Antibodies are proteins used by the immune system when harmful substances such as bacteria or viruses (called antigens) enter the body, to either tag a foreign body for attack by other parts of the immune system or by neutralizing the target directly. Production of antibodies is the main goal of the immune system.

Antibodies are created by a type of white blood cell called plasma and take two forms; a soluble form that is secreted from the cell and membrane form that attaches to the surface of the cell. Those antibodies that attach to the surface of cells create memory cells, which remain in the body and remember the specific antigen, so the body knows how to quickly respond. Although antibodies generally have the same structure, the tip that binds to antigens differs greatly, allowing the immune system to identify a large variety of antigens. Antibodies identify and react to antigens in a few ways: one, they bind to the pathogen to prevent it from entering or damaging cells, two, they coat the pathogen in order to stimulate removal, and three, they trigger other autoimmune responses to help destroy the pathogen.

Before the birth of a child, antibodies are provided by the mother. When the infant is born, antibodies that naturally occur in the blood stream become specialized as the body is exposed to antigens, however, antibodies do exist naturally in the body regardless of exposure to outside substances.

Issues can arise when the immune system produces antibodies that attack healthy tissue.