Antigen presenting cell (APC) is specialized cell of our immune system that can recognize the foreign antigen or pathogenic substances and engulf them by endocytosis and break them down into small peptides and after this whole process they present these peptides (or the parts of the antigen) to other immune cells (mainly the T-lymphocytes) and induces immune response for these extracellular antigen. They present antigen by forming a complex of antigen and MHC molecule which is expressed on the surface of APC. So the main function of antigen presenting cell is antigen processing and presentation.
CELLS THAT SERVE AS APC: Many different cells of our immune system serve as APC, but dendritic cells, macrophages and B-lymphocytes are serve as the principal antigen presenting cells which present antigen mainly to the T-lymphocytes. After the interaction with Ag-MHC complex T-cell become activated and differentiated into mature or effector cells (both helper and cytotoxic T-cell) and triggered the further immune response.
LOCATION OF APC: Generally, antigen presenting cells are located on lymphoid tissue, connective tissues, and in blood circulation. Sometimes some APCs (dendritic cells) are present on skin and mucosal epithelium.
PROPERTIES OF APC: The noticeable characters of APC are
- These are nucleated cells containing major histocompatibility complex (MHC) on the surface or the plasma membrane. The MHC molecule present on APC are mainly class-II MHC. Which form a complex with the antigenic particles, endocytosed by the APCs and helps to express it on the surface to another cells.
- These are naturally phagocytic cells according to their function.
ANTIGEN PRESENTATION: The main function of an Antigen presenting cell is antigen presentation.
- Firstly, antigen presenting cells recognize and engulf the extracellular antigen or pathogenic material through endocytosis and enters it into a vessel containing many types of degradative enzymes, called phagosome. After that the phagosome fuses with cellular lysosome and form phagolysosome, containing degradative lysosomal enzymes. Through this process the breakdown of antigen or pathogen into small peptides is done.
- These peptide antigen (derived after the degradation of pathogen like virus, bacteria and other microbes) then processed and presented to the immature or naïve Helper T-cell and Cytotoxic T-cell through MHC molecule. It uses different MHC molecule in different antigen presentation pathways.
- The process of antigen presentation by APC is occur in different pathways -a. cytosolic pathway, and b. endocytic pathway. And they present two types of protein antigens – Exogenous and Endogenous antigens.
- In Cytosolic Pathway, endogenous antigens are synthesized inside the infected cells and these are presented through class-I MHC molecule. Naïve Cytotoxic T-cells (containing CD8 marker) only can recognize the Ag with Class-I MHC present on the surface of APC. They come and bind to the Ag-MHC-I complex by the receptor (TCR).
- In Endocytic Pathway, the extracellular antigen which derived from the degradation of extracellular pathogens, are presented to the immature or naïve T-helper cell containing CD4 marker. T-helper cell only can recognize the antigen binding with MHC-II molecule expressed on the surface of the antigen presenting cells. T-helper cell interact with the APC by binding with the Ag-MHC complex through T-cell receptor, and become activated or mature and trigger the further immune response.
Antigen presenting cells are referred as this for their function of antigen processing and presentation.