Antigen presenting cells or APCs are B cells, dendritic cells, or macrophages that can activate the T cells by presenting antigens on their surfaces. Remember that macrophages and dendritic cells participate in phagocytosis, while B cells mainly make antibodies. B cells also present antigens to T cells. All APCs will have MHC II molecules on their surfaces. They ingest antigens and spit them back out on their surface to present to other immune cells. Dendritic cells are classic APCs. They recognize a pathogen and attach to it, eventually internalizing it. There are lysosomes that digest most of the pathogen, except for parts that are separated by proteases in order to become epitopes that will cause an immune response to happen. Only selected epitopes are presented by the APCs. They ultimately get attached to the MHC II molecules, which is where the presentation happens. MHC I molecules are found on all nucleated cells, presuming they are healthy cells. NK cells and other immune cells recognize the MHC I molecules, leaving the cells with these molecules on them alone. Infected cells are associated with pathogen-specific antigens attached to their MHC I molecule, which marks them for destruction.
T LYMPHOCYTE FUNCTION Humoral immunity is associated with mainly extracellular pathogens. Cells that are already infected with a pathogen need to be gotten rid of by T cells. T cells are responsible for cell-mediated immunity. T cells are made in the bone marrow but mature in the thymus, where they are referred to as thymocytes before they become mature. T cells need to be properly selected by the thymus before they can be released. This is called thymic selection. The first step involves the making of T-cell receptors necessary for APC activation. Those that are defective and do not make these receptors are killed through apoptosis. This is called negative selection. Next, there is positive selection. Those cells that do not interact with the body’s MHC molecules are selected out. The third step involves removing cells that react too much to the self MHC molecules. This prevents things like autoimmunity, which involves accidentally recognizing the self-cells as being foreign. This is also referred to as central tolerance.
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