Although the recruitment of CD8+ T cells by exogenous antigens (such as those derived from S. mansoni) primarily seems to be unexpected, recent studies have demonstrated that CD8+ T cells are prone to respond to extracellular antigens in infectious diseases [55, 56] via bystander activation triggered by persistent antigenic stimulation, cytokines milieu [52, 57, 58], or interaction with antigen-presenting cells (APC) that acquire exogenous antigens by phagocytosis and present them throughout MHC class I molecules [59–62]. This evidence concerns the gene CD8A and infectious disease.