Key Functions of Antigen Presenting Cells

Antigen presenting cells (APCs) are vital players in the immune system, bridging innate and adaptive responses. They capture and present antigens to T cells, shaping the body’s ability to recognize and respond to pathogens while maintaining tolerance to self.

1. Dendritic cells

   • Serve as the primary antigen-presenting cells (APCs) that capture, process, and present antigens to T cells.
   • Found in tissues and migrate to lymph nodes to activate naïve T cells, initiating adaptive immune responses.
   • Express high levels of major histocompatibility complex (MHC) molecules and co-stimulatory signals necessary for T cell activation.
   • Play a crucial role in distinguishing between self and non-self antigens, influencing tolerance and immunity.
   • Involved in the cross-presentation of exogenous antigens to CD8+ T cells, enhancing cytotoxic responses.

2. Macrophages

   • Act as versatile APCs that can phagocytose pathogens and present antigens to T cells.
   • Found throughout the body, they play a key role in both innate and adaptive immunity.
   • Produce cytokines that influence the immune response and can polarize T cells into different subsets (e.g., Th1, Th2).
   • Exhibit plasticity, adapting their function based on the microenvironment and signals received.
   • Essential for tissue homeostasis and repair, as well as the clearance of dead cells and debris.

3. B cells

   • Function as APCs primarily for T helper (Th) cells, presenting processed antigens via MHC class II molecules.
   • Upon activation, they differentiate into plasma cells that produce antibodies specific to the presented antigen.
   • Contribute to the humoral immune response, providing long-term immunity through memory B cells.
   • Can also act as antigen scavengers, enhancing their own activation and the overall immune response.
   • Involved in the regulation of immune responses through cytokine production and interaction with other immune cells.

4. Langerhans cells

   • Specialized dendritic cells located in the skin and mucosal tissues, playing a key role in the first line of defense.
   • Capture and present antigens from the skin to T cells in lymph nodes, initiating immune responses.
   • Express unique markers (e.g., Langerin) that distinguish them from other dendritic cells.
   • Involved in maintaining skin homeostasis and tolerance to harmless antigens, such as allergens.
   • Can migrate to lymph nodes upon activation, influencing the adaptive immune response.

5. Follicular dendritic cells

   • Located in the germinal centers of lymphoid follicles, they are crucial for B cell activation and maturation.
   • Present antigens to B cells in a form that promotes affinity maturation and class switching.
   • Do not express MHC class II molecules, distinguishing them from other APCs, and instead use complement receptors to capture antigens.
   • Play a role in the formation and maintenance of memory B cells, contributing to long-term immunity.
   • Influence the selection of high-affinity B cells during the germinal center reaction, essential for effective antibody responses.