Fig. 1.

The adaptive immune system. It includes T (CD4⁺ and CD8⁺) and B cells. CD8⁺ T cells differentiate into cytotoxic T cells after recognizing antigen presented by MHC class I. CD4⁺ T cells in contrast recognize antigens presented by MHC class II on antigen-presenting cells. Once activated by their antigen and the cytokine milieu (each lineage of CD4⁺ T cells has a combination of cytokines required to differentiate), as well as upon receiving costimulatory signals, CD4⁺ T cells differentiate in the different lineages: Treg, Th1, Th2, and Th17 cells, which produce a combination of cytokines as indicated. Finally, CD4⁺ T cells help B cells to differentiate into plasma cells once they encounter a specific antigen and plasma cells produce antigen specific antibodies.