This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Rheumatic Fever, Subject name: Pathology
The mechanism of the development of Acute Rheumatic Fever is which of the following? 
Select and explain. 
A. Cross reactivity with exogenous antigen, B. Innocent bystander effect, C. Due to toxin secretion by streptococci, D. Release of pyrogenic cytokines
A: Cross reactivity with exogenous antigen
Answer: A: Cross reactivity with exogenous antigen 
Explanation: Acute rheumatic fever results from immune response to group A streptococci (Strep. pyogenes) which cross-reacts with host tissues. The antibodies directed against the M proteins of streptococci cross react with the self antigens in the hea. In addition, CD4+ T cells specific for streptococcal peptides also react with self proteins in the hea and produce macrophage activating cytokines. So, the damage to the hea tissue is a combination of antibody and T-cell mediated reactions