Antigen Presenting Cells

APCs are vital in initiation of the immune response. An APC is a cell that displays a foreign antigen complexed with MHC on its cell surface. Its major responsibility is to present these foreign antigens to T cells. T cells can identify this complex using their T-cell receptors (TCRs). There are three main types of APCs: dendritic cells (DCs), macrophages, and activated B cells. DC are present in tissues that are in contact with the environment, such as the skin and the lining of the nose, lungs, stomach, and intestines. They are responsible for antigen phagocytosis. After phagocytosis, they express the foreign antigen on their cell surface and then migrate to the lymphoid tissues to interact with T and B cells to initiate the immune response. Macrophages' main role is in the removal of pathogens and necrotic debris. However, like DCs, macrophages also phagocytize antigens and express them on their cell membranes to present to T cells in order to initiate an immune response. The first time an antigen is encountered, the DCs and macrophages act as the primary APCs. However, if the same antigen is encountered again, memory B cells become the most important APC because they initiate the immune response quickly after antigen presentation. It appears that both the DCs and macrophages have the most activity in terms of allore-cognition.

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