Antigen-presenting cell

Antigen presentation

Antigen-presenting cells (APCs) are a diverse group of immune system cells that play an essential role in initiating the body's immune response by presenting antigens to T cells. Antigens are substances that the immune system recognizes as foreign or dangerous, such as parts of pathogens, cancer cells, or transplanted tissue. By displaying these antigens on their surface, APCs help T cells recognize and respond to specific threats.

Types of Antigen-presenting Cells

There are several types of APCs, each with unique functions and locations within the body. The main types include:

• Dendritic cells: Considered the most potent APCs, dendritic cells are crucial for initiating T cell responses. They capture antigens at sites of infection or inflammation, process them, and then migrate to the lymph nodes to present the antigens to T cells.
• Macrophages: These cells are found throughout the body and are involved in phagocytosis, engulfing and digesting cellular debris, foreign substances, microbes, and cancer cells. After digesting these materials, macrophages present antigens from these pathogens to T cells.
• B cells: While primarily known for producing antibodies, B cells can also function as APCs. They present antigens to helper T cells, which is crucial for the activation of B cells and the production of high-affinity antibodies.

Antigen Presentation

Antigen presentation involves the display of antigen fragments bound to major histocompatibility complex (MHC) molecules on the cell surface. There are two main types of MHC molecules involved in antigen presentation:

• MHC class I molecules: Present on almost all nucleated cells, these molecules present antigens from within the cell (e.g., viral peptides) to CD8+ T cells (cytotoxic T cells), which can then directly kill infected cells.
• MHC class II molecules: Expressed on professional APCs, these molecules present antigens that have been engulfed and processed to CD4+ T cells (helper T cells), which then help coordinate the immune response by activating other immune cells.

Role in the Immune Response

The interaction between APCs and T cells is a critical step in the adaptive immune response. By presenting antigens to T cells, APCs help the immune system distinguish between self and non-self, and determine the appropriate immune response. This can include the activation of T cells to kill infected or cancerous cells, the stimulation of B cells to produce antibodies, or the initiation of other immune mechanisms to eliminate the threat.

Clinical Significance

Understanding the role of APCs in the immune system has important implications for vaccinology, autoimmune diseases, allergies, and cancer immunotherapy. For example, vaccines often aim to enhance the presentation of specific antigens to stimulate a protective immune response. In autoimmune diseases, dysregulation of antigen presentation can lead to the immune system attacking the body's own tissues. In cancer, some therapies aim to boost the ability of APCs to present tumor antigens, thereby enhancing the immune system's ability to target and destroy cancer cells.