Antigen-antibody reaction or antigen-antibody interaction is a particular chemical interaction between antibodies generated by B cells of the white blood cells and antigens during the immune reaction. The process of agglutination combines antigens and antibodies.
It is the basic biological process that the body uses to defend itself against various foreign particles like viruses and their toxic chemicals. An antigen-antibody complex is formed in the blood when antibodies specifically and strongly bind to antigens. The immunological complex is subsequently transferred to cellular systems where it can be eliminated or deactivated.
Table of Contents
- Antigen (Ag)
- Antibody (Ab)
- Antigen-Antibody Reaction
- Types of Antigen-Antibody Reaction
- Frequently Asked Questions (FAQs)
(Anti = opposite; gen = anything that causes)
Immunogens are any foreign substances that, once entering our bodies, frequently cause a sequence of immunological responses. While others, known as haptens, require the assistance of other molecules (carrier proteins) to activate an immunological response. All of the immunogens and haptens are referred to as antigens.
- They could be polysaccharides, lipids, proteins, or peptides.
- An epitope is an antibody-binding location.
An antibody is a component that the immune system produces in response to antigens. Thus, antigens result in the production of antibodies. They act together to exhibit an immunological response. The general characteristics of an antibody are as follows:
- An antibody is also known as an immunoglobulin (Ig)
- They are Y-shaped
- Generated by plasma B-cells.
- Paratope is the name of the antigen binding site.
- Five types: IgG, IgA, IgM, IgE, and IgD.
Also, read: Role of Antibodies
Antigens and antibodies combine specifically with each other. Antigen-Antibody reaction is the term used to describe this interaction between them. Ag-Ab reaction is a common acronym for it. These serve as the building blocks of humoral or antibody-mediated immunity.
These reactions serve as the foundation for the detection of both specific and non-specific Ags, such as enzymes that cause non-specific diseases. Serological reactions are referred to as Ag-Ab reactions when they occur in vitro.
Stages of Antigen-Antibody Reaction
There are three stages to the interactions between Ag and Ab.
- The first stage of the reaction entails the formation of the Ag-Ab complex.
- The second stage results in visible phenomena like agglutination, precipitation, etc.
- The third stage involves the destruction of Ag or neutralisation of Ag.
Properties of Antigen-Antibody Reaction
- Significantly specific reaction
- Occurs in a noticeable manner
- Non-covalent reactions (Ionic bonds, Van der Waals forces, Hydrophobic interactions, Hydrogen bonds)
- Antibodies and antigens are not denatured
- Affinity: This refers to how strongly an antigen binds to a certain antigen-binding site on an antibody.
- Avidity: It is a more general concept than affinity. It represents the Ag-Ab complex’s total strength. It depends on:
- The antibody’s affinity
- Antibody and antigen valencies (the number of binding sites)
- How epitopes and paratopes are structurally arranged.
- Cross-Reactivity: This term describes an antibody’s capacity to bind to similar epitopes on other antigens.
Types of Antigen-Antibody Reaction
The types of antigen-antibody reactions are as follows:
- Precipitation Reaction
- Agglutination Reaction
- Complement Fixation
- ELISA – Enzyme-Linked ImmunoSorbent Assay
An insoluble precipitate of Ag-Ab complex is produced when a soluble Ag and its Ab combine in the presence of an electrolyte (NaCl) at a specific pH and temperature. Precipitin is the Ab that causes precipitation, and the reaction is termed a precipitation reaction.
The precipitation reaction occurs in both liquid and gel media.
- Liquid Precipitation: An antigen-antibody reaction is carried out by adding increasing amounts of antigen to tubes containing a constant amount of antibody. Precipitation results from the combined reaction of the antigen and antibody.
- Gel Precipitation: Petri plates or plates with agar gel or a similar gel are used in these methods. In the gel system, both Ag and Ab rapidly diffuse in all directions. A zone of equivalency, observed as visible precipitation, will form at a specific point depending on the diffusion rate and concentration of reactants.
Multiple bands develop in complex Ag or Ab preparations. They fall into two methods: single diffusion and double diffusion.
The particles are clustered or agglutinated when a certain Ag is combined with its Ab in the presence of electrolytes at an appropriate temperature and pH. The clumps of cellular Ag formed by the serum’s Ab are known as agglutinins.
Agglutinogens are the name for the aggregated particulate antigens.
- Slide Agglutination: This is a fast and convenient way to identify the presence of agglutinating antibodies.
- Tube Agglutination: This is a common technique for estimating the quantity of Ab. A constant volume of the Ag suspension is introduced after serially diluting the Ab-containing serum with saline in multiple small test tubes.
A control tube is retained that contains no antiserum. The tubes are incubated up until observable agglutination. The tube demonstrating the highest agglutination is known as the titre.
- Passive Agglutination Test: This test is equivalent to the haemagglutination test, but the physical characteristics of the reaction are different.
A carrier particle has Ag coated on its surface, making the reaction more sensitive by assisting in the transformation of a precipitation process into an agglutination reaction. RBC, latex particles, or bentonite can be used as carrier particles. Sometimes, tanned RBC (polystyrene coated RBC) might be used.
Some non-specific, unstable fresh serum components known as complements are required for the lysis of RBC or microorganisms.
Every person has the 11 proteins that comprise the complement system. They attach to the Fc subunit of Ab in the Ag-Ab complex. Complement fixation tests make use of the Ag-Ab complex’s capacity to fix complement.
In the first step, the test Ag and the antiserum, which have been heated to 56°C to inactivate complement, are combined with a known quantity of complement. This is incubated for 18 hours at 4°C.
If the serum contains Ab that is specific for the Ag, an Ag-Ab complex will develop and fix the complement.
Fluorescence is the ability to absorb light rays of a certain wavelength and emit light rays of a different wavelength.
Fluorescent dyes emit intense visible light when exposed to UV radiation.
In 1942, Albert Coons and coworkers demonstrated how labelled dyes could be coupled to antibodies, allowing for the detection of antigens using these labelled dyes.
Commonly used dyes include:
The most used label for immunofluorescence operations is fluorescein, an organic dye that absorbs blue light (490 nm) and emits a strong yellow-green fluorescence (517 nm).
As a strong emitter of red fluorescence and an effective light absorber (30 times more effective than fluorescein), phycoerythrin is often used as a label for immunofluorescence.
ELISA – Enzyme-Linked Immunosorbent Assay
In 1971, enzyme-labelled Ag’s and Ab’s were created as serological reagents for the testing of antibodies and antigens.
When compared to radioimmunoassay (RIA), they are more simple, sensitive, affordable, and risk-free.
The ligand used is a molecule that is covalently attached to an enzyme like peroxidase, beta-galactosidase, alkaline phosphatase, etc. and is capable of detecting the Ab.
There are three types of ELISA:
- Indirect ELISA: HIV can be detected using the indirect ELISA method. The surface of the microtiter plates is coated with envelope proteins developed using recombinant technology. Unbound proteins are washed out when suspect serum is added.
- Sandwich ELISA: This method is used for determining whether there is Ag in a sample. The suspect serum is added and given time to react after the well has been coated with Ag-specific Ab. Unbound Ag is removed from the wells through washing.
The next step is to add a labelled Ab against a different Ag epitope. Washing is used to remove unbound Ab’s, followed by the addition of coloured substrate and colour development. The colour intensity is directly proportional to the Ag concentration in the serum.
- Competitive ELISA: Competitive ELISA is another variant for estimating antigen concentrations. In this method, the antibody is initially incubated in solution with the sample containing the antigen.
An antigen-coated microtiter well is then filled with the antigen-antibody mixture.
The free antibody available to attach to the antigen-coated well will reduce when more antigen is present in the sample. In an indirect ELISA, the addition of a secondary antibody (Ab2), coated with an enzyme, specific for the primary antibody’s isotype can be utilised to estimate how much primary antibody is attached to each well.
Thus, we conclude this discussion with a brief overview of the antigen-antibody reaction. Specific interactions between the antibodies and antigens result in combination, known as Antigen-Antibody reaction or Antigen-Antibody interaction.
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