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29.2.3 Adaptive immunity to extracellular bacteria

The immunity that plays major role against extracellular bacteria is the humoral or antibody mediated immunity as it prevents the infection by neutralizing the toxins. Usually polysaccharide antigens are prototypic thymus-independent antigens and humoral immunity is the basic line of defense against polysaccharide-rich encapsulated bacteria. The antibodies in such cases defend the body by neutralization, opsonization, phagocytosis and stimulation of complement system. Extracellular bacteria also stimulate the production of CD4+ helper T cells which induces inflammation and phagocytic activity. Besides this, these antigens may cause some mutational disorders and also the affected individual may have reduced immune response towards microbial infections.

29.2.4 Some important definitions

Septic shock- It is the after effect of some gram positive and gram negative bacterial infections represented by intravascular coagulation.

Superantigen - Some bacterial toxins or antigens stimulate non-specific activation of T-cells leading to polyclonal T-cell activation. These antigens are called superantigens.

29.2.5 Immune evasion by extracellular bacteria

1) Polysaccharide antigens or encapsulated bacteria are more lethal as compared to a strain devoid of capsule because they resist phagocytosis.

2) Capsulated bacteria inhibit alternate pathway of complement system due to the presence of sialic acid.

3) One more way of evading immune response by extracellular bacteria is due to the genetic edition of surface antigens. E.g. surface antigen of some specific bacteria is contained in their pili. Pili contain a protein antigen called “ pilin” and this pilin undergoes gene variation. Pili are the structures of bacteria responsible for bacterial adhesion to host cells.