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8.3 Antigen recognition

8.3.1 Specificity - Antibodies are very specific to an antigen and can even understand the minute difference between almost similar antigens. It may however happen that an antibody may bind to different but structurally similar antigen and this phenomenon is termed as a cross-reaction .

8.3.2 Diversity – Diversity determines the ability of antibody to bind specifically to a large number of different antigens. The pool of antibodies with different specificities describes the antibody repertoire .

8.3.3 Affinity maturation - The efficiency of antibody bonding to antigen is measured in terms of affinity and avidity. Some modification is required in structure of V region of antibodies during T cell dependent humoral immune response to antigens so that the antibodies having high affinity can be generated. B-cells that are responsible for generating high affinity antibodies preferentially bind to the antigen due to selection and become the prominent cells with each antigen antibody reaction. This mechanism is termed as affinity maturation, and it leads to an increase in binding affinity of antigen and antibody as antibody mediated response develops further.

8.4 Effector functions of antigen antibody reaction  

1)  As two or more Fc portions are required to stimulate effector functions so effector functions are carried out only by molecules with bound antigens and not with free Ig.

2)  Fc region of the antibody molecules play a critical role in effector stimulation, so antibody isotypes varying in Fc region can be easily distinguished on the basis of interactions they carry.

3)  Distribution of antibody molecules through different tissues is decided solely by the constant region in the heavy chain of an antibody molecule. This directed distribution through constant region of heavy chain is the reason behind IgA presence in mucosal secretions or recruitment of other antibodies to a particular tissue.

4)  In antibody mediated immune response, variation in the isotypes of antibodies decides the ways to eliminate antigen from the body. In addition, isotype switching or class switching also has some role in it. e.g. antibody response to bacteria and viruses is carried out by IgG antibodies but switching to IgG isotype can also lengthen the humoral response because it has the longest half life period among all the antibodies.

*Isotype - The presence of variations in the constant regions of the immunoglobulin heavy and light chains are called isotypes. Five heavy chain isotypes and two light chain isotypes are present in humans.