Which molecule is responsible for antigen recognition and presentation?

The molecule responsible for antigen recognition and presentation is MHC Class II. MHC Class II molecules are primarily expressed on professional antigen-presenting cells (APCs) such as dendritic cells, macrophages, and B cells. These molecules play a crucial role in the immune response by presenting processed antigenic peptides to CD4+ T helper cells.

When a pathogen is engulfed by an APC, it is processed into peptide fragments that fit into the groove of MHC Class II molecules. The MHC Class II-peptide complex then travels to the cell surface, where it can interact with T cell receptors on CD4+ T helper cells. This interaction is fundamental for the activation of T cells, leading to a coordinated immune response.

While MHC Class I also plays an important role in antigen presentation, it primarily presents endogenous antigens to CD8+ cytotoxic T cells, which differs from the function of MHC Class II focusing on exogenous antigens. CD4 is a co-receptor that assists in the recognition of the MHC Class II-peptide complex but does not independently present antigens. Antibodies, on the other hand, are produced by B cells in response to antigens and are involved in neutralizing pathogens but are