<prompt> What is the mechanism of acute rheumatic fever -Cross reactivity with endogenous antigenInnocent by slender effectDue to toxin secretion by streptococciRelease of pyrogenic cytokines  <response>: Ans. is a i.e., Cross reactivity with endogenous antigen Acute Rheumatic fever results from immune response to group A streptococci.The main factor responsible for virulence in infection with streptococcal A group is M protein.The body mounts an immune response against streptococci A organism by producing antibodies directed against the M protein.Antibodies directed against the M protein of streptococci have been shown to cross react with self antigens in the heartThe streptococcal M protein and the human myocardium appears to be identical antigenically.Epitopes present in the cell wall membrane and the A, B, C repeat regions of the M protein are immunologically similar to molecules in human myosin, tropomyosin, keratin actin, laminin, vimentin.This molecular mimicry is the basis for autoimmune response that leads to Acute Rheumatic fever. 