An antibody, also known as an immunoglobulin, is a protein found in the blood or body fluid of an animal. These proteins have special receptors that allow them to bind to foreign substances known as antigens. Their purpose is to identify and neutralize antigens so that they cannot make the host organism sick. Antibodies make up the core of the immune system, acting like shock troops to quickly quell incursions from antigens.
The chemical structure of antibodies can get quite complex, but the short version is that they are Y-shaped structures composed of linked chains of polypeptides known as light and heavy chains. The string of amino acids determines what class the antibody is in, and also which antigens it can bind to. Each can bind with only one antigen, in a system that could be compared to a lock and key. Receptors at the tips of the structures allow it to bind to a particular antigen.
Some antibodies float freely in the blood, produced by the B cells as needed. B cells know when to produce more of these proteins because they have membrane-bound ones that stick to their surfaces at all times. These antibodies act as invasion detectors, alerting the B cell when they detect an antigen they bind to so that the B cell can trigger the production of more proteins to fight the antigen.
Some examples of antigens include bacteria and viruses. The body develops antibodies when it is initially exposed to the antigen, and it stores them for future usage. Some antigens are wily, capable of changing their genetic code just enough so that future generations will not interact with antibodies because the lock and key do not match. Others are dependable and common enough that people can be vaccinated against them by being introduced to a small sample of the antigen so that their bodies are prepared to recognize and fight the antigen in the future.
Occasionally, the production of antibodies goes haywire. In autoimmune disorders, the body develops antibodies to itself, and they will start to attack substances that are actually created by the body under the mistaken impression that these substances are harmful. People may also develop the proteins when they are exposed to certain normally harmless compounds, like food, pet dander, or dust mites. When the body meets these substances again, it will produce more and trigger an allergic reaction as the body attempts to fight the perceived antigen.