Antigen-presenting cells

Definition

Antigen-presenting cells (APCs) are specialized immune cells that capture, process, and present antigens to T cells, which is essential for initiating the adaptive immune response. By displaying antigens on their surface using major histocompatibility complex (MHC) molecules, APCs help activate T cells and stimulate both cellular and humoral immunity. They play a critical role in bridging the innate and adaptive immune systems.

5 Must Know Facts For Your Next Test

1. Antigen-presenting cells include dendritic cells, macrophages, and B cells, all of which can process and present antigens to T cells.
2. APCs express MHC class I and class II molecules; MHC class I presents to CD8+ cytotoxic T cells, while MHC class II presents to CD4+ helper T cells.
3. The activation of T cells by APCs is crucial for the development of specific immune responses against pathogens.
4. APCs also provide necessary co-stimulatory signals through surface molecules that help fully activate T cells during the immune response.
5. The efficiency of antigen presentation by APCs can influence the magnitude and duration of the adaptive immune response.

Review Questions

• How do antigen-presenting cells contribute to the activation of T cells?
  • Antigen-presenting cells contribute to T cell activation by processing and presenting specific antigens on their surface using MHC molecules. When a T cell's receptor recognizes an antigen-MHC complex, it binds to the APC. This initial recognition is essential but not sufficient for full T cell activation; APCs must also provide additional co-stimulatory signals. This two-step process ensures that T cells respond appropriately to foreign antigens while maintaining tolerance to self-antigens.
• Discuss the differences between MHC class I and MHC class II molecules in relation to antigen presentation.
  • MHC class I molecules present endogenous antigens from proteins synthesized within the cell, primarily to CD8+ cytotoxic T cells. These molecules are found on nearly all nucleated cells. In contrast, MHC class II molecules present exogenous antigens obtained from outside the cell, primarily to CD4+ helper T cells. MHC class II is expressed mainly on professional antigen-presenting cells like dendritic cells, macrophages, and B cells. This distinction is vital for determining the appropriate immune response.
• Evaluate the importance of antigen-presenting cells in shaping both cellular and humoral immunity.
  • Antigen-presenting cells are crucial for shaping both cellular and humoral immunity because they initiate and direct T cell responses, which in turn influence B cell activation and antibody production. By presenting different types of antigens and providing necessary signals, APCs help determine whether the immune response will be primarily cellular (involving cytotoxic T cells) or humoral (involving antibody production by B cells). Their role is pivotal in ensuring that the immune system responds effectively to pathogens while maintaining tolerance to self-antigens, preventing autoimmune diseases.

Related terms

Major Histocompatibility Complex (MHC): A set of molecules displayed on cell surfaces that present antigens to T cells, playing a vital role in immune recognition.

Dendritic Cells: A type of antigen-presenting cell that is highly efficient at processing and presenting antigens, often found in tissues that interface with the external environment.

T Lymphocytes: A type of white blood cell that plays a central role in cell-mediated immunity; they recognize antigens presented by APCs.