CD4 and infection: Although continued maturation of memory B cell responses and immunity following natural infection appears to last up to six months or longer [5,[24], [25], [26], [27], [28]], antibody titers have been found to decay by 4 to 6 months post-infection [24], with CD4+ and CD8+ T cell-specific responses declining with a half-life of 3-5 months [25,28,29].