CD4 and cancer: T cells are largely divided into two subtypes: T helper (Th) cells, also known as CD4+ cells, that participate in the activation of other immune cell types (B cells, cytotoxic cells and macrophages) by the release of cytokines; and T cytotoxic (Tc) cells, also known as CD8+ cells or cytotoxic T lymphocytes (CTLs), that kill cancer and virus-infected cells.