Basic T-cell development is illustrated emphasizing the major T-cell and NK-cell subsets, and the distinction of the innate from the adaptive immune system. Some more specific T-cell subsets such as the CD4+ T follicular helper cells or cytotoxic CD4+ cells are not illustrated. CLP indicates common lymphoid precursor; DN, double negative (CD4−, CD8−); DP, double positive (CD4+, CD8+).