Antigen-Antibody Interactions

If the interaction between antigen and antibody were totally random, one would expect the concentrations of free antigen, free antibody and bound Ag-Ab complex to all be equivalent. In other words,

Therefore, the greater the K, the stronger the affinity between antigen and antibody. These interactions are the result of complementarity in shapes, hydrophobic interactions, hydrogen bonds and Van der Waals forces.

ANTIGEN-ANTIBODY RATIOS

Experimentally, if one adds a known concentration of antibody to a tube and then adds increasing amounts of the specific antigen, the Ag-Ab complexes will begin to precipitate. If one continues to add increasing amounts of antigen, the complexes will begin to dissolve and return to solution. The following graph illustrates this process.

The left portion of the graph (tubes 1-3) illustrates "Antibody Excess", since not all of the antibody that is available to bind to antigen has actually bound antigen. The right portion of the graph (tubes 5-7) illustrates "Antigen Excess", where there is not enough antibody to bind to all of the available antigen. In the middle (tube 4) is a region known as "Equivalence". Here, the ration of antigen to antibody is perfect, so that all the antigen molecules and all of the antibody molecules are part of a complex. These are interesting experimental observations that do have relevance to situations occurring in the human body. For example:

• Antibody excess might occur when a person is exposed to a virus from which they have recently recovered. Hence, their body would contain a relatively large concentration of antiviral antibodies. These antibodies could quickly act to block cell receptors on the viral surface and prevent adsorption to host cells, thereby preventing disease.
• Antigen excess might occur early in the first infection by a microorganism. A person would have relatively few antibodies and these would form complexes but they would be very small. Such small complexes probably would not be phagocytosed or removed by the kidneys and could become lodged near tissue surfaces. Later, when antibody becomes available, the size of the complexes can increase leading to effective elimination by phagocytes or tissue damage where the smaller complexes had become lodged. Click here for more information.
• Equivalence would occur when a person is exposed to an agent to which they have circulating antibodies. The correct ratio of antigen to antibody would produce extensive lattice formation, leading to enhanced phagocytosis, opsonization or agglutination, effectively eliminating the foreign agent.

Click the button to illustrate the process

CROSSREACTIVITY

Crossreactivity can occur when two (or more) antigens share similar structural features. Consider three different antigens, as shown on the right. Antibody produced in response to Ag1 is very specific and would, therefore, have a large affinity constant (K) when combining with Ag1. However, Ag2 is similar in shape to Ag1 and is capable of interacting with anti-Ag1 antibody via two of three sites. The interaction between Ab and Ag2 is not as strong as the interaction between Ab and Ag 1 (i.e. K is much smaller) but is still strong enough to allow binding. Hence, Ag1 and Ag2 are said to cross-react. Ag3, in contrast, cannot interact very well with anti-Ag1 antibody and would have a K value so low that significant binding would not occur. Ag3, therefore, would not cross-react with Ag1. Would antibody produced in response to Ag2 bind Ag3? Would antibody produced in response to Ag2 bind Ag1?

Crossreactivity also forms the basis for several diagnostic tests. For example, infection with Treponema pallidum (syphilis) causes the production of antibodies that cross-react with a substance found in cardiac muscle, cardiolipin. Since it is much easier to obtain pure cardiolipin than pure Treponemal antigens, this cross-reaction is used to test for syphilis (Wassermann test). Likewise, antibodies produced against certain Rickettsia cross-react with antigens from Proteus. Since the latter are much easier to obtain, they can be used to test for the former.