Antibodies
Antibodies (Immunoglobulins) 1. Introduction 2. Structure 3. Functions 4. Overview of the 5 classes of Igs 5. Antibody diversity 6. Antibody preparation
1. Introduction Antibodies A group of glycoproteins produced by B cells, which bind antigens with high specificity, often termed “immunoglobulins”. 1-1. Distribution 1-2. Heterogeneity
Antibody response
1-1. Distribution of antibodies Serum and tissue fluids Mediating the humoral immune response Surface of B cells Acting as B cell antigen receptor (BCR)
Surface and secreted antibodies
1-2. Heterogeneity of antibodies to fractions of normal serum many classes and types specific for an antigen epitope
2. The structure of antibodies 2.1. Basic structure of antibody 2.2. Antibody fragments 2.3. Structure of 5 classes of antibodies
2-1. Basic Structure of Antibody (antibody unit) Basic sructure of Ab is a 4-chain unit Variable region and constant region Immunoglobullin domains
Basic sructure of Ab is a 4-chain unit 2 identical light chains (L chain) 2 identical heavy chains (H chain) The L and H chains are linked by disulphide bonds, forming a Y-shaped molecule 2 kinds of L chain: , , forming 2 types of antibodies 5 kinds of H chain: , , , and , forming 5 classes of antibodies.
Structure of IgG
5 classes of human Abs depending on the structure of the heavy chain
Variable region and constant region All chains have one variable and one constant region. V region The amino-terminal part of the chain, consisting of 100–110 amino acids, are found to vary among different antibodies V region can be divided into hypervariable region (complementarity determining region, idiotypic determinant) and framework region C region The carboxyl-terminal part of the molecule, called the constant (C) region
Some functions of the domains Domains of Antibody A disulphide bond enclosed peptide loop L chain: VL and CL H chain: VH, CH1, CH2. CH3 (IgG, IgA, IgD; IgM and IgE have got an extra CH4) Some functions of the domains VL and VH: binding an antigenic epitope specificaly CH2: binding the C1q of complement. CH3 (or CH4 of IgM and IgE): binding to the Fc receptor on effector cells.
The basic structure of IgG1
2-2. Antibody fragments Antibody can be proteolytically cleaved to yield to fragments Fab fragment (fragment that binds antigen) antigen binding part of antibody Fc fragment (fragment that crystallized) effector part of antibody
2-3. Structure of 5 classes of antibodies IgG, IgE, IgD and serum IgA are of a typical four-chain structure (monomer). IgM is a pentamer of the basic four-chain units (composed of 5 four-chain units), these units are linked by a peptide called J chain. Secretory (sIgA) is a dimer of two basic four-chain units, these units are linked by the J chain, A peptide called secretory component is also needed for sIgA traverses epithelial cell layers.
3. Functions of antibodies Antibodies are bifunctional molecules 3.1 V region binding to antigen (epitope) specifically 3.2 C region effector functions
3-1. Functions of V region Variable regions account for unique antigen binding specificities Antibody on B cell surface (BCR) B cells recognize antigen (epitope) specifically Antibody in serum and interstitial fluid Neutralizing bacterial toxin, preventing viral attachment to host cells
Constant regions mediate different effector functions 3-2. Functions of C region Constant regions mediate different effector functions Complement activation (classical pathway) Binding to Fc receptors on effector cells - ADCC - Opsonization - Type I hypersensitivity Other functions - placental transfer, epithelial transfer
Antibody dependent cellular cytotoxicity (ADCC) PMNs, monocytes, macrophages, NK cells, eosinophils can kill antibody-coated target cells directly. Fab of Ab binds the antigen of the target cell, and the Fc of Ab binds to the FcR on the effector cells, mediating the lysis of the target cells.
Opsonization Process of making a microbe easier to phagocytose. Antibody coating the microbe will facilitate the phagocytes to engulf it. Fab of Ab binds the antigen of the antigen, and the Fc of Ab binds to the FcR on the phagocytes, facilitating the phagocytosis of the antigen
IgE mediating type I hypersensitivity The Fc of IgE binds to FcR on mast cells and basophils, and the Fab binds antigen, mediating the hypersensitivity
4. Overview of the 5 classes of antibodies IgG IgM IgA IgD IgE
IgG The major Ig in serum and tissue fluid, 70-75% of total Igs IgG is a monomer of a 4-chain unit The predominant Ig produced in secondary response The most important Ig involved in elimination of pathogens and mediation of inflammation (complement activation, opsonization, ADCC) Placental transfer, IgG is the only Ig class which is of placental transport to fetus
IgM 10% of the total Ig pool, in serum (intravascularly) A pentamer of 4-chain units, with the largest molecular weight The predominant Ig in primary response. Associated with immune response to antigenically complex, blood-borne infectious organisms (A potent activator of complement)
IgA 15-20% of the total Ig, 2 forms of IgA (serum IgA and secretory IgA) Serum IgA is a monomer of 4-chain unit, secretory IgA is a dimer of 4-chain units The secretory IgA is the predominant Ig seromucous secretions (saliva, colostrum, milk, tracheobronchial and genitourinary secretions) involved in seromucous defense
IgD 1% of the serum Ig, but a major Ig on B cells A monomer of a 4-chain unit biological function remains unclear, but BCR IgE Scarce in serum, but on the surface of basophils and mast cells involved in immunity against some parasites, and mediating the allergic diseases
5. Antibody diversity Isotype: the genetic variants are present in all individuals of a species, e.g. Ab classes and types. Allotype: the genetic variation between individuals within a species. Idiotype: the variation in the viable domain, particularly in hypervariable regions, eg. Abs produced in an individual can bind a range of antigens specifically.
6. Preparation of antibodies Ployclonal antibody conventional antibody, obtained from an animal immunized with antigen (with many epitopes), mixture of antibodies produced by many B cell clones Monoclonal antibody (mAb) antibody of a single specificity (monospecific), produced by a B cell clone specific for an epitope. Antibody engineering