Structure and Classification of Immunoglobulin:
Immunoglobulins (Ig) otherwise called as antibodies are soluble glycoproteins produced in the membrane bound form on B lymphocytes to recognize and bind to antigens. They are present in serum, tissue fluids, or on cell membranes. B cells are only cells which synthesize antibodies. Antibodies serve as adaptors for immune system effector molecules by linking antigens to receptor molecules. So mainly antibodies carry out two main functions that are to recognize and bind to foreign material and to trigger elimination of foreign material through complex system comprising different proteins. Study of antibodies and how they react with antigens is called as serology.
Molecular Structure of Antibodies:
Antibody is Y-shaped molecule comprising of three equal fragments connected to each other by flexible hinge. Two of these fragments known as Fab (fragment antigen binding) are identical to one another are engaged in antigen binding and other fragment called as Fc (fraction crystallisable) binds to effector molecules. These fragments were recognized after proteolytic digestion with papain split antibody molecule in two fragments in hinge region prior to hydrogen-hydrogen inter-chain disulfide bond. Antibody molecule is flexible therefore permitting binding to different arrays of multivalent antigen.
The essential structure of immunoglobulins revealed the four chain unit polypeptide including two identical light and heavy chains.
Light chains ( L ) have molecular weight of approx 24 KD and heavy chains ( H ) molecular weight of between 55 -70 KD. Heavy chains are connected to one another using disulphide bonds, whereas each heavy chain links the light chain by disulphide and non covalent bonds.
Heavy and light chains comprise of repeating, homologous units of 110 amino acid residues in length which fold independently to form the immunoglobulin domain. Both heavy and light chains have regions known as variable and constant regions represented as CH and CL and VH and VL where subscript denotes heavy or light chain. The amino acid sequences of variable region of both heavy and light chains comprises of 110 amino acid residues while in case of heavy chain constant region, the amino acids differ , between 110 to 440 residues.
Classification of Antibodies:
Immunoglobulins may be categorized in five distinct classes based on differences in size, amino acid sequence, carbohydrate content and charge. Different classes are IgA, IgE, IgD, IgG and IgM. Some of these classes may be divided in subclasses egIgG (ϒ) has four subclasses that is IgG1, IgG2, IgG3 and IgG4 and IgA(α) has 2 subclasses -IgA1 and IgA2. Unique hinge region exists for every IgG subclass. Other immunoglobulin kinds IgE, IgM and IgD don't have subclasses. Structure of heavy chain of Ig molecule is significant in defining isotype. Other terms utilized in classifying immunoglobulins are allotypes and idiotypes. Amino acid sequence and three dimensional structure of constant region of the Ig molecule determines the allotype , though, allotypes illustrates genetic differences within same species, as a result all members of the species won't have any particular allotype. Idiotype states variation in amino acid sequence and three dimensional structure of the Ig molecule and the antigen binding specificity of any particular antibody.
There are two classes or isotypes of light chains known as Lambda (λ) and Kappa (k) after Greek alphabet. Only one specific kind of light chain is present in antibody molecule but never one of each. These two isotypes of light chains can be differentiated based on carboxy terminal of constant region.
Structure - Function Relationship of Antibodies:
Different roles are played by different classes of immunoglobulins. Essentially immunoglobulins recognize and bind to antigens and trigger off effector functions therefore ensuring that effector functions are particularly targeted towards elimination of that particular antigen. All immunoglobulin isotypes except IgD play the dual role which is to bind to antigen and show one or more effector functions. There are certain characteristics needed in order for antibody to be able to carry out these roles. These features comprise diversity, specificity, affinity and avidity. Antibodies are able to differentiate subtle differences between antigens and these differences applies to identification of all classes of molecules e.g. two linear protein determinants could be differentiated from each other based on substitution of the single amino acid sequence that may or may not influence secondary structure. Identification of this fine specificity is significant as biochemical constituents of all living organisms are essentially associated so that antibodies produced against microbial molecules don't react with structurally similar molecules. In some cases although, antibodies produced against specific antigen reacts with structurally similar antigen and this is what is considered as cross-reaction. Diversity is generated from large numbers of antibodies essential to bind antigen consequently of genetic mutations which may bring about random variations in structure in hypervariable regions of both heavy and light chains. It is essential which antibodies bind tightly to antigens to be able to eradicate pathogen, thus high affinity binding antibodies are produced by changes which take place during somatic mutation in antigen stimulated B lymphocytes. During humoral immune response the isotype of antibody influences response of that antibody to the antigen, and this is due to stimulation of B clone cells would create different isotypes with identical V domains therefore identical antigen binding sites. This is known as isotype switching where naive B cells would generate IgM and IgD with identical binding sites on activation. In isotype switching, there is change in kind of constant region heavy chain (CH) or antibody isotype generated by B cell while V regions and antigen specificity remains same. Different isotypes and subtypes may be generated from original B cells consequently of isotype switching.
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