The immune system is composed of a complex network of cells that defend the body against foreign antigens such as bacteria, virus or tumor cells. White blood cells such as neutrophils, eosinophils, basophils, T cells, B cells, macrophages, and dendritic cells are the type of cells in the immune system. The immunity can be classified into two as innate immunity and adaptive immunity based on the type of recognition of foreign antigens by the immune system. Innate immunity generates a non-specific immune response to any type of pathogen. Adaptive immunity generates a specific immune response based on the pathogen. Antigen-presenting cells such as macrophages and dendritic cells are responsible for the recognition of antigens.
Key Areas Covered
1. What are Dendritic Cells
– Definition, Structure, Function
2. How Do Dendritic Cells Recognize Foreign Antigens
– Processing and Presenting Antigens
Key Terms: Antigen-Presenting Cells, Cytokines, Dendritic Cells, Epitopes, Foreign Antigens, Helper T Cells, MHC Class 2 Molecules, T Cell Receptors
What are Dendritic Cells
Dendritic cells are the most efficient antigen presenting or processing cells of the immune system of the body. They can be identified within the lymphatic tissues, mucosa, and the skin. They present antigens to the T cells for the initiation of immune response. Since dendritic cells present antigens to T cells, they are known as professional antigen-presenting cells. A dendritic cell is shown in figure 1.
Figure 1: Dendritic Cell
The immature dendritic cells are known as veiled cells and possess large cytoplasmic veils. They grow branched projections at a certain stage of development. Once activated by an antigen, dendritic cells migrate to the lymph nodes to interact with helper T cells.
How Do Dendritic Cells Recognize Foreign Antigens
Dendritic cells engulf foreign antigens by phagocytosis, forming a vesicle known as phagosome. The fusion of a lysosome that contains hydrolytic enzymes with the phagosome starts the intracellular digestion of the foreign antigen. The resultant small pieces of peptides are known as epitopes and these epitopes bind to the MHC class 2 molecules that enter into the vesicle, forming MHC-peptide complexes. Generally, MHC class 2 molecules bind with the exogenous antigens. The MHC-peptide complexes are released from the vesicles and bind to the outer surface of the cell membrane, becoming an antigen-presenting cell. The antigens are recognized by the specific T cell receptors of the CD4+ helper T cells that bind with the MHC complex of dendritic cells. The processing and presenting antigens by dendritic cells is shown in figure 2.
Figure 2: Processing and Presenting Antigens by Dendritic Cells
T cell receptors are the molecules found on the surface of T cells and they recognize the antigens bound to the MHC complex on dendritic cells. Upon binding, the T cells activate a series of biochemical events, stimulating the secretion of specific cytokines. The cytokines activate both T cell proliferation and the production of antibodies by B cells.
Conclusion
Dendritic cells are a type of antigen-presenting cells that are involved in the initiation of a specific immune response. They engulf foreign antigens and produce epitopes through intracellular digestion, presenting them on the cell membrane with the help of MHC class 2 molecules. These epitopes are recognized by the T cell receptors in the CD4+ helper T cells and secrete cytokines that specifically initiate an adaptive immune response.
Reference:
1. Janeway, Charles A, and Jr. “Antigen Recognition by T Cells.” Immunobiology: The Immune System in Health and Disease. 5th Edition., U.S. National Library of Medicine, 1 Jan. 1970, Available here.
Image Courtesy:
1. “Dendritic cell revealed” By National Institutes of Health (NIH) – National Institutes of Health (NIH) (Public Domain) via Commons Wikimedia
2. “2216 Antigen Processing and Presentation” By OpenStax College – Anatomy & Physiology, Connexions Web site, Jun 19, 2013 (CC BY 3.0) via Commons Wikimedia
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