Antigen-presenting cells

Definition

Antigen-presenting cells (APCs) are specialized immune cells that process and present antigens to T cells, initiating an adaptive immune response. They play a critical role in bridging innate and adaptive immunity by capturing, processing, and presenting foreign substances to T lymphocytes, thus activating them to respond effectively to pathogens.

5 Must Know Facts For Your Next Test

1. APCs include various cell types such as dendritic cells, macrophages, and B cells, each with unique functions in the immune response.
2. The interaction between APCs and T cells occurs primarily in lymphoid tissues, where APCs present antigens along with co-stimulatory signals necessary for T cell activation.
3. APCs can present antigens in the context of MHC class I or MHC class II molecules, which determine whether CD8+ or CD4+ T cells are activated.
4. APCs play a vital role not only in pathogen defense but also in maintaining tolerance to self-antigens, helping prevent autoimmune diseases.
5. The effectiveness of an adaptive immune response heavily relies on the ability of APCs to efficiently capture and present antigens, influencing the magnitude and duration of the immune response.

Review Questions

• How do antigen-presenting cells activate T cells, and why is this activation crucial for the adaptive immune response?
  • Antigen-presenting cells activate T cells by processing foreign antigens and presenting them on their surface using Major Histocompatibility Complex (MHC) molecules. This presentation occurs alongside necessary co-stimulatory signals provided by the APCs. The activation of T cells is crucial for the adaptive immune response because it enables them to recognize and respond specifically to pathogens, leading to targeted attacks against infections and the formation of immunological memory.
• Discuss the differences between MHC class I and class II molecules in relation to their roles in antigen presentation by APCs.
  • MHC class I molecules are found on nearly all nucleated cells and present endogenous antigens (from inside the cell) to CD8+ cytotoxic T cells. In contrast, MHC class II molecules are primarily expressed on professional antigen-presenting cells like dendritic cells and macrophages, presenting exogenous antigens (from outside the cell) to CD4+ helper T cells. This distinction is important because it determines which type of T cell is activated and influences the nature of the immune response that follows.
• Evaluate how the function of antigen-presenting cells contributes to both effective immunity against pathogens and tolerance to self-antigens.
  • Antigen-presenting cells play a dual role in both activating effective immunity against pathogens and maintaining tolerance to self-antigens. By presenting foreign antigens alongside necessary co-stimulatory signals, APCs ensure a robust activation of T cells against invaders. Conversely, they also participate in educating developing T cells in the thymus and present self-antigens under non-inflammatory conditions, which helps prevent autoimmunity by promoting tolerance. This balance is essential for a well-regulated immune system that can distinguish between harmful pathogens and the body’s own tissues.

Related terms

T cells: A type of lymphocyte that plays a central role in the adaptive immune response, including helper T cells that assist other immune cells and cytotoxic T cells that kill infected or cancerous cells.

Major Histocompatibility Complex (MHC): A set of molecules displayed on cell surfaces that present processed antigens to T cells, critical for the recognition of foreign substances and activation of the immune response.

Dendritic cells: A type of antigen-presenting cell known for their ability to capture and present antigens efficiently, playing a key role in initiating the adaptive immune response.