Significant progress has dramatically improved our understanding of TH cell biology with a number of new subsets recently being identified and discussed in this review, as well as emerging effector TH cell phenotypes, such as granzyme B-expressing cytolytic CD4+ T cells found in certain viral infections [232] and subsets of TH cells dedicated to production of IL-21 (TH21) [233], that have not yet been shown to be a distinct effector lineage and are not discussed here but warrant further study. Here, CD4 is linked to viral infectious disease.