Etymologically the word antigen is of Greek origin. The prefix "anti" which means opposite and "geno" which refers to generate. Antigen is that substance that when introduced into the body causes it to make antibodies to fight it. Example: viruses, fungi, parasites, etc. Antigens are almost always foreign and toxic particles that, when they enter the body immediately, bind to a certain antibody, this antibody has the ability to destroy it.
There are other opinions regarding what an antigen can be, it is said that it is an expert molecule in causing a response in the immune system through the activation of lymphocytes, these are commonly of protein origin although they can also be carbohydrates. In order to classify antigens we must know their nature, once we determine their origin we can classify them in the following way. The exogenous antigens are those that enter the body from outside, for example through an injection, inhalation or through the intake.
The endogenous antigens are those that have been produced in the center of a cell due to viral and bacterial infections, once this antigen is present within the cell is be recognized by lymphocytes were activated and began secreting a toxin time loss the death of the infected cell. Auto-antigens are those that the immune system identifies, and that occurs frequently in patients suffering from some type of autoimmune disease. The tumor antigens are those that are located on the surface of tumors.
