- The combination of antigens (Ag) and antibodies (Abs) occur specifically and in observable manner.
- There is various usefulness of this type of reaction. Some of them are:
- A basis for the formation of antibody mediated immunity
- Helps generally during the cases of infectious diseases.
- Besides, in tissue injury related to some types of hypersensitivity and autoimmune diseases.
- Diagnosis of infections in the laboratory.
- Identification of the infectious agents and non-infectious antigens.
- In surveys related to epidemiology, these reactions have great role.
- Helps in screening the population with particular infection.
- Enzymes like non-infectious antigens are identified.
- Antigens or antibodies can be detected or quantitated.
- In vitro, the antigen and antibody reactions are called serological reactions.
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Stages of Ag-Ab’s reactions
- In three stages, antigen-antibody reactions occur generally. They are:
i) Primary stage
- It is the stage when Ag and Abs react for the first time.
- Visible effects cannot be noticed here.
- Occurs rapidly even in low temperature also.
- Reaction follows laws of thermodynamics and general laws of physical chemistry.
- Reaction is generally reversible in nature.
- Presence of weaker intermolecular forces like Van der Waal’s forces, ionic and hydrogen bonds.
- Due to these forces, combination between Ag and Abs gets affected.
- No involvement of firmer covalent bond.
- Detection of primary reaction can be done by different physical and chemical methods.
- Done by estimation of free and bound antigens or antibodies separately.
- All occurs in the reaction mixture.
- Various markers are also used such as radioactive isotopes, fluorescent dyes or ferritin.
ii) Secondary stage
- Secondary stage generally follows the primary stages.
- In some instances, it may not follow also.
- Several demonstrable events occur in this stage, such as,
- Lysis of cells
- Killing of live antigens
- Neutralization of toxins
- Neutralization of other biologically active antigens
- Fixation of complement
- Immobilization of motile organisms
- Enhancement of phagocytosis
- All of the above reactions were discovered one by one.
- There was belief of involving different type of antibody in different reactions.
- Thus, designation of antibody was done on the basis of the reaction they were thought to produce.
- Agglutinin was designated for antibody causing agglutination, precipitin for precipitation and so on.
- Likewise, antigens were called agglutinogen and precipitinogen respectively.
- A single antibody is responsible to cause the various above mentioned reactions.
- Not only that, most of the serological reactions is also the result of single antibody.
- Whereas, an antigen may be responsible in stimulating production of different classes of immunoglobulins.
- These immunoglobulins are different in reaction capacities and various other properties as well.
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iii) Tertiary stage
- Formation of chain reactions occurs in some Ag-Abs reactions occurring in vivo.
- This results in neutralization or destruction of any injurious antigens or to tissue damage.
- These types of reactions are called tertiary reactions.
- Humoral immunity against infectious diseases, clinical allergy and some immunological diseases falls under this reaction category.
Features of Antigen-antibody reactions
- An antigen combines with its homologous antibody and vice versa only i.e., reaction of antigen and antibody is specific.
- But, sometimes cross reaction may occur due to relatedness or antigenic similarity.
- Entire molecules are involved in reaction, not fragments.
- It means, when reaction occurs with antibody, whole antigenic determinant present in a large molecule or on a ‘carrier’ particles are agglutinated.
- In reaction, denaturation of neither antigen nor antibody occurs.
- Combination, generally occurs at the surface i.e., only surface antigens are relevant immunologically.
- Those antibodies are generally protective in nature that combines with the surface antigens of infectious agents.
- There is no single participation of antigen or antibody in forming agglutinates or precipitates i.e., both take part in formation of them.
- Combination of antigens and antibodies occurs in varying proportions. Multi-valency is shown by both antigens and antibodies.
- Antigens can have valency up to hundreds whereas only IgM shows five or ten valencies and remaining antibodies are bivalent in nature.
- Firm but reversible type of combination occurs. The strength of firmness or union is affected by the affinity and avidity of the reaction.
- The degree or intensity with which antigen are attracted towards antibody molecules is called affinity.
- It is a function of closeness of fit between an epitope of antigen and paratope of antibody.
- Paratope is the antigen combining site present in the antibody molecule.
- Affinity gives quantitative measurement of the strength by which antibody binds to epitope.
- Antibodies having low-affinity binds to the antigens weakly and readily dissociates.
- But antibodies having high affinity are bound to the antigens tightly and for longer period.
- When antigen-antibody complexes are formed, bonding becomes stronger which is called avidity.
- Within biological systems, it can also be taken as a better measure of binding capacity.
- Antigenic determinants on a virus or bacterial cells when reacts with antibody can be one example of it.
- Pentameric form of IgM secreted though has low affinity compared to IgG, can bind antigens more effectively due to its high avidity which results from higher valency.
Stages and Features of antigen-antibody reactions