Key Functions of Antigen Presenting Cells to Know for Immunobiology

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.