Final answer:
MHC II molecules bind peptides in the endosomal/lysosomal pathway and are composed of two protein chains, both spanning the plasma membrane. They are expressed on antigen-presenting cells including macrophages, dendritic cells, and B cells.
Step-by-step explanation:
MHC II molecules assembled in ER, but unlike MHC I, they bind peptides in the endosomal/lysosomal pathway after being exported from the ER. While both MHC I and MHC II molecules are vital for antigen presentation to T cells, they differ in their structure and antigen processing pathways.
MHC II molecules are composed of two protein chains (an α and a β chain) that both span the plasma membrane, and they present processed foreign antigens to helper T cells. These antigen-presenting cells include macrophages, dendritic cells, and B cells. The antigen-binding cleft of an MHC II molecule, where it binds to the foreign peptides, is formed by domains α1 and β1.