Cellular and molecular basis of the humoral immune response

An antigen, e.g. a bacterium, virus or toxin, may stimulate the differentiation of B cells into plasma cells. There are two different pathways by which B cells can be activated to differentiate into plasma cells:

1. T-independent response. In this case, B cells will be activated even in the absence of T cells. There are two types of T-independent antigens: Tl-'l antigens activate a proportion of B cells poly-clonally, irrespective of the specificity of their antigen receptor. Tl-2 antigens are normally large polymeric molecules with repeating antigenic determinants (e.g. polysaccharides with identical subunits), such as the capsular antigens of microbial organisms, which directly cross-link the B-cell antigen receptor.

2. T-dependent response. In this case, the response depends on both T and B cells recognizing the antigen. In such a response, the differentiation of the B cells depends on the interaction with T cells.

In an immune response which is independent of T cells, the antibodies produced are mainly of the IgM class. Even in a T-dependent response, B cells first develop into plasma cells which secrete IgM antibodies. However, in a T-dependent response, factors secreted by the T cells (cytokines) will direct B cells to switch to other Ig classes: IgG, IgA or IgE. Consequently the B cell product (antibody) will acquire the biological properties characteristic of each Ig class.

Maturation of the immune response

In a T-dependent response, the affinity of the secreted antibody for the antigen increases. This is referred to as affinity maturation of the immune response. Analysis of the primary structure of antibody molecules has shown that the increase in antibody affinity correlates with an accumulation of somatic mutations in the variable region of the antibody molecule. Antigen-activated B cells migrate into the primary follicles of the lymphoid organs. There, proliferation of B cells leads to the formation of ger minal centers. During clonal expansion in the germinal centers the hypermutation mechanism which is activated diversifies the B cell repertoire. Intraclonal diversity is generated by nucleotide substitutions specifically introduced into the H and L chain variable region genes. Antigen presented on Follicular Dendritic Cells (FDC) permits selection of the few high-affinity variants which may then differentiate into memory and plasma cells. In this way through hypermutation and selection an affinity maturation of the immune response is ensured and the affinity of the secondary response antibodies is generally higher than that of the primary response antibodies. The immune system 'learns' from the first contact with an antigen, so that at a second encounter the humoral immune response will not only be of greater quantity but also of greater quality.

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