MCAT Content / Immune System / Antigen Presentation

Antigen presentation

Topic: Immune System

Antigen presentation broadly consists of pathogen recognition, phagocytosis of the pathogen or its molecular components, processing of the antigen, and then the presentation of the antigen to naive (mature but not yet activated) T cells. The ability of the adaptive immune system to fight off pathogens and end an infection depends on antigen presentation.

T cells must be presented with antigens in order to perform immune system functions. The T cell receptor is restricted to recognizing antigenic peptides only when bound to appropriate molecules of the MHC complexes on APCs, also known in humans as Human leukocyte antigen (HLA).

Several different types of T cells can be activated by APCs, and each type of T cell is specially equipped to deal with different pathogens, whether the pathogen is bacterial, viral or a toxin. The type of T cell activated, and therefore the type of response generated, depends on which MHC complex the processed antigen-peptide binds to.

MHC I molecules present antigen to CD8+ cytotoxic T cells, while MHC class II molecules present antigen to CD4+ helper T cells. With the exception of some cell types (such as erythrocytes), Class I MHC is expressed by almost all host cells. Cytotoxic T cells (also known as TC, killer T cell, or cytotoxic T-lymphocyte (CTL)) are a population of T cells that are specialized for inducing the death of other cells. Recognition of antigenic peptides through Class I by CTLs leads to the killing of the target cell, which is infected by the virus, intracytoplasmic bacterium, or are otherwise damaged or dysfunctional. Additionally, some helper T cells will present their antigen to B cells, which will activate their proliferation response.


Key Points

• Antigen presentation consists of pathogen recognition, phagocytosis of the pathogen or its molecular components, processing of the antigen, and then the presentation of the antigen to naive T cells.

• The T cell receptor is restricted to recognizing antigenic peptides only when bound to appropriate molecules of the major histocompatibility complex (MHC), also known in humans as human leukocyte antigen (HLA).

• Helper T cells receive antigens from MHC II on an APC, while cytotoxic T cells receive antigens from MHC I. Helper T cells present their antigen to B cells as well. Dendritic cells, B cells, and macrophages play a major role in the innate response and are the primary antigen-presenting cells (APC).


Key Terms

cytotoxic: A population of T cells specialized for inducing the deaths of other cells

adaptive immune system: a specific type of immunity developed over time

T cell: a lymphocyte, from the thymus, that can recognize specific antigens and can activate or deactivate other immune cells

antigen: a substance that binds to a specific antibody; may cause an immune response

antigen-presenting cell (APC): an immune cell that detects, engulfs, and informs the adaptive immune response about an infection

Major histocompatibility complex (MHC): a group of genes that code for proteins found on the surfaces of cells that help the immune system recognize foreign substances

MHC I: molecules are expressed on all nucleated cells and are essential for the presentation of normal “self” antigens. Cells that become infected by intracellular pathogens can present foreign antigens on MHC I as well, marking the infected cell for destruction.

cytotoxic T cell: a T lymphocyte that kills cells that are damaged

MHC II: molecules are expressed only on the surface of antigen-presenting cells (macrophages, dendritic cells, and B cells). Antigen presentation with MHC II is essential for the activation of T cells

Helper T cells: help activate B cells to secrete antibodies and macrophages to destroy ingested microbes and activate cytotoxic T cells



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