CD4+ T cell recognition of fungal peptide allows naive B cells to activate and become B cell blasts, leading to somatic hypermutation and memory B cells with high affinity to fungal antigens. The association between Epstein-Barr virus (EBV) and multiple sclerosis (MS) suggests this mechanism is not important because EBV infects and activates resting naive B cells without CD4+ T cell help. Here, CD4 is linked to myeloid sarcoma.