Which antigen is present on all nucleated body cells?

MHC I, or Major Histocompatibility Complex class I, is indeed present on all nucleated body cells. This complex plays a crucial role in the immune response by presenting endogenous antigens, which are typically pieces of proteins synthesized within the cell, to CD8+ cytotoxic T cells. This presentation is vital for the recognition of infected or abnormal cells by the immune system.

Since all nucleated cells produce and express MHC I molecules on their surface, they can inform the immune system about their current state. This allows the immune response to detect and eliminate cells that are infected with intracellular pathogens like viruses or that have become cancerous.

Other choices, such as MHC II, are primarily expressed on professional antigen-presenting cells (like dendritic cells, macrophages, and B cells) and are involved in presenting exogenous antigens to CD4+ T helper cells, but they are not found on all body cells. MHC III refers to different components of the MHC gene cluster and is not directly involved in antigen presentation. CD antigens are a broad class of surface molecules used as markers for various immune cells and are not universally present on all nucleated cells. Thus, MHC I is the correct answer as it