T cells, antigen presentation and function T cells have multiple biological roles, including cytotoxic and helper B-cell roles. These two distinct roles are carried out by different T cell subtypes that can be recognized by monoclonal antibodies by recognizing markers on the cell surface. All mature T cells express CD3 molecules on their surface. T cells of helper B cells express CD4 molecules on their surface, while cytotoxic T cells express CD8 molecules on their surface. Both CD4 and CD8 T cells play an important role in the body’s defense against infectious antigens. cd4 T cells are particularly important in acquired immunodeficiency syndrome, where depletion of CD4 T cells leads to the development of this severe immunodeficiency. cd8 T cells play an important role in lysing pathogen-infected cells and killing tumor cells. both cd4 and cd8 T cells secrete cytokines, and these cytokines are important mediators. The genes encoding human MHC molecules are a group of genes tightly linked on chromosome 6. The encoded protein products play an important role in the immune system. The protein products encoded by these genes are called human leukocyte antigens (HLA); there are two types of MHC protein molecules, called class I molecules and class II molecules, in the process of T cell recognition. Class I molecules are expressed on the surface of all nucleated cells and present antigenic peptides to CD8 T cells. Class II molecules are only found in B cells, phagocytes, dendritic cells and other specialized presenting cells and present antigenic peptides to CD4 T cells. Class I and class II molecules are expressed on the surface of cells associated with antigenic peptides. In the absence of pathogens or foreign antigens, antigenic peptides bound to MHC can be derived from the host’s own endogenous proteins, which would then play an important role in the maintenance of immune self-tolerance. In contrast, after exogenous proteins are presented to the APC (e.g. in case of infection), the generated small peptides are expressed on the cell surface by these cells. Activation of T cells in the lymph node or spleen is generally possible only when a suitable APC surface-expressed peptide is encountered in complex with an MHC molecule. Signal 2 is often referred to as a costimulatory signal and can only be provided by mature APCs. This costimulatory signal is a critical link in the regulation of the T cell response because if the T cell receives only signal 1, its activation is terminated and it becomes an incompetent T cell. Dendritic cells (DCs) are a large and important class of APCs that can be in both immature and mature states. Immature DCs settle in the peripheral tissues of the body, take up their surroundings through phagocytosis, and process them to express pattern recognition receptors (PRRs) on their surface. When a pathogen invades the tissue, immature DC cells engulf the pathogen and are activated by signals generated through PRRs. After activation, immature DC cells become mature DC cells, and mature DC cells cease phagocytosis, but processing and extraction of the engulfed antigen is enhanced and transported to the lymph nodes where T cells accumulate, and the expression of genes encoding molecules that generate co-stimulatory signals is upregulated. The result is the appearance of mature DCs with antigen bound to MHC on the surface at the lymph nodes, which deliver signal 1 and signal 2 to antigen-specific T cells.