infections
Diagnosis of Infectious Disease
Antigen tests
Antigens are substances that can trigger an immune response in the body. Microorganisms (and all cells) have antigens on their surface and inside them. Each type of microorganism has different, unique antigens. Thus, finding one of those antigens means that specific microorganism is present. Antigen tests detect the presence of a microorganism directly, so that doctors can diagnose an infection quickly, without waiting for a person to produce antibodies in response to the microorganism. Also, these tests can be used in people whose immune system cannot produce many antibodies, such as people who have recently had bone marrow transplantation or who have AIDS. To do antigen tests, doctors take a sample from a person and mix it with a test antibody to the suspected microorganism. If there are antigens from that microorganism in the person's sample, they attach to the test antibody. Different methods can be used to detect the antigen-antibody combination. But whatever method is used, the presence of the antigen means that the microorganism is present and probably is the cause of the infection. 