What self-antigen is found on all nucleated cells in the body?

MHC class I molecules are the self-antigens found on all nucleated cells in the body. They play a crucial role in the immune system by presenting endogenous antigens (such as those derived from proteins synthesized within the cell) to CD8+ cytotoxic T cells. This interaction is vital for the detection and elimination of infected or malfunctioning cells, such as those that may be infected by viruses or have undergone malignant transformation.

While MHC class II molecules, CD4, and CD8 also play important roles in the immune response, they do not meet the criteria specified in the question. MHC class II molecules are primarily expressed on professional antigen-presenting cells (such as dendritic cells, macrophages, and B cells) and are involved in presenting exogenous antigens to CD4+ helper T cells. CD4 and CD8 are co-receptors that assist T cell activation but are not themselves presented as self-antigens on cells throughout the body. The presence of MHC class I on all nucleated cells ensures that the immune system can continuously monitor cellular health and respond appropriately to threats.