The term antigenicity describes the ability of an antigen to produce an immune response in the body, and then bind to T-cells or antibodies after this response occurs. When an antigen, or infectious microbe, is introduced to the body, it usually triggers a response from the immune system. If the antigen is then able to bind to the white cells and antibodies produced by this immune response, it is said to have high antigenicity and is relatively difficult to expel from the body. Antigens that have trouble binding to white cells and antibodies are said to have low antigenicity and are relatively easy for the immune system to overcome.
While antigenicity is generally seen as a harmful quality in microbes, it can sometimes be used to the advantage of humans and animals. Immunogenicity is a subset of antigenicity that can be observed when an antigen causes an immune response but does not bind to white blood cells and antibodies. This kind of action often occurs when humans and animals receive vaccines. The antigens in these injections are very weak or dead, but they still force the body to try to protect itself. Later, when a strong version of the virus tries to attack the body, its antigenicity will probably be low because the body's immune response to it will be ready for it.
When antigens with strong antigenicity are able to bind themselves to cells sent from the immune system, it may be very difficult for the body to fight them. The antigens in these cases are attacking the systems in the body meant to keep it healthy and strong, which could cause very serious health problems. For instance, if pollen enters the body it may cause an immune response wherein the patient sneezes, coughs, or experiences swollen nasal and throat passageways. The pollen, which is the antigen here, may then bind to the white blood cells that are trying to expel it, neutralizing them temporarily. This may then cause histamine response to continue, weakening the patient.
In cases like the pollen example, medication is usually needed to help re-balance bodily function. Allergic reactions are generally treated with antihistamines, and sometimes steroid treatments. The antihistamines calm the histamine response, because the bound white blood cells are unable to, and the steroid treatments reduce any inflammation.
Different people react in various ways to certain substances, meaning something that is an antigen to one person may not affect another. Peanuts and other tree nuts are an example of this. Some people can eat peanuts without incident, while others cannot even smell peanut butter without having an adverse reaction. For this reason, people and companies in the business of developing new foods, cosmetics, medicines, and other products must perform antigenicity tests before releasing these products to the public. Such tests examine any immune response the product causes, allowing the manufacturer to know whether or not the product is safe for the general public.