Which molecules are found on every body cell that present antigen and signal cytotoxic T cells to destroy infected cells?

MHC I molecules are indeed present on almost all body cells and play a crucial role in the immune response. These molecules are responsible for presenting endogenous antigens—peptides derived from proteins synthesized inside the cell. When a body cell becomes infected by a virus or intracellular bacteria, these pathogens produce proteins that are processed into small peptides. MHC I binds to these peptides and displays them on the cell surface.

This presentation serves as a signal to cytotoxic T cells (also known as CD8+ T cells), which patrol the body for infected or abnormal cells. When a cytotoxic T cell recognizes the foreign peptide presented by MHC I, it can initiate a response leading to the destruction of the infected cell. This mechanism is a vital part of the adaptive immune system, allowing the body to target and eliminate cells harboring intracellular pathogens.

In contrast, while MHC II molecules are important for presenting antigens to helper T cells (CD4+), they are not found on every body cell but are primarily expressed by antigen-presenting cells such as macrophages and dendritic cells. Antibodies and complement proteins are components of the humoral immune response but do not present antigens on cell surfaces in the same manner as MHC I. Thus,