B-Lymphocyte Antigens - The Humoral Adaptive Immune System

B-LYMPHOCYTE ANTIGENS

A B-cell antigen is a substance or molecule specifically interacting with an antibody, and which may lead to the further production of antibody and an immunological response. Typically B-lymphocyte antigens are proteins within which the epitopes each consist of clusters of 5–20 amino acid residues. B-lymphocyte epitopes arise most commonly from the three-dimensional folding of proteins (i.e. conformational epitopes), although they may also consist of a sequential linear sequence of amino acids within the polypeptide chain (linear epitopes). As a general rule, there is a gradient of increasing immunogenicity with increasing molecular weight of protein. Further, the higher the structural complexity of the protein or polypeptide antigen, the higher the level of immunogenicity it is likely to exhibit. Thus, a polypeptide comprising a single amino acid such as polylysine will be expected to be a weaker immunogen than a protein of equivalent molecular weight made up of a diverse range of amino acids.

Polysaccharides tend not to be good immunogens for B-lymphocytes. When a polysaccharide serves as the sole immunogen, the humoral response obtained is termed ‘T-cell-independent’ because the polysaccharide does not elicit helper T-lymphocyte cooperation. The consequences of a T-cell-independent humoral response include the lack of production of memory B-cell populations and the lack of synthesis by the plasma cell of the full range of antibody subclasses, i.e. T-cell-independent humoral responses mainly involve the production of IgM antibody. For improved immunogenicity carbohydrate antigens are conjugated to proteins which allow a more effective ‘T-cell dependent’ humoral response, i.e. one that affords the generation of memory B-cell populations and of the synthesis of the full range of antibody subclasses. This strategy is used in a number of current vaccine products, e.g. meningococcal group C conjugate vaccine contains the capsular polysaccharide antigen of Neisseria meningitidis group C conjugated to Corynebacterium diphtheriae protein. Pure nucleic acid and lipid serve as very poor antigens.