MHC class II cellular immune cascade. Exogenous peptide antigens such as fVIII are processed through MHC class II mechanisms. Antigen-presenting cells take fVIII into endocytic vesicles where it is bound to an MHC class II molecule. Bound peptides are then presented on the surface of the cell to specific T-cell receptors (TCR) on CD4⁺ T lymphocytes. In response to antigen presentation, T lymphocytes elaborate cytokines and up-regulate several surface molecules. These surface molecules interact with corresponding proteins on B lymphocytes, leading to maturation of B cells and antibody formation. APC indicates antigen-presenting cell; CD4, CD4⁺ T lymphocyte; B cell, B lymphocyte; MHC, major histocompatibility complex; TCR, T-cell receptor; CD, cluster of differentiation; IL, interleukin. Copyright G.C. White II.