Antibody, illustration. This Y-shaped protein is made up of four protein chains. The two longest (heavy) chains are identical and form the Y-shaped backbone of the structure. The two shorter (light) chains are also identical and interact with the heavy chains at the branches of the Y. Antibodies defend the body against infection. Their branched regions bind to antigens, such as viral or bacterial proteins, neutralising their activity or initiating immune reactions against them.