FAS and infection: In sum, the in vivo existence of these relationships could be accounted for based on the concept that upregulation of CD95/Fas receptor surface expression (resulting, for example, from lymphocyte activation in response to infection) represents a critical mechanism of induction of apoptosis in Т lymphocytes upon interaction with death ligands and, along with this, the observed change in CD95 may have distinct functional significance for CD4+ and CD8+ T cell subsets.