Antigen-presenting cells

Definition

Antigen-presenting cells (APCs) are a critical component of the immune system that capture, process, and present antigens to T cells, thus playing a vital role in initiating the adaptive immune response. They include specialized cells such as dendritic cells, macrophages, and B cells, which have the ability to display fragments of pathogens on their surface using molecules called major histocompatibility complex (MHC) proteins. This interaction activates T cells, leading to a coordinated immune response that targets specific pathogens.

5 Must Know Facts For Your Next Test

1. Antigen-presenting cells are essential for activating naive T cells, which then proliferate and differentiate into effector T cells that help eliminate infections.
2. APCs can present antigens through two classes of MHC molecules: Class I MHC presents to CD8+ cytotoxic T cells, while Class II MHC presents to CD4+ helper T cells.
3. Dendritic cells are often considered the most efficient APCs due to their ability to capture and present antigens from pathogens in tissues to initiate strong immune responses.
4. In addition to presenting antigens, APCs also provide co-stimulatory signals necessary for full T cell activation, ensuring a robust immune response.
5. Dysfunction in antigen presentation can lead to immune system disorders, including autoimmune diseases and reduced effectiveness of vaccines.

Review Questions

• How do antigen-presenting cells initiate the adaptive immune response, and why is their role critical?
  • Antigen-presenting cells initiate the adaptive immune response by capturing and processing antigens from pathogens, then presenting these antigens on their surface using MHC molecules. This presentation is crucial because it activates naive T cells, allowing them to recognize specific antigens and proliferate into effector T cells. Without APCs effectively presenting antigens, the adaptive immune system would be unable to respond appropriately to infections.
• Discuss the differences between Class I and Class II MHC molecules in the context of antigen presentation by APCs.
  • Class I MHC molecules present endogenous antigens from within the cell to CD8+ cytotoxic T cells, primarily displaying pieces from viral or intracellular pathogens. In contrast, Class II MHC molecules present exogenous antigens from external sources to CD4+ helper T cells. This distinction is important because it helps coordinate different arms of the immune response, allowing for targeted actions against specific types of pathogens based on how they were encountered.
• Evaluate the consequences of impaired antigen presentation by APCs on overall immune function.
  • Impaired antigen presentation by antigen-presenting cells can lead to significant consequences for immune function. If APCs cannot effectively present antigens or provide necessary co-stimulatory signals, T cell activation may be insufficient, resulting in a weakened adaptive immune response. This dysfunction can contribute to increased susceptibility to infections, difficulties in controlling existing infections, or even the development of autoimmune diseases where the body mistakenly targets its own tissues due to lack of proper antigen discrimination.

Related terms

Major Histocompatibility Complex (MHC): A set of molecules displayed on cell surfaces that are essential for the recognition of foreign molecules by the immune system, helping T cells identify infected or abnormal cells.

T Cells: A type of white blood cell that plays a central role in cell-mediated immunity, with subtypes including helper T cells and cytotoxic T cells, both crucial for responding to pathogens.

Dendritic Cells: A specific type of antigen-presenting cell that is highly effective at capturing and processing antigens, serving as a crucial link between the innate and adaptive immune systems.