An antibody, also known as an immunoglobulin, is a protein component of the immune system that circulates in the blood and recognizes foreign substances like bacteria and viruses, and neutralizes them. Antibodies are produced by B cells, which are specialized white blood cells, and they bind to antigens, which are foreign substances that enter the body, in order to eliminate them from the system. Each type of antibody is unique and defends the body against one specific type of antigen. Antibodies are heavy proteins of about 10 nm in size, arranged in three globular regions that roughly form a Y shape. Antibodies are located in various areas of the body, including the skin, lungs, tears, saliva, and even breast milk. They can be used to combat autoimmune conditions and cancer, confer passive immunity, and have diagnostic applications.