2: Proteolytic digestion of immunoglobulin G (IgG). Pepsin treatment produces a dimeric F(ab)2 frag-

ment. Papain treatment produces monovalent Fab fragments and an Fc fragment. The F(ab)2 and the Fab

fragments bind antigen, but lack a functional Fc region.


Table 5.1 Comparative features of immunoglobulin isotypes

Property IgG IgA IgM IgD IgE

Light chain Kappa or lambda

Kappa or lambda

Kappa or lambda Kappa or lambda

Kappa or lambda

Heavy chain Gamma ( ) Alpha ( ) Mu ( ) Delta ( Epsilon ( )

Serum concentration (mg/mL)

12 2 1.2 0.03 0.0003

Percentage 75% 10% –15% 5% –10% – –

Sedimentation coe!cient

7S 7S, 11S 19S 7S 8S

Molecular weight 150,000 160,000 900,000–1,000,000 180,000 190,000

Half-life (day) 23 6 5 3 2

Placental transfer + (IgG1, IgG3, IgG4)

– – – –

Presence in milk + + – – –

Carbohydrate percentage (%)

3 8 12 13 12

Heat stability (56°C)

+ – + + –

Location (mostly) Serum, extravascular

Transport across epithelium

Serum (intravascular) B cell membrane

Serum, extra-vascular

Principal biological activities

Complement #xation

+ + (alternate pathway)

+++ – –

Neutralization + – – – –

Opsonization + – +++ – –

Phagocytosis + + –/+ – –

Binds to mast cells and basophils

– – – – +

Local immunity – + (secretory IgA)

– – –

Examples Antiviral, antitoxic, opsonin, protects fetus and newborn

Immunity against enteric, polio and in$uenza virus

ABO red cell antibodies, rheumatoid factor, antibodies against OAg (Enterobacteriaceae)

– Mediator of immediate- hypersensi-tivity reaction


molecule of molecular weight 146 to 160 kDa (7S) in serum and is abundant compo-nent of the secondary humoral response. This class of Ig is not only found in the blood-stream, but also in extravascular spaces. It contains less carbohydrate than other Igs. It has a half-life of approximately 23 days. It is also transported across the placenta and is therefore, responsible for passive immunity in the fetus and neonate. Passively adminis-tered IgG, suppresses the homologous anti-body synthesis by a feed back mechanism. This process is utilized in the immunization of women by the administration of anti-RhD IgG during delivery.

There are four subclasses of IgG isotypes in man (IgG1, IgG2, IgG3 and IgG4), each one is distinguished by a minor variation in the amino acid sequences in the C-region and by the numbers and location of disulfide bridges. The four subclasses are distributed in human serum, IgG1 (65%), IgG2 (23%), IgG3 (8%) and IgG4 (4%) (Fig. 5.4).

Immunoglobulin G participates in most immunological reactions such as comple-ment fixation (IgG1 and IgG3), precipitation, neutralization of toxins and viruses. IgG1 and IgG3 are capable of interacting with the Fc receptors on macrophages and therefore, acting as efficient opsonins.

Fig. 5.3: Schematic representation of an IgG molecule, indicating the location of the constant and the variable regions on the light and heavy chains


Immunoglobulin A

Immunoglobulin A is the second most abun-dant class of immunoglobulin constitute about 10% to 13% of all serum immunoglobulins. The normal serum level is 0.6 to 4.2 mg per mL. It has a half-life of 6 to 8 days. IgA is found in two forms in the body�in serum, where it occurs principally as monomer (160 kDa, 7S) and on secretory surfaces, where it exists as a dimeric molecule (385 kDa, 11S).

The dimeric form is known as secretory IgA (sIgA) (Fig. 5.5) and is found in associa-tion of J chain and with secretory compo-nent; the latter is involved in the transport of IgA to the secretory surfaces. Secretory com-ponent is non-covalently associated with the IgA molecules in the sIgA complex. sIgA is the main Ig in the secretions such as milk, saliva and tears and in the secretions of respi-ratory, intestinal and genital tracts. It protects mucous membranes from attack by bacteria and viruses.

There are two subclasses of IgA; IgA1 and IgA2 distinguished by their distribution and arrangement of disulfide bonds. IgA is the

predominant form found in serum, where as IgA1 and IgA2 isotypes are present in roughly equal amounts in IgA.

Immunoglobulin A is the component of the secondary humoral response. The prin-cipal antigens that elicit an IgA response are microorganisms in the gut or on the airways. IgA cannot cross the placental barrier, but however, sIgA can be passed to the neonate through milk. IgA does not fix complement, but can activate the alternative complement pathway. It promotes phagocytosis and intra-cellular killing of microorganisms.

Immunoglobulin M

Immunoglobulin M constitutes 5% to 8% of serum Ig with a normal level of 0.5 to 2 mg per mL. It has a half-life of about 5 days. It is a heavy molecule (19S; molecular weight 900,000 to 1,000,000, hence called the mil-lionaire molecule). It has a pentameric struc-ture comprising five identical four chain units (Fig. 5.6), i.e. it has 10 identical binding sites. The �mu� heavy chains has five domains, V

H plus 4 C regions (C 1, C 2, C 3, C 4)

and lacks a hinge region. The pentameric structure is stabilized by disulfide bonding

Fig. 5.4: General structure of the four subclasses of human IgG, which differ in the number and arrange-ment of the interchain disulfide bonds (thick brown lines) linking the heavy chains. A notable feature of human IgG3 is its 11 interchain disulfide bonds.


between adjacent C 3 domains and by the presence of Joinez (J) chain. Though theoreti-cally 10 antigen-binding sites are there, only five antigen-binding sites react with antigen probably due to steric hindrance.

Immunoglobulin M is the principal com-ponent of primary immune response. Be-cause of its large size (970 kDa, 19S), it is located mainly in the bloodstream. As it is not transported across the placenta, the pres-ence of IgM in the fetus indicates intrauter-ine infection and its detection is useful to the diagnosis of congenital infections such as syphilis, rubella, human immunodefi-ciency virus (HIV) infection and toxoplas-mosis. IgM antibodies are relatively short lived, disappears earlier than IgG. Hence, their demonstration in serum indicates re-cent infection. Treatment of serum with 0.12 M 2-mercaptoethanol selectively de-stroys IgM without affecting IgG antibodies.

The isohemagglutinins (anti-A, anti-B) and many other natural antibodies to micro-organisms are IgM. Antibodies to typhoid O antigen (endotoxin) and Wassermann reac-tion (WR) antibodies in syphilis are also of this class. It is efficient in both opsonization and complement fixation.

Immunoglobulin D

Immunoglobulin D structurally resembles IgG. The concentration is about 0.03 mg per mL of serum. It has a half-life of about 3 days. IgD acts as an antigen receptor, when present

Fig. 5.5: Structure of IgA (both monomer and dimer)

Fig. 5.6: Structure of human IgM (pentamer)

on the surface of certain B lymphocyte. Two subclasses, IgD1 and IgD2 have been de-scribed (Fig. 5.7).

Immunoglobulin E

Immunoglobulin E is 8S molecule (molecular weight is about 190,000) with a half-life of 2 days. Normal serum contains only traces. It exhibits unique properties such as heat lability and affinity towards surface of mast cells. The Fc region of IgE binds to the recep-tor for the antigen on the surface of mast cell and basophil. The resulting antigen-antibody complex triggers immediate (type 1) hyper-sensitivity reaction by releasing the media-tors. Serum IgE increased in anaphylactic reaction and helminthic infection. IgE is also known as reagin (Fig. 5.8).


Abnormal Immunoglobulins

Apart from antibodies other structurally simi-lar proteins were seen in some pathological conditions as well as some time in healthy individuals.

Bence-Jones protein was the first abnor-mal protein found in multiple myeloma. The characteristic feature of this protein is that it is coagulated at 50 C, but redissolved off at 70 C. These proteins are light chains (K or L) of Ig molecules.

Myeloma is a plasma cell dyscrasia where there is unchecked proliferation of one clone of plasma cells producing one type of Ig in excessive quantity. Such Ig are called mono-clonal antibodies. The myeloma may be IgG, IgA, IgD and IgE, when they involve plasma cells producing respective Igs. Involvement of

Fig. 5.7: Structure of IgD

IgM producing plasma cell lead to Walden-

strom macroglobulinemia.

MAJOR HISTOCOMPATIBILITY COMPLEX MOLECULES

Major Histocompatibility Complex Syngeneic Human Leukocyte Antigen

Major histocompatibility complex class I: It consists of a heavy peptide chain of 43 kDa, non-covalently linked to a smaller 11 kDa peptide called 2-microglobulin. The part of the heavy chain is organized into three globular domains ( 1, 2 and 3), which protrudes from the cell surface. The hydro-phobic portion anchors the molecule on the cell membrane and a short hydrophilic sequence (arm) the COOH portion into the cytoplasm. Class I molecules are virtually present in all cells except the villous tropho-blast. MHC class I molecules are the prin-cipal antigens involved in graft rejection.

Major histocompatibility complex class II: These are also transmembrane glycoproteins

Fig. 5.8: Structure of IgE


consisting of and peptide chains of mo-lecular weight 34 kDa and 24 kDa respec-tively. Both chains are folded to give two domains ( 1, 2 and 1, 2). Class II mole-cules are particularly associated with B cells and macrophages, but can be induced on capillary endothelial cells by -interferons.

T cell receptors: The T cell receptors (TCR) complex is a combination of the antigen rec-ognition structure (TCR) and cell activation machinery (CD3). There are two types of T cell antigen receptors, the TCR1 consisting of / chains and the TCR2 consisting of / chains. T cells expressing TCR1 ( / cells) are more primitive T cells, generally restricted to mucosal epithelium and other tissue loca-tions and are important for stimulating innate and mucosal immunity. TCR2 is expressed on most T cells ( / T cells) and these cells are primarily responsible for antigen activat-ed immune responses.

B cell receptors: Ig serve as B cell receptors (BCRs). B cells bear receptors that are com-posed of two identical H chains and two identical L chains. In addition, secondary components (Ig and Ig ) are closely as-sociated with the primary receptor and are thought to couple it to intracellular signaling pathways (Refer Fig. 5.1).

Immunoglobulin Specificities

Immunoglobulins are protein in nature, hence antigenic. They exhibit immunologi-cal specificities. The antigenic specificities, which distinguish the different classes and subclasses of immunoglobulins present in all normal individual of a given species are termed isospecificities.

The antigenic specificities, which distin-guish the Ig of same class between different groups of individuals in the same species are called allotypic specificities. They are under genetic control. The examples are Gm (V

H),

25 types and Inv (KL), three types.

Antigenic specificities exclusive to each Ig molecule is known as idiotypic specificities. An idiotype is a unique antigenic determinant of the hypervariable region, produced by spe-cific clone of antibody-producing cells. An anti-idiotypic antibody reacts with V domain of the specific Ig molecule that induced it.

STUDY QUESTIONS

Essay Questions

1. Name the molecules, which recognize the antigen. Describe the structure and functions of IgG.

Short Notes

1. Secretory immunoglobulin. 2. IgM. 3. IgE. 4. MHC I molecule. 5. MHC II molecule.

Match the Antibody Class with its Characteristics

Characteristics Immunoglobulin

class

Crosses placental barrier IgM

Millionaire molecule IgG

Takes part in anaphylaxis IgA

Local immunity IgE

SUGGESTED READING

1. Daniel P Stites. Basic and Clinical Immunology, 8th edition. USA: Lange (Medical Book); 2007.

2. Goldsby RA, Kindt Thomas J, Osborn Barbara A Kuby. Immunology, 6th edition. New York: WH Freeman and company; 2007.

3. Greenwood D, Slack R. Medical Micro-biology. 15th edition. 1997.

4. Jawetz. Melnick and Adelberg�s Medical Microbiology, 25th edition. USA: McGraw Hill, Lange Basic Science; 2010.

5. Male David, Brostoff Jonathan, Roth David B, et al. Immunology, 7th edition. Mosby: Elsevier; 2006.


6. Peakman M, Vergani D. Basic and Clinical Immunology, 1st edition; 1997.

7. Thao Doan, Roger Melvold, Susan Viselli, et al. Lippincott�s illustrated reviews: Immuno-

logy, 1st Indian print. Baltimore, USA: Lippincott Williams and Wilkins; 2008.

8. Tortora, Funke, Case. Microbiology an Introduction, 6th edition; 1998.

Antigen recognition molecule

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