Question

APCs express MHC class II molecules, which are presented to a helper T cell after an antigen peptide from a processed exogenous antigen has been associated with them. The helper T cell then recognizes the MHC class II/antigen complex presented by the APC. Why do these APCs also express MHC class I molecules?

Answer 1

APCs express MHC class I molecules alongside MHC class II to present endogenous antigens to cytotoxic T cells, addressing threats from intracellular pathogens such as viruses. The ability to present to both helper and cytotoxic T cells enables a comprehensive immune response.

Step-by-step explanation

Antigen-presenting cells (APCs) express both MHC class I and MHC class II molecules for the different roles they play in the immune response. While MHC class II molecules are crucial for presenting antigens to helper T cells, APCs also express MHC class I molecules because they are essential for the presentation of endogenous antigens to cytotoxic T cells (CTLs). This presentation to CTLs is relevant for the immune response against intracellular pathogens, such as viruses, which are presented by APCs as well as all other nucleated cells of the body. Hence, expressing both types of MHC allows APCs to present antigens to both helper and cytotoxic T cells, catering to both exogenous and endogenous pathogenic threats. The naïve T cells expressing CD8 engage with MHC I molecules and differentiate into CTLs, which are capable of directly killing infected cells, while CD4+ T cells interact with MHC II molecules and become helper T cells, aiding in the overall immune response.

Additionally, all nucleated cells in the body can present antigens via MHC I, thereby signaling an immune response when they are infected with intracellular pathogens, indicating the pervasive need for these molecules in the immune surveillance system.