Antigen-Antibody Reactions

Antigen-Antibody Reactions

Antigens-antibody, is a combination with each other, specifically and in an observable manner.

Uses of Antigens and antibodies

The reactions between antigens and antibodies have several purposes:

  • In the body, they form the basis of antibody-mediated immunity in infectious diseases, or of tissue injury in some types of hypersensitivity and autoimmune diseases.
  • In the laboratory, they help in the diagnosis of infections.
  • In epidemiological surveys, they assist in the identification of infectious agents and non-infectious antigens such as enzymes and in screening the population for a particular infection.
  • ln general, these reactions can be used for the detection and quantification of either antigen or antibodies.
  • Antigen-antibody reactions in vitro are known as serological reactions.

Stages of Antigens and antibodies

The reactions between antigens and antibodies occur in three stages:

Primary stage

This is the initial interaction between antigens and antibodies, without any visible effects. This reaction is rapid, occurs even at low temperatures and obeys the general laws of physical chemistry and thermodynamics. The reaction is reversible, the combination between antigen and antibody molecules being affected by weaker inter moleculars such as vander Waal’s force, ionic bonds and hydrogen bonding, rather than by the firmer covalent bonding.

The primary reaction can be detected estimating free and bound antigens or antibodies separately in the reaction mixture by a number of physical and chemical methods, including the use of markers such as radioactive isotopes, florescent dyes or furritin.

Secondary stage

In most, but not all, instances, the primary stage is followed by the secondary stage, leading to demonstrable events such as Precipitation, agglutination, lysis of cells, killing of live antigens, neutralisation of  toxins and other biologically active antigens, fixation of complement, immobilisation of motile organisms and enhancement of phagocytosis.

When such reactions were discovered one by one, it was believed that a different type of antibody was responsible for each type of reaction, and the antibodies came to be designated by the reactions they were thought to produce)·

Thus the antibody causing agglutination was called agglutinin, that causing precipitation precipitin, and so on, and the corresponding antigen, agglutinogen, precipitinogen, and so on. It is true that a single antibody can cause precipitation, agglutination and most of the other serological reactions and an antigen can stimulate the production of different classes of immunoglobulins which differ in their reaction capacities as well as in other properties.

Tertiary stage

Some antigen-antibody reactions occurring in vivo initiate chain reactions that lead to neutralisation or destruction of injurious antigens, or to tissue damage. These are tertiary reactions and include humoral immunity against infectious diseases as well as clinical allergy and other immunological diseases.

General features of antigen-antibody reactions

Antigen-Antibody Reactions

  • The reaction is specific, an antigen combining only with its homologous antibody and vice versa. The specificity, however, is not absolute and cross-reactions may occur due to antigenic similarity or relatedness.
  • Entire molecules, and not fragments, react. When an antigenic determinant present in a large molecule or on a carrier particle reacts with its antibody, whole molecules or particles are agglutinated.
  • There is no denaturation of the antigen or the antibody during the reaction.
  • The combination occurs at the surface. Therefore, it is the surface antigens that are immunologically relevant. anitibodies to the surface antigens of infectious agents are generally protective.
  • The combination is firm but reversible. The firmness of the union is influenced by the affinity and avidity of the reaction:


Affinity refers to the intensity of attraction between the antigen and antibody molecules. It is a function of the closeness of fit between an epitope and the antigen-combining region of its antibody (paratope). Affinity is a quantitative measure of binding strength between an antibody and an epitope. Low affinity antibodies bind antigens weakly and tend to dissociate readily whereas high-affinity antibodies bind antigens more tightly and remain bound longer.


Avidity is the strength of the bond after the formation of the antigen-antibody complexes and is a better measure of its binding capacity within biological systems (for example, the reaction of an antibody with antigenic determinants on a virus or bacterial cell) than the affinity of its individual binding sites. Secreted pentameric IgM often has lower affinity than IgG, but the high avidity of IgM, resulting from its higher valence, enables it to bind antigens effectively.

  • Both antigens and antibodies participate in the formation of agglutinates or precipitates.
  • Antigens and antibodies can combine in varying proportions, unlike chemicals with fixed valencies. Both antigens and antibodies are multivalent. Antibodies are generally bivalent, though IgM molecules may have five or ten combining sites. Antigens may have valencies up to the hundreds.

Measurement of antigen and antibody

Many methods are available for the measurement of the antigens and antibodies participating in the primary, secondary and tertiary reactions. Measurement may be in terms of mass (for example, mg nitrogen) or more commonly as units or titre.

The antibody titre of a serum is the highest dilution of the serum that shows an observable reaction with the antigen the particular test. The titre of a serum is influnenced by the nature and quantity of the antigen and the type and conditions of the test. Antigens may also be titrated against sera.

Two important parameters of serological tests are sensitivity and specificity:


Sensitivity refers to the ability of the test to detect even very minute quantities of antigen or antibody. When a test is highly sensitive, false negative results will be absent or minimal.


Specificity refers to the ability of the test to detect reactions between homologous antigens and antibodies only, and with no other. In a highly specific test, false positive reactions are absent or minimal. In general, the sensitivity and specificity of a test are in inverse proportion.

Originally, reagents for serological tests were prepared by individual laboratories, leading to batch variation, and lack of reproducibility and comparability. The commercial availability of readymade standardised test kits has simplified test procedures, improved quality and greatly enlarged their scope and use.

References and Sources


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