Antibodies, also called immunoglobulins, are blood proteins synthesized by certain cells of the immune system (B lymphocytes and plasma cells) in response to the intrusion of a foreign body (antigen) into the body. It is more precisely a variety of protein, serum globulins, with the special property of combining with one or more foreign substances present in the body. Antibodies play an essential role in the body's defences and are present not only in the blood, but also in other fluids in the body.