CD4 and infection: CD4+ T cells are classified in three subsets: i) naïve T cells that have not yet been exposed to an antigen, ii) helper T cells that have been activated by a specific antigen and produce cytokines that can be toxic to the target cells or can stimulate other T and B cells and iii) memory T cells, which have encountered an antigen during a prior infection and at a second exposure can mount a faster and stronger immune response.