Figure 1.

Regulation of immune responses by antigen-presenting cells (APCs). APCs such as dendritic cells acquire antigens from pathogens or malignant cells. These antigens are then presented in context with MHC molecules to either CD8+ T cells (cytoxoxic T lymphocytes) or to CD4+ T helper cells. Upon DC-T-cell contact, T cells become activated and acquire specificity for the antigen presented by the APCs. CD8+ T cells travel via the peripheral bloodstream to tumor sites or virally infected cells (target cells) and upon cell contact exert their destruction. Activated CD4+ T cells augment CD8+ T-cell responses through release of cytokines such as IL-2. They can also mediate antibody responses against tumors by activation of B-cells and plasma cells. DC, dendritic cell; MHC, major histocompatibility complex.