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Each antibody binds to a specific antigen; this works like a lock and key.

1. Fragment antigen binding region
2. Fragment crystallizable region
3. Heavy chain (blue) with one variable (V_H) domain followed by a constant domain (C_H1), a hinge region, and two more constant (C_H2 and C_H3) domains.
4. Light chain (green) with one variable (V_L) and one constant (C_L) domain
5. Antigen binding site (paratope)
6. Hinge regions

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.