3a. Viral infection → no cell-mediated immune response → antibody production → macrophages producing virus accumulate around vessels and in the interstitium of the serous surfaces and tissues throughout the body → virus enters these tissues → virus complexes with antibodies → complexes deposit on vessel walls and attract complement → damage to vessel walls → leakage of protein rich fluid into body cavities → effusive or wet form of FIP which may lead to dyspnea and/or cardiac tamponade.

3b. Viral infection → Weak or ineffective cell-mediated immune response → less florid macrophage response in tissue and poorer virus production → The noneffusive or dry form of FIP ensues.