Immunological aspects of infections part 17

Immune System


In the following discussion, streptococci, particularly S.pyogenes, are used as the example of a bacterial infection, but many other organisms produce a similar response. Streptococcal antigens include specific toxins such as streptolysins O and S that lyse the blood and tissue cells and pyrogenic exotoxin, which act as superantigens to overstimulate the host responses. There are also specific enzymes such as hyaluronidase  and streptokinase, which help promote  the spread of infecting streptococcus. Perhaps most important is the M protein, a cell surface antigen of the group A streptococcus that allows the bacteria to evade immune defenses (especially  neutrophils and complement). One way in which  M protein functions is to bind host factor H, which prevents  complement C3 from depositing on the streptococcal surface. Since efficient phagocytosis by neutrophils requires interaction with its C3 receptor, factor H prevents this interactions.

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