Antigen presenting cells (APC)

Antigen presenting cells or APCs - This lecture explains about the properties of Professional antigen presenting cells such as macrophage, dendritic cells and B cells. It also explains the importance of antigen presenting cells or APC to engulf a pathogen through phagocytosis and then presenting it to the whole immune system so that cell mediated and humoral immune response can build up.
Mature, naive B- and T cells, as well as precursors of APC (antigen presenting cells, including monocytes/macrophages and dendritic cells) from the bone marrow emigrate from the central lymphatic organs. Lymphocytes travel mainly via the bloodstream. APC leave the bloodstream to widely roam tissues. Eventually, all types of cells meet again at the peripheral lymphatic organs: lymph nodes, GALT/Peyer plaques and tonsils, BALT and spleen.

Lymph nodes seem static in the microscope, but should better be compared to the transit area of a big international airport, with oodles of cells arriving and leaving all the time. Lymph nodes have several inlets and an outlet. Afferent lymphatic vessels reaching the most peripheral lymph nodes transport the interstitial fluid filtrated from blood capillaries. With the lymph flow, dendritic cells loaded with ingested material drift to the lymph nodes, e. g., Langerhans cells from the skin. In case of an infection, lymph flow increases dramatically, carrying with it pathogens and their antigenic molecules, outside and inside of activated macrophages and dendritic cells. Thus, a lymph node is a local command center with continuous real-time information on the antigenic situation in the periphery. From the blood, lymphocytes constantly enter the lymph node via specialized high endothelial venules. B cells migrate to areas near the cortex, and, if activated, form follicles with germinal centers. There, specialized "follicular dendritic cells" immobilize immune complexes with their Fc- and complement receptors, so that the antigens are "visible" to the proliferating B cells. T cells wander to adjacent paracortical areas. Some activated B cells that already have differentiated to plasma cells, and more macrophages, sit in the lymph node's medulla. Each lymph node has an efferent vessel connecting to the next lymph node and, eventually, via the thoracic duct to the blood.

(Caution: "dendritic cells" and "follicular dendritic cells" are completely different cell types that obtained similar names (dendritic = tree-like) because of their morphological appearance. Dendritic cells are specialized APC ingesting antigen in the periphery and presenting processed antigen on MHC II to T cells. Follicular dendritic cells sit in germinal centers and use complement receptors and Fc receptors to fix antigen-containing immune complexes on their outer surface for B cells to see.)

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