Antibodies (also called immunoglobulins) are large Y-shaped proteins that can stick to the surface of bacteria and viruses. They are found in the blood or other body fluids of vertebrates. Antibodies are the key element in the adaptive immune system.
The antibody recognizes a unique part of the foreign target called an antigen. Each tip of the "Y" of an antibody contains a structure (like a lock) that fits one particular key-like structure on an antigen. This binds the two structures together.