When a macrophage endocytoses an antigen, which class of MHC protein does it use to present the antigen?

When a macrophage endocytoses an antigen, it uses Class II Major Histocompatibility Complex (MHC) proteins to present the antigen. This process is integral to the immune response.

Class II MHC proteins are specifically designed to present processed antigens derived from extracellular sources, which are typically taken up by immune cells such as macrophages, dendritic cells, and B cells through a process called endocytosis or phagocytosis. After the antigen is engulfed, it is processed within the cell, and fragments of the antigen are displayed on the surface of the macrophage using Class II MHC proteins.

This presentation occurs primarily to CD4+ T helper cells, which recognize the antigen-MHC II complex and initiate a strong immune response by helping activate other immune cells. This interaction is crucial in mounting an effective adaptive immune response, highlighting the specialized role of Class II MHC proteins in the immune system.