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Antigen-presenting cells
Antigen-presenting cells (APC) are specialized white blood cells that help fight off foreign substances that enter the body. These cells send out signals to T-cells (other immune system cells) when an antigen enters the body. Each type of T-cell is specially equipped to deal with different pathogens, which may be a bacteria, virus or toxin.First, the APC engulfs the antigen. Enzymes inside the APC break down the antigen into smaller particles. The processed antigens are transported to the surface of the APC, bound with either an MHC (major histocompatibility complex) class I or class II molecule. This complex forms epitopes (part of a foreign substance that can be recognized by the immune system), which the T-cell receptor (TCR) recognizes and binds to.
The APCs are divided into two categories - professional and non-professional APCs. Professional APCs express MHC class II while non-professional APCs express MHC class I. Only professional APCs are able to activate a helper T-cell that has never encountered its antigen before. There are three main types of professional antigen-presenting cells - macrophages, dendritic cells and B-cells. These cells are able to engulf the antigen quickly during a process called phagocytosis. Once the T-cell recognizes and binds to the MHC molecule complex, the APC sends out an additional co-stimulatory signal to activate the T-cell.
Non-professional APCs include fibroblasts (skin cells), thymic epithelial cells, thyroid epithelial cells, glial cells (brain cells), pancreatic beta cells and vascular endothelial cells.
While almost every cell in the body is technically an APC because they can present antigens to T-cells, the term is usually used to describe professional APCs.
Related Terms
Antigen, antigen-processing cell, apoptosis, cytokines, cytotoxic T-cells, endocytic, endocytosis, epitopes, helper T-cells, immune, immune defense system, immune reaction, immune response, immune system, interferons, lymphocytes, major histocompatibility complex, MHC, MCH class I, MCH class II, non-professional antigen-presenting cell, non-professional antigen-processing cell, non-professional APC, pathogen, pattern recognition receptors, phagocyte, phagocytosis, professional antigen-presenting cell, professional antigen-processing cell, professional APC, PRRs, T-cell, T-lymphocyte, white blood cell.
Natural Standard Bottom Line Monograph, Copyright © 2009 (www.naturalstandard.com). Commercial distribution prohibited. This monograph is intendedfor informational purposes only, and should not be interpreted as specific medical advice. You should consult with a qualified healthcare provider before making decisions about therapies and/or health conditions.
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