CD8A and tetanus: Less is known about bystander activation of CD4+ T cells than of CD8+ T cells, but unrelated memory CD4+ T cells were shown to be activated after repeat tetanus vaccination via bystander activation (77), and multiple cytokines sharing a common receptor gamma chain were found to induce CD154/CD40 ligand expression by human CD4+ T lymphocytes via a cyclosporin A-resistant pathway (78).