Which MHC class of molecule is necessary for antigen recognition by CD4-positive T cells?

CD4-positive T cells, also known as helper T cells, require Major Histocompatibility Complex (MHC) class II molecules for antigen recognition. MHC class II molecules are specifically designed to present processed exogenous antigens, which are typically derived from extracellular pathogens. When a pathogen is engulfed by antigen-presenting cells (APCs) such as dendritic cells, macrophages, or B cells, it is digested into peptide fragments that are then loaded onto MHC class II molecules.

Once the MHC class II-peptide complex is presented on the surface of the APC, it can be recognized by the CD4 T cell receptor. This interaction is critical for T cell activation, leading to the expansion of T cell populations and the production of cytokines that help orchestrate the immune response. Consequently, MHC class II molecules play a vital role in the adaptive immune system, especially in the activation of help T cells that assist in the responses of B cells and other T cells.