CD8A and bacterial infectious disease: In the case of most viral and bacterial infections, the invading pathogen peptides, generated in the antigen presenting cell, will stimulate adaptive immune cells via MHC class I molecules, for viral infections and intracellular bacteria, or MHC class II molecules for extracellular bacterial infections.18 Activation of cytotoxic CD8+ T lymphocytes by MHC class I presentation of peptides leads to killing of the infected cells through a number of mechanisms.