But the adaptive immune system is slow and can take several days before two key cell types—B cells and T cells—are brought into play.
T cells are further grouped into two sub-types, CD4+ and CD8+ cells. CD4+ are helper T cells that help the activity of other immune cells by releasing cytokines. The cytokines prime the maturation of B cells, which become plasma cells and produce antibodies to neutralize the pathogen.
CD8+ cytotoxic T cells, on the other hand, directly kill infected cells.
Once the adaptive immune system has vanquished the invader, a pool of long-lived memory T and B cells are made. These memory lymphocytes remain dormant until the next time they encounter the same pathogen. This time, though, they produce a much faster and stronger immune reaction. Memory is the key feature of the adaptive immune system, enabling long-term protection.
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( If you get a MMR or BCG vaccine, other parts of your immune system that fight a wide variety of viruses with more blunt force weapons, get boosted)
