The Interaction of Antibodies with Antigens

Antibodies are proteins The interaction occurs: These photos show one type of interaction — precipitation — between antibodies and antigen.

In the human body, this binding can literally be life-saving.

The capsule that surrounds pneumococci protects them from phagocytosis. (Pneumococci that fail to make a capsule — "R" forms — do not cause disease [Link].

If the appropriate antibodies are present in the body, they combine with the capsule. Coated with protein instead of polysaccharide, the pneumococci are now easy to ingest.

These photomicrographs show phagocytosis of antibody-coated pneumococci.

(From W. B. Wood, M. R. Smith, and B. Watson, Journal of Experimental Medicine 84:387, 1946.)

In the days before antibiotics, the start of antibody production by the immune system of the patient marked the turning point in the progression of the disease.

The Antigen-Combining Site

The diagram on the right shows the primary structure of an IgG antibody. Different IgG antibodies differ most markedly at the so-called hypervariable regions (shown in red): In the three-dimensional (tertiary) structure of the molecule, the 6 hypervariable regions are brought close together and make up the antigen-combining site. For this reason, the hypervariable regions are also called complementarity determining regions (CDRs).
Link to a discussion

The diagram on the left (courtesy of A. G. Amit) shows Marked are the

The second image (also courtesy of Dr. Amit) is a space-filling model of the same antibody with the light chain in yellow, the heavy chain in blue. The view is looking down on the epitope-binding surface (left) and the epitope on lysozyme (right). In the antigen-antibody complex, the two surfaces fit snugly together. The 17 amino acid residues that contact lysozyme in the antigen-antibody complex are numbered (1–7 on the L chain, 8–17 on the H chain. The 16 amino acid residues of lysozyme that contact the antibody; that is, that make up its epitope are also numbered. Number 14 is Gln-121.

The complementarity of the antigen-binding site and the epitope, i.e., determine how strongly the two bind together.

The strength of the binding of an antibody to its antigen is called its affinity. Affinity is discussed in a separate page. Link to it.

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18 May 2011