CD74 represents a type II integral membrane protein known as MHC Class II-associated invariant chain (Mr 43, 41, 35, 33 kDa). It is moderately expressed on B cells, and weakly on monocytes and macrophages. CD74 is also found on a subset of activated T cells, Langerhans cells, dendritic cells and some epithelial cells. CD74 may play a role in antigen processing and presentation through intracellular transport and expression of Class II molecules on the cell surface.1-4