Who primarily expresses MHC II molecules?

MHC II molecules are primarily expressed by specialized antigen-presenting cells (APCs), which include macrophages and B cells. These cells play a crucial role in the immune response by presenting extracellular antigens to CD4+ T helper cells, effectively activating them.

Macrophages are highly versatile immune cells that can phagocytize pathogens and present the resulting antigens via MHC II. B cells, on the other hand, can also take up specific antigens through their B cell receptors, process these antigens, and present them using MHC II molecules to help activate T cells. This interaction is vital for the adaptive immune response, leading to the activation of various immune pathways, including antibody production.

Understanding the roles of these cells helps clarify why the focus is on macrophages and B cells regarding MHC II expression. While T cells interact with MHC II, they are not responsible for presenting these molecules; instead, they respond to antigen presentation from professional APCs like macrophages and B cells. Other immune cells do not express MHC II as a standard function, which further emphasizes the specificity of macrophages and B cells in this context.