Antibodies

 

An antibody matches an antigen as a key matches a lock.
Belong to a family of large molecules known as immunoglobulins.
Immunoglobulins are proteins, 150,000 - 900,000 kd. - each has two identical heavy polypeptide chains and two identical light chains, shaped to form a Y.
The tips of the Y's arms vary greatly from one antibody to another. This creates a pocket uniquely shaped to enfold a specific antigen. This is called the variable (V) region.
The stem of the Y links the antibody to other participants in the immune defenses. This area is identical in all antibodies of the same class, and is called the constant (C) region.