Video Transcript
The figure represents the structure
of an antibody. Where does an antigen bind?
To identify where an antigen binds
on an antibody, let’s first look at its various components. An antibody, sometimes called an
immunoglobulin, is a soluble protein made up of four polypeptide chains. There are two identical longer
chains, known as heavy chains, that make up the central portion of the antibody and
two identical shorter chains on the exterior, known as the light chains. The chains are held together by
disulfide bonds, one of which is labeled with the letter A. The disulfide bonds hold the chains
together in a characteristic Y shape. And at the base of this Y is a
receptor binding site, which allows the antibody to bind to surface receptors on
cells, particularly other immune system cells, such as macrophages. At the top of each branch of the Y
is an antigen binding site. The two antigen binding sites bind
to a single specific antigen based on their shape, much like a key fits into a
specific lock. Because there are two identical
antigen binding sites, a single antibody can bind to two identical antigens
simultaneously. Therefore, on this diagram, the
sites where antigens can bind are B and C.
