Cellular and humoral immune responses of the adaptive immune system. Activated T cells accomplish cellular immune responses, and B cells and antibodies mediate humoral immune responses. Exogenous antigen activates B cells by binding to the B-cell receptor (BCR) and T cells by binding to the T-cell receptor. However, TCR binding only occurs when antigen peptide is presented by antigen-presenting cells in association with major histocompatibility complex (MHC) proteins. The TCR is either associated with CD4 or CD8, depending on the T-cell type. Endogenous antigens are presented via MHC proteins to cytotoxic T cells, which destroy the host cell together with the intracellular pathogen. The immune response involves two paths, one using B cells (humoral immune response) and one using cytotoxic T cells (cellular immune response). Millions of different B and T cell types exist to recognize millions of different antigens. 



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