Antigen-antibody interaction- Complement fixation

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In this article, I briefly describe complement fixation, which is an antigen-antibody interaction.

Antigen-antibody interaction

The antigen-antibody interaction is a bimolecular association. This does not lead to an irreversible chemical alteration in either the antibody or the antigen. This association involves many non-covalent interactions between the antigenic determinant (epitope) of the antigen and the variable-region (VH/VL) domain of the antibody molecule. The antigen-antibody binding is based upon weak and non-covalent interactions like hydrogen bonds, hydrophobic interactions, electrostatic forces, and Van der Walls interactions. Thus, a sturdy antigen-antibody interaction requires a large number of such weak interactions. These interactions can only take place if the antigen and antibody molecules are close enough for some of the individual atoms to fit into complementary recesses.

Affinity and avidity

A very close fit between antigen and antibody increases the strength of their interaction. The sum of the attractive and repulsive forces operating between the antigenic determinant and the combining site of the antibody is known as affinity. The affinity of an antibody for a specific epitope is the combined strength of the non-covalent interactions between a single antigen-binding site on an antibody and the epitope.

The strength of multiple interactions between a multivalent antibody and antigen is called avidity. When complex antigens containing multiple repeating antigenic determinants are mixed up with antibodies containing multiple binding sites, the interaction of an antibody with an antigen at one site will increase the probability of a reaction between those two molecules at a second site. Avidity is more than the sum of the individual affinities. When comparing affinity and avidity, affinity defines the strength of interaction between antibody and antigen at single antigenic sites. In contrast, avidity defines the overall stability or strength of the antibody-antigen complex. The strength of the antibody-antigen complex is controlled by three major factors, i.e., antibody-epitope affinity, the valence of both the antigen and antibody; and the structural arrangement of the interacting parts.

Specificity and cross-reactivity

The specificity of an antigen-antibody reaction is the ability of an individual antibody combining site to react with only one antigenic determinant. It also defines the ability of a population of antibody molecules to react with only one antigen. An antibody can interact with its antigen, thus making the antigen-antibody reactions highly specific. A strong antigen-antibody interaction depends on a very close fit between the antigen and antibody, which requires a high degree of specificity.

The antigen–antibody interaction is highly specific. However, sometimes the antibody elicited by one antigen can cross-react with an unrelated antigen, called cross-reactivity. Cross-reactions arise because the cross-reacting antigen has an epitope, which is structurally similar to one on the immunizing antigen.

Cross-reactivity

Cross-reactivity is often observed among polysaccharide antigens that contain similar oligosaccharide residues. The glycoproteins expressed on red blood cells are the ABO blood group antigens. Subtle differences in the terminal residues of the sugars attached to these surface proteins distinguish the A and B blood group antigens. An individual lacking one or both of these antigens will have serum antibodies to the missing antigens. Thus, individuals belonging to blood groups O and B, have anti-A antibodies in their serum. Similarly, individuals belonging to blood groups O and A, have anti-B antibodies in their serum.

Individuals belonging to blood group AB, are believed not to have anti-A nor anti-B antibodies because they express both antigens on their red cells. The antibodies are induced by exposure to cross-reacting microbial antigens present on common intestinal bacteria. Microbial antigens elicit the blood group antibodies, which will cross-react with similar oligosaccharides present on foreign red blood cells providing the basis for blood typing tests, and accounting for the necessity of compatible blood types during blood transfusions. A number of viral and bacterial antigens elicit antibody that cross-reacts with the host-cell components. This results in a tissue damaging reaction. Some vaccines also exhibit cross-reactivity.

Types of antigen-antibody interaction

There are mainly six types of antigen-antibody interaction and can be categorized as

Complement fixation

This is an immunological medical test that can be used to detect the presence of either specific antibody or antigen in a patient’s serum. The activity of blood serum that completes the action of antibody is called complement. The complement system is a system of serum proteins that react with antigen-antibody complexes.

The basic steps of complement fixation test (figure) are:

  • In the first step, serum is isolated from the patient.
  • The complement proteins in the patient’s serum must be destroyed and replaced with a known amount of standardized complement proteins. As the patient’s serum naturally has different levels of complement proteins, so the replacement of complement proteins help to negate any effect on the test.
  • The serum is heated in such a way that all of the complement proteins but none of the antibodies within it are destroyed.
  • A known amount of standard complement proteins are then added to the serum.
  • Then, the antigen of interest is added to the serum.
  • If the patient’s serum contains antibodies against the antigen of interest, then a complex of antigen-antibody will be formed that will fix the complement (figure). The complement proteins will react with these complexes and are used up.
Figure: Complement fixation

Addition of sheep red blood cells to the serum

The serum is added with sheep red blood cells (sRBCs), which have been pre-bound to anti-sRBC antibodies. When complement fixation occurs, there will be no complement left in the serum to react with the sRBC-antibody complexes. However, if the patient’s serum contains no antibodies against the antigen of interest, the complement will not be depleted. It will react with the sRBC-antibody complexes, thus lysing the sRBCs. In this way, spilling their contents into the solution and turning the solution pink (figure). The pink color of the solution confirms the test as negative.

Applying the same principle, antigen can also be detected in a patient’s serum. In this case, the patient’s serum is supplemented with specific antibody to induce formation of complexes. Then, addition of complement and indicator sRBC is done as performed in the above mentioned antibody detection procedure.

Conclusion

The antigen-antibody interaction is a bimolecular association. A sturdy antigen-antibody interaction requires a large number of such weak interactions. The strength of the antibody-antigen complex is controlled by three major factors. These factors include antibody-epitope affinity, the valence of both the antigen and antibody; and the structural arrangement of the interacting parts. An antibody can interact with its antigen, thus making the antigen-antibody reactions highly specific.

Complement fixation is an immunological medical test that can be used to detect the presence of either a specific antibody or antigen in a patient’s serum. The complement system is a system of serum proteins that react with antigen-antibody complexes. Following the procedure of complement fixation, antigen or antibody can be detected in a patient’s serum.

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