ANTIGEN PRESENTATION & THE MHC SYSTEM (HLA)
Antigen presentation is a cornerstone of adaptive immunity, enabling T lymphocytes to recognise pathogens with extraordinary precision. Because T cells cannot detect free-floating pathogens or antigens on their own, the body relies on specialised molecules, major histocompatibility complex (MHC) proteins, to display processed antigen fragments on the surfaces of cells. This mechanism ensures that T cells respond only to infected or abnormal cells, maintaining immune accuracy and preventing unnecessary activation. The MHC system, called human leukocyte antigen (HLA) in humans, is also central to transplant compatibility, immune tolerance and autoimmune disease development.
What You Need to Know
Antigen presentation is the process by which cells display peptide fragments to T lymphocytes, allowing the immune system to recognise infection, malignancy, or abnormal cellular activity. This process depends on major histocompatibility complex molecules, known in humans as the human leukocyte antigen (HLA) system, which bind antigenic peptides and present them on the cell surface. Without antigen presentation via MHC molecules, T cells cannot be activated, regardless of how much antigen is present in the body.
There are two main classes of MHC molecules, each specialised for presenting different types of antigen to different T cell populations:
MHC class I is expressed on almost all nucleated cells and presents endogenous antigens, peptides generated inside the cell, typically from viruses or intracellular pathogens, to CD8⁺ cytotoxic T cells
MHC class II is expressed only on professional antigen-presenting cells such as dendritic cells, macrophages, and B cells, and presents exogenous antigens, peptides derived from material taken up from outside the cell, to CD4⁺ helper T cells
These two pathways allow the immune system to monitor both internal cellular health and external environmental threats. MHC class I presentation enables cytotoxic T cells to identify and eliminate infected or malignant cells directly, while MHC class II presentation activates helper T cells that coordinate broader immune responses. Activated CD4⁺ T cells provide signals that support antibody production by B cells, enhance macrophage microbicidal activity, and guide the overall direction of the immune response.
Beyond the Basics
Antigen Processing Pathways
Before antigens can be presented to T cells, they must be processed into short peptide fragments that fit into the binding groove of MHC molecules (immune proteins). This processing occurs through two distinct intracellular pathways, determined by whether the antigen originates inside or outside the cell. Each pathway is tightly regulated and directs antigen presentation to a specific T cell population.
Endogenous Antigen Processing and MHC Class I
The endogenous pathway processes proteins generated within the cell, including viral proteins produced during infection or abnormal proteins generated by malignant transformation. These intracellular proteins are broken down by the proteasome, a large enzymatic complex responsible for degrading unwanted or damaged proteins into short peptides.
The resulting peptides are transported into the endoplasmic reticulum by transporter associated with antigen processing proteins, known as TAP transporters. Within the endoplasmic reticulum, peptides bind to newly synthesised MHC class I molecules. Once a stable MHC class I peptide complex is formed, it is transported to the cell surface. This allows CD8⁺ cytotoxic T cells to survey cells for evidence of intracellular infection or malignancy and eliminate affected cells when abnormal peptides are detected.
Exogenous Antigen Processing and MHC Class II
The exogenous pathway is used by professional antigen-presenting cells to process material taken up from outside the cell. Extracellular pathogens and proteins are internalised through phagocytosis or endocytosis and enclosed within vesicles. These vesicles fuse with lysosomes, forming acidic compartments where antigens are degraded into peptides.
MHC class II molecules are directed into these vesicles, where peptide loading occurs. The resulting MHC class II peptide complexes are then transported to the cell surface and recognised by CD4⁺ helper T cells. Activation of helper T cells supports antibody production by B cells, enhances macrophage microbicidal activity, and drives cytokine secretion that coordinates broader immune responses.
Antigen-Presenting Cells
While many cells express MHC class I, only professional antigen-presenting cells express MHC class II and provide the additional signals required for full T cell activation. Dendritic cells are the most potent antigen-presenting cells, capturing antigens in peripheral tissues and migrating to lymph nodes to activate naïve T cells. Their ability to initiate primary immune responses places them at the centre of adaptive immunity.
Macrophages present antigens primarily in inflamed tissues, reinforcing T cell responses at sites of active infection. B cells present antigens that bind specifically to their surface immunoglobulin receptors, allowing precise interaction with helper T cells during antibody-mediated immune responses. Together, these cells ensure that antigen presentation occurs in the appropriate anatomical and immunological context.
Costimulation and T Cell Activation
Recognition of antigen alone is not sufficient to activate T cells. Antigen-presenting cells must also deliver costimulatory signals through surface molecules such as CD80 and CD86, which bind to CD28 on T cells. This second signal confirms that antigen recognition is occurring in a context of danger rather than tolerance.
In the absence of costimulation, T cells enter a state known as anergy, meaning they remain alive but unresponsive. This mechanism plays a key role in maintaining self-tolerance and preventing autoimmune activation. When both antigen recognition and costimulation are present, T cells proliferate and differentiate. Helper T cells begin secreting cytokines that coordinate immune activity, while cytotoxic T cells develop into effector cells capable of targeted cell killing.
Genetic Diversity of the HLA System
The human leukocyte antigen system is one of the most genetically diverse regions of the human genome. Each individual expresses a unique combination of HLA molecules, allowing populations to present a wide range of antigenic peptides derived from different pathogens. This diversity enhances population-level immune protection but introduces challenges in clinical settings.
In organ and tissue transplantation, differences in HLA molecules between donor and recipient can lead to strong immune recognition and graft rejection. For this reason, HLA typing is essential for procedures such as bone marrow transplantation, where close matching significantly reduces the risk of immune-mediated complications. Understanding HLA diversity is therefore central to both immune defence and clinical transplantation practice.
Clinical Connections
Effective antigen presentation is essential for immune protection, and disruption of this process has clear clinical consequences. Defects in MHC class I expression or peptide loading reduce the ability of CD8⁺ cytotoxic T cells to detect infected or malignant cells, increasing susceptibility to viral infections. In contrast, impaired MHC class II function limits activation of CD4⁺ helper T cells, weakening antibody production, macrophage activation, and coordination of adaptive immunity. These defects are associated with recurrent infections and poor vaccine responses, even when antigen exposure occurs.
Several clinical patterns arise directly from altered antigen presentation and MHC function:
Increased viral infection risk associated with reduced MHC class I expression
Impaired antibody responses linked to MHC class II deficiency or dysfunction
Autoimmune disease associated with specific HLA alleles that influence peptide presentation
Transplant rejection driven by T cell recognition of mismatched MHC molecules
Autoimmune diseases such as type 1 diabetes, rheumatoid arthritis, and coeliac disease show strong associations with particular HLA types. These associations arise because certain HLA molecules present self-derived peptides in a way that permits activation of autoreactive T cells. Once tolerance is breached, sustained T cell activation drives chronic inflammation and tissue damage. Understanding HLA associations therefore helps explain why autoimmune disease risk varies between individuals and populations.
Antigen presentation also plays a central role in transplant medicine and cancer immunity. In organ and bone marrow transplantation, mismatched MHC molecules are recognised as foreign by recipient T cells, triggering graft rejection unless immunosuppression or close HLA matching is used. In cancer, tumour cells may downregulate MHC class I expression to avoid cytotoxic T cell recognition. This strategy increases vulnerability to natural killer cells, which detect reduced MHC I as a danger signal.
Concept Check
What are the main differences between MHC class I and class II molecules in terms of structure and function?
How does the endogenous pathway process intracellular antigens for presentation on MHC class I?
What role do dendritic cells play in initiating adaptive immune responses?
Why is costimulation essential for T cell activation, and what happens when it is absent?
How does genetic diversity in HLA genes influence both immunity and transplantation?