MHC class II (I-A/I-E): ProductsMajor Histocompatibility Complex (MHC) class II is a heterodimeric transmembrane protein that is expressed on antigen-presenting cells including dendritic cells, macrophages, and B cells. Mouse MHC class II is expressed as multiple isotypes encoded by four genes located in the mouse MHC. MHC class II alpha and beta subunits are assembled into heterodimers in the endoplasmic reticulum, where they are complexed to an invariant chain (Ii). Following synthesis, MHC class II is further processed in the MHC class II compartment, a specialized endosome. Here, the Ii chain is digested and peptide antigens derived from extracellular proteins are loaded into the groove between the alpha and beta MHC class II subunits. Antigens presented on MHC class II bind to epitope-specific T Cell Receptors (TCR) expressed by CD4+ helper T cells. In addition to presenting peptide antigens to T cells, MHC class II can also initiate intracellular signaling pathways in antigen-presenting cells that modulate their proliferation, maturation, and apoptosis.