In addition, recent studies of Ag-specific T cell responses to vaccines, or after certain infections (reviewed in [13], [14]), have revealed an enormous phenotypic heterogeneity and functional diversity within the CD4 T cell population, emphasizing the importance of the quality (i.e. the combination of different functions at a single cell or cell population level) of a T cell response, as opposed to its simple magnitude or response of a single population, in correlation with clinical outcome. Here, CD4 is linked to infection.