Final answer:
MHC I molecules are found on all nucleated cells and present self-antigens and nonself pathogens to effector T cells. MHC II molecules are found on macrophages, dendritic cells, and B cells and present abnormal or nonself pathogen antigens for T cell activation.
Step-by-step explanation:
MHC I and MHC II are two classes of MHC molecules involved in adaptive immunity. MHC I molecules are found on all nucleated cells and present normal self-antigens as well as abnormal or nonself pathogens to effector T cells. On the other hand, MHC II molecules are only found on macrophages, dendritic cells, and B cells, and they present abnormal or nonself pathogen antigens for the initial activation of T cells.
The structures of MHC I and MHC II molecules are different. MHC I molecules consist of a longer α protein chain and a smaller β₂ microglobulin protein, while MHC II molecules are composed of an α and a β chain of approximately similar length. Both MHC I and MHC II molecules have an antigen-binding cleft near the top portion of the dimer, but the specific domains involved in forming the cleft differ between the two types.
In terms of antigen presentation, MHC I molecules present intracellular antigens that are processed in the cytosol and brought into the endoplasmic reticulum by the transporter associated with antigen processing (TAP) system. The processed antigens then interact with MHC I molecules and are transported to the cell surface. MHC II molecules, on the other hand, present antigens that are processed and presented on the surface of antigen-presenting cells (APCs) like macrophages, dendritic cells, and B cells.