Antibodies are a group of molecules that bind to specific proteins (antigens). Once an antibody is bound with its antigen, it signals to the rest of the immune system to destroy the cell to which that antigen belongs. Antibodies begin life as receptors on a type of white blood cell called a B cell. They are shed in vast numbers in a soluble form when the B cell encounters the specific antigen to which its receptors bind.

MS Glossary
All About Multiple Sclerosis