Final answer:
MHC I molecules are expressed on all nucleated cells and present normal "self" antigens as well as abnormal or nonself pathogens to T cells. MHC II molecules are expressed on immune cells only and present abnormal or nonself pathogen antigens for the activation of T cells.
Step-by-step explanation:
MHC I molecules are expressed on all nucleated cells and are essential for presentation of normal "self" antigens. Cells that become infected by intracellular pathogens can present foreign antigens on MHC I as well, marking the infected cell for destruction. On the other hand, MHC II molecules are expressed only on the cells of the immune system, specifically cells that affect other arms of the immune response. These cells are called "professional" antigen-presenting cells, and they include macrophages, dendritic cells, and B cells. MHC II molecules present abnormal or nonself pathogen antigens for the initial activation of T cells.