A antigen is a substance that, when introduced into an organism, triggers a defensive reaction. In this way, antigens can produce antibodies, used by the immune system for the identification and neutralization of viruses, bacteria and other pathogens.
Antigens are usually molecules that, once arranged inside the body, cause the immune system (also know as immune system) generate antibodies. This is because, by recognizing the antigen as something strange, fight it.
Molecules bacteria or from virus they can act as antigens that bind to specific antibodies in the body. For the immune system, the antigen poses a threat and therefore acts accordingly.
Sometimes antigens are formed on the own Body, as in the case of bacterial or viral toxins. There are also antigens that come from the environment. Therefore it can be differentiated between the endogenous antigens and the exogenous antigens.
In other words, the origin of the antigens gives rise to a classification. The exogenous are those that enter the body, that is, they come from outside. For this they use ingestion, an injection or inhalation. In general, the immune system response to these types of antigens is not manifested by symptoms.
The exogenous antigens are received by the presenting cells (known by the acronym CPAs) through the mechanism called endocytosis and then they are processed into small parts, which are then passed on to the helper T lymphocytes. These are activated and begin to secrete cytokines, a substance that can also lead to the activation of cytotoxic T lymphocytes, cells that produce antibodies.
With respect to endogenous antigens, they are those that are generated within a cell, either because of its normal metabolism or as a consequence of a bacterial or viral infection. Intracellular viruses are an example of an exogenous antigen that once inside the body becomes endogenous. This is a particular case that can be released into the bloodstream when the affected cell is destroyed.
Their fragments they occur on the cell surface in a larger histocompatibility complex. If they are recognized by activated cytotoxic T lymphocytes, then the secretion of toxins that can cause cell death begins. So that the cytotoxic cells do not eliminate the normal cells that have proteins from the body, the T lymphocytes are eliminated from the repertoire through the negative selection, which is also known as immune tolerance.
There is, on the other hand, the concept of autoantigen, a protein normal or a complex that the immune system can recognize. It should be noted that it can also be RNA or DNA. This takes place in people who suffer from an autoimmune disease, since under normal conditions it should not happen thanks to the aforementioned immune tolerance. Among the factors that most frequently trigger this incorrect activation of the immune system are genetics.
It is important to bear in mind that antigens are often used for the preparation of vaccines. Vaccination consists of introducing an antigen into the body for the activation of the immune system, thus achieving disease prevention.
The process involves the person being exposed to an antigen from an infectious agent so that the immune system creates antibodies and is responsible for activating defenses. Thus, if in the future the subject is confronted with the infectious agent capable of producing the disease, you will already have the immune mechanisms to oppose the infection and prevent it from thriving.