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Antigen presentation is a process in the body's immune system by which macrophages, dendritic cells and leukocytes capture antigens and then carry those antigens to T-cells. Once a T-cell has been attuned to an antigen it will then circulate throughout the bloodstream, seeking out that antigen and destroying it.==Introduction== The basis of adaptive immunity lies in the capacity of immune cells to distinguish between the body's own cells, and the unwanted invaders. The host’s cells express “self” antigens that identify them as such. These antigens are different from those on the surface of bacteria ("non-self" antigens) or on the surface of virally infected host cells (“missing self”). The adaptive immune system is triggered through a process known as antigen presentation. With the exception of some cell types, such as non-nucleated cells (including erythrocytes), all cells are capable of presenting antigen and activating the adaptive response. Some cells, however, are specially equipped to present antigen, and to prime naive T cells. Dendritic cells and B-cells (and to a lesser extent macrophages), while playing a a major role in the innate response, also act as professional antigen presenting cells (APC). These professional APCs are equipped with special immunostimulatory receptors that allow for enhanced activation of T cells. Several different types of T cell can be activated by professional APCs, and each type of T cell is specially equipped to deal with different pathogens, whether the pathogen is bacterial, viral or a toxin. The type of T cell activated, and therefore the type of response generated, depends, in part, on the context in which the antigen was first encountered by the APC.
Exogenous Antigens Dendritic cells engulf exogenous (outside the cell) pathogens, such as bacteria, parasites or toxins in the tissues and then migrate, via chemotactic signals, to the T cell enriched lymph nodes. During migration, DCs undergo a process of maturation in which they lose most of their ability to engulf other pathogens and develop an increased ability to communicate with T-cells in the lymph nodes. The DC also uses enzymes to chop the pathogen into smaller pieces. In the lymph node the DC will display these small pieces of the pathogen on its surface by coupling them to a receptor called the Major histocompatibility complex, or MHC (also known in humans as Human leukocyte antigen (HLA)). This MHC:antigen complex is then recognized by T-cells passing through the lymph node. Exogenous antigens are usually displayed on MHC Class II molecules, which interact with CD4+ helper T-cells. CD4+ Lymphocytes, or helper T cells, are immune response mediators, and play an important role in establishing and maximizing the capabilities of the adaptive immune response. Endogenous Antigens Endogenous antigens (from inside the cell) are produced by viruses replicating within a host cell. Like the DC, the host cells use enzymes to digest virally associated proteins. The infected cell displays these small pieces of the virus, coupled to MHC, on its surface, to T-cells. Endogenous antigens are typically displayed on MHC Class I molecules, which interact with CD8+ cytotoxic T-cells. With the exception of some cell types, such as non-nucleated cells (including erythrocytes), Class I MHC is expressed by all host cells. Cytotoxic T cells (also known as TC, killer T cell, or cytotoxic T-lymphocyte (CTL)) are a sub-group of T cells which are capable of inducing the death of other cells, and are thus defined as "cyto"- or cell -toxic. CTLs function to kill cells that are infected with viruses (and other pathogens), or are otherwise damaged or dysfunctional. See also | ||||||||
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