Which class of major histocompatibility complex proteins can any cell present an antigen on?

Class I major histocompatibility complex (MHC) proteins are present on the surface of nearly all nucleated cells in the body. These proteins play a crucial role in the immune response by presenting endogenous antigens, which are typically proteins synthesized within the cell. When a cell is infected by a virus or if it becomes cancerous, the altered or foreign proteins produced by these processes can be displayed on Class I MHC molecules.

The presentation of these antigens allows cytotoxic T cells (CD8+ T cells) to recognize and target infected or abnormal cells for destruction. This is essential for the immune system's ability to identify and eliminate intracellular pathogens and tumor cells.

In contrast, Class II MHC proteins are primarily expressed on professional antigen-presenting cells, such as dendritic cells, macrophages, and B cells, and they present exogenous antigens to helper T cells (CD4+ T cells). Class III and Class IV do not correspond to recognized categories of MHC proteins in the context of antigen presentation. Thus, Class I MHC proteins are the only group that can be found on all nucleated cells, making them critical for the universal recognition of intracellular antigens by the adaptive immune system.