Antigen-Presenting Cells (APCs)

Definition

Antigen-presenting cells (APCs) are specialized immune cells that capture, process, and present antigens to T cells, initiating an immune response. These cells play a vital role in bridging the innate and adaptive immune systems, as they present peptide fragments of antigens on their surface using major histocompatibility complex (MHC) molecules. The interaction between APCs and T cells is crucial for activating T cells, which then proliferate and differentiate into effector cells that help eliminate pathogens or infected cells.

5 Must Know Facts For Your Next Test

1. APCs include various cell types such as dendritic cells, macrophages, and B cells, each playing unique roles in antigen presentation.
2. APCs can process both exogenous antigens (from outside the cell) and endogenous antigens (produced within the cell), presenting them via different MHC classes.
3. Dendritic cells are the most potent type of APC and are primarily responsible for initiating T cell responses in lymphoid organs.
4. The interaction between an APC and a T cell requires co-stimulatory signals in addition to the specific recognition of the antigen-MHC complex.
5. APCs not only activate T cells but also play a role in maintaining tolerance to self-antigens, preventing autoimmunity.

Review Questions

• How do antigen-presenting cells contribute to the activation of T cells?
  • Antigen-presenting cells contribute to T cell activation by capturing antigens, processing them into peptide fragments, and presenting these fragments on their surface using major histocompatibility complex (MHC) molecules. When a T cell recognizes its specific antigen-MHC complex, it binds to the APC. This binding is not enough on its own; additional co-stimulatory signals from the APC are also required to fully activate the T cell, leading to its proliferation and differentiation into effector cells.
• Compare the roles of different types of antigen-presenting cells in the immune response.
  • Different types of antigen-presenting cells play distinct roles in the immune response. Dendritic cells are crucial for capturing antigens in peripheral tissues and migrating to lymph nodes, where they effectively activate naive T cells. Macrophages are essential for presenting antigens to memory T cells and can also phagocytose pathogens. B cells can act as APCs by presenting antigens they have bound through their B cell receptors, thus contributing to both humoral immunity and providing secondary signals to T helper cells during the activation process.
• Evaluate the importance of antigen-presenting cells in maintaining immune tolerance and preventing autoimmune diseases.
  • Antigen-presenting cells are critical in maintaining immune tolerance by ensuring that the immune system does not overreact to self-antigens. They achieve this by presenting self-antigens in a manner that promotes tolerance rather than activation of T cells. For example, when self-antigens are presented with insufficient co-stimulatory signals or in an inappropriate context, T cells may become anergic or undergo apoptosis. This mechanism helps prevent autoimmune diseases by reducing the likelihood of an immune attack on the body's own tissues, highlighting the delicate balance between immune activation and tolerance.

Related terms

Major Histocompatibility Complex (MHC): A set of molecules displayed on cell surfaces that present antigenic peptides to T cells, essential for the recognition and response of the adaptive immune system.

Dendritic Cells:

A type of APC that is particularly efficient at capturing antigens in tissues and migrating to lymph nodes to activate T cells.

T Cell Activation: The process by which T cells are stimulated to proliferate and differentiate in response to antigens presented by APCs, leading to an effective immune response.