2.5.5 The Role of Antigen-Presenting Cells

What Are Antigen-Presenting Cells (APCs)?

1. Definition:

• APCs are specialised cells that process and display antigens on their surface bound to major histocompatibility complex (MHC) molecules.

2. Examples of APCs:

• Macrophages: Engulf pathogens and present their antigens.
• Dendritic cells: Highly efficient APCs found in tissues that encounter external pathogens (e.g., skin, mucosa).
• B lymphocytes: Present antigens to helper T cells during the immune response.

Steps in Antigen Presentation:

1. Pathogen Uptake:

• APCs engulf pathogens via phagocytosis or endocytosis.

2. Antigen Processing:

• The pathogen is broken down into smaller fragments inside the APC.
• These fragments (antigens) are loaded onto MHC molecules within the cell.

3. Antigen Presentation:

• The MHC-antigen complex is displayed on the surface of the APC.
• This allows the APC to interact with T lymphocytes, particularly helper T cells.

4. T Cell Activation:

• T cells bind to the antigen-MHC complex via their T cell receptor (TCR).
• This activates the T cell, triggering the immune response.

Role of APCs in the Immune Response:

1. Activation of T Cells:

• APCs present antigens to helper T cells, which release cytokines to activate other immune cells, including cytotoxic T cells and B cells.

2. Coordination of Immune Response:

• APCs act as a link between the innate and adaptive immune systems, ensuring a targeted response.

3. Triggering Immunological Memory:

• APCs help activate memory T cells, ensuring a faster response during subsequent infections.

Importance of Antigen Presentation:

• Non-Specific to Specific Response:
  • APCs bridge the innate immune response (non-specific) and the adaptive immune response (specific).
• Targeted Defence:
  • By presenting specific antigens, APCs enable the immune system to target pathogens effectively.
• Immunological Tolerance:
  • APCs present self-antigens to prevent the immune system from attacking the body's own cells (in most cases).

Diseases Related to APC Dysfunction:

1. Autoimmune Diseases:

• APCs presenting self-antigens may mistakenly activate T cells, causing the immune system to attack the body's tissues.

2. Cancer:

• Cancer cells may evade immune detection by downregulating antigen presentation.

3. HIV/AIDS:

• HIV targets helper T cells, impairing their interaction with APCs and weakening the immune system.