Types of Antigen-Antibody Reaction

The types of antigen-antibody reaction interactions are specific biochemical interactions between antibodies produced by B cells (a subset of white blood cells) and antigens when they approach a distance of several nanometers. This reaction results in agglutination, a process that binds paratopes of antibodies to particular antigens on their topes and sets off a series of immune responses to act against the respective antigens for their elimination or destruction. In this article, we will look into the types of Antigen-Antibody Reaction and the stages, and properties of antigen-antibody reactions.

Antigen (Ag) or Immunogen

(Anti= against; gen=thing that causes)

Any foreign substances that, when enter our body, occasionally trigger a series of immune responses are called immunogens. On the other hand, some of them do not directly elicit an immune response but need the help of some other molecules (carrier proteins) to do so and are called haptens. The immunogens and haptens are collectively referred to as antigens.

They could be peptides, proteins, lipids, or, polysaccharides. The region on an antigen that binds to an antibody is known as an epitope.

Antibody (Ab) or Immunoglobulin (Ig)

An antibody is a component that the immune system generates in response to the presence of antigens. Thus, it is the antigens that stimulate the production of antibodies, and together they trigger an immune response.

Abs are glycoproteins and its structure is Y-shaped. They are produced by plasma B-cells. The antigen binding site is called a paratope. Abs are five types: IgG, IgA, IgM, IgE, and IgD (Pneumonics: “GAMED”).

Antigen-Antibody Reaction

The interaction between antigen and antibody is known as antigen-antibody reactions. The reaction is quite specific, and an antigen reacts only with antibodies generated by itself or with closely related antigens. Antibodies identify epitopes on antigens. In general, the better the fit of the epitope (in terms of geometry and chemical character) to the antibody binding site, the more favorable the interactions will be formed between the antibody and antigen and the affinity of the antibody will be higher for antigen.

Stages of Antigen-Antibody Reaction

The interaction between an antigen and an antibody occurs in three stages:

1. Primary Stage: The first stage involves the formation of the Ag-Ab complex. It is rapid and and reversible without any visible effects.
2. Secondary Stage: It leads to observable phenomena such as agglutination or precipitation It is slow and irreversible with visible effects.
3. Tertiary Stage: It involves the neutralization or destruction of the antigen.

Types of Antigen-Antibody Reaction

The different types of antigen-antibody reactions include:

1. Precipitation Reaction

In the presence of an electrolyte (NaCl) at a specific pH and temperature, the reaction between soluble antigen and soluble antibody forms an insoluble precipitate of the Ag-Ab complex. For the reaction to take place, the proportions of Ag and Ab in the mixture must be equivalent. The antibody that causes precipitation is called a precipitin, and the reaction is known as a precipitation reaction. Precipitation reactions can occur in both liquid and gel media.

Radial Immunodiffusion (Mancini) Precipitation test

In this test:

• Antibody is incorporated into the agar gel during pouring.
• Various dilutions of the antigen are inserted into punched holes in the agar.
• Antigen diffuses into the gel, reacting with the antibody.
• At equivalence, a ring of precipitation forms.
• The quantity of antibody remains constant.
• The ring diameter is proportional to the log of antigen concentration.
• Different concentrations of a standard antigen generate a standard curve.
• The test is quantitative, determining antigen amounts in unknown samples.
• It’s commonly used in clinical labs to determine immunoglobulin levels in patient samples.

2. Agglutination Reaction

In the presence of electrolytes at an optimum temperature and pH, when a particulate antigen interacts with its corresponding antibody, the particles clump or agglutinate. The clumps of cellular antigen formed by the serum’s antibody are known as agglutinins. The reaction occurs more effectively with IgM antibody. In the case of sensitivity, Agglutination reactions are more sensitive than Precipitation reactions.

Types of Agglutination Test

• Slide Agglutination: This is a rapid and convenient method to identify the presence of agglutinating antibodies. Antigen is placed on slide where antibody is mixed to agglutinate. It is specific test designed to detect oxidase positive gonococcal colonies based on the non-immune reactivity between Fc portion of IgG and staphylococcal protein (SP) A. It is also done for Widal test (for typhoid fever and parathyphoid fever).
• Tube Agglutination: This is a common method for determining the quantity of antibody. A constant volume of the antigen suspension is added after serially diluting the antibody-containing serum with saline in several small test tubes. A control tube is kept that does not contain antiserum. The tubes are incubated until observable agglutination occurs. The tube showing the highest agglutination is known as the titre.

3. Complement Fixation

Tests for antigen/antibody complexes that rely on the consumption of complement are known as complement fixation tests and are used to quantify antigen/antibody reactions. This test will only work with complement fixing antibodies (IgM and IgG are best). Antigen/antibody complexes can also be measured by their ability to fix complement because an antigen/antibody complex will “consume” complement if it is present, whereas free antigens or antibodies do not. Wasserman’s Test is a most common Complement Fixation test used for detection of Syphilis.

4. Immunofluorescence

Fluorescence is the property of absorbing light rays of one wavelength and emitting light rays of a different wavelength. Fluorescent dyes emit bright visible light when subjected to ultraviolet radiation.

Immunofluorescence is a technique whereby an antibody labelled with a fluorescent molecule (fluorescein or rhodamine or one of many other fluorescent dyes) is used to detect the presence of an antigen in or on a cell or tissue by the fluorescence emitted by the bound antibody.

• Direct Immunofluorescence: Here, the antibody specific to the antigen is directly attached with the fluorochrome.
• Indirect Immunofluorescence: Here, the antibody specific for the antigen remains unlabeled and a second anti-immunoglobulin antibody directed toward the first antibody is tagged with the fluorochrome. Indirect fluorescence is more sensitive than direct immunofluorescence since there is amplification of the signal.

5. ELISA – Enzyme-Linked ImmunoSorbent Assay

ELISA is one of the sensitive techniques for the detection of the presence of antigen or antibody and quantification as well in case of clinical diagnosis of various diseases such as Ebola, AIDS, Pernicious anaemia, different parasitic infections, etc. Labelling involves the use of enzymes.

Compared to radioimmunoassay (RIA), they are simpler, cheaper, more sensitive, and safer.

The ligand used is a molecule that is covalently bound to an enzyme such as alkaline phosphatase, peroxidase, beta-galactosidase, etc., and is capable of detecting the antibody.

Types of ELISA

• Indirect ELISA: The indirect ELISA method can be used to detect HIV. Using recombinant technology, envelope proteins are coated on the surface of the microtiter plates. Unbound proteins are washed out when suspect serum is added.
• Sandwich ELISA: This technique is used to determine the presence of antigen in a sample. After the well has been coated with antigen-specific antibody, the suspect serum is added and given time to react. Unbound antigen is removed from the wells through washing. The next step is to add a labelled antibody against a different antigen epitope. Washing is used to remove unbound antibodies, which is then followed by the addition of a coloured substrate and subsequent colour development. The intensity of the colour is directly proportional to the concentration of antigen in the serum.
• Competitive ELISA: It is another method for the determination of antigen concentrations. In this method, the antibody is first incubated in solution with the sample containing the antigen. The antigen-coated microtiter well is then filled with the antigen-antibody mixture. If there is more antigen present in the sample, the amount of free antibody available to attach to the antigen-coated well will decrease. In an indirect ELISA, the addition of a secondary antibody (Ab2), coated with an enzyme, specific for the isotype of the primary antibody can be used to determine how much primary antibody is attached to each well.

Properties of Antigen-Antibody Reaction

The following are the properties of antigen-antibody reaction:

Affinity

It is the strength with which one antigen binds on a single antigen-binding site on an antibody.

• Low-affinity Ab: Bind Ag weakly and dissociates readily.
• High-affinity Ab: Bind Ag tightly and remain bound longer.

Avidity

It is a measure of the overall strength of the Ag-Ab complex. It depends on the affinity of the antibody, valency (no. of binding sites) of antibody and antigen and the structural arrangement of epitopes and paratopes.

Cross-Reactivity

It is the ability of an antibody to bind to similar epitopes of different antigens i.e., the antibodies elicited by one Ag can cross-react with unrelated Ag if they share identical epitopes or have similar chemical properties.

Conclusion: Types of Antigen-Antibody Reaction

In conclusion, the antigen-antibody reaction involves specific interactions between antibodies produced by B cells and antigens, leading to agglutination and subsequent immune responses. Understanding the types of antigen-antibody reaction is crucial in various diagnostic and research applications. The antigen-antibody reaction plays a critical role in the immune system’s ability to recognize and eliminate pathogens and foreign substances, as well as in the development of immunity against future infections.

FAQs on Types of Antigen-Antibody Reaction

What are the 4 types of Antigens?

There are four main types of antigens: Exogenous antigens, Endogenous antigens, Autoantigens and Neoantigens.

What are 5 types of Antibodies?

There are five main types of antibodies, each with specific roles in the immune response: IgG, IgA, IgM, IgE, and IgD (Pneumonics: “GAMED”).

What is the Antigen-Antibody Response?

It is a fundamental process in the body’s defence against pathogens and foreign substances. It involves the interaction between antigens and antibodies produced by the immune system.

What are the Different types of Bonds in Antigen-Antibody Interactions?

The main types of bonds involved in antigen-antibody interactions are: Non-covalent bonds ( hydrogen bonds, van der Waals forces, ionic bonds, and hydrophobic interactions) and Covalent bonds.

What is an Antigen-Antibody Reaction also known as?

An antigen-antibody reaction is also known as an immune reaction and it involves the binding of antigens to antibodies.

What does an Antigen-Antibody Reaction Involves?

An antigen-antibody reaction is a specific chemical reaction that occurs during an immune response between antigens and antibodies produced by B cells of white blood cells. Antigens and antibodies are combined during the agglutination process.

What is the Difference Between an Antigen and an Antibody?

An antigen is any substance that prompts our body to trigger an immune response against it. Antibodies are Y-shaped proteins that the body produces when it detects antigens.

What is Direct and Indirect Antigen-Antibody Reaction?

Direct antigen-antibody reaction happens when antibodies bind directly to antigens, whereas indirect reaction involves a two-step process where antibodies first attach to antigen-bound molecules.

What is the Agglutination Antigen-Antibody Reaction?

Agglutination antigen-antibody reaction occurs when antibodies bind to antigens, causing them to clump together, aiding in the detection and elimination of harmful substances in the body.

What is Antigen Made of?

Antigens are made of proteins, but they can also be composed of other molecules such as carbohydrates, lipids, or nucleic acids. These molecules are recognized by the immune system as foreign and can trigger an immune response.