Produced by the body's immune system, antibodies, also known as immunoglobulins, are Y-shaped proteins that help identify and remove antigens and other foreign targets such as bacteria and viruses. An antigen is a substance that causes the immune system to react by releasing antibodies. Antigens that enter the body from an external source are exogenous, while antigens produced within cells are endogenous.
Examples of antigens include bacteria, viruses, fungi, foreign blood cells from transplanted organs and common allergens such as dust, bee stings, pollen and animal dander. Antigens are typically large protein molecules but also include inorganic substances such as toxins and chemicals. A subclass of antigens, called autoantigens, are proteins or protein complexes that are not typically attacked by the immune system, except in patients with an autoimmune disorder such as lupus.
Antibodies bind to an antigen at the binding site located at the tip of the Y shape. The human body has thousands of antibodies, and each take on a different form at the binding site, which allows each antibody to bind with a specific set of foreign antigens. There are five major classifications of antibodies, each with a different purpose. The most common and the smallest is the IgG class, which circulates in the blood and can move across cell membranes into cells. IgG antibodies attach to bacteria or viral antigens, resulting in the activation of other immune cells that kill the antigens.