Immunology and Serology Topic: Antigen - labsstudies

Immunology and Serology Topic: Antigen

Immunology and Serology Topic: Antigen


Antigen is refers to any substance capable of eliciting an immune response (e.g., the production of specific antibody molecules).

An antigen (Ag) by definition is capable of combining with the specific antibodies formed by its presence.

Antigens are foreign proteins or fragments of proteins that enter the host body through infections. In some cases, however, the body’s own proteins can act as antigens and cause an autoimmune response. Antigens are found on the surface or inside bacteria and viruses. These antigens are isolated and can be used to create vaccines.


are defined as any substance capable of eliciting an immune response.

An antigenic determinant or epitope is a specific site to which an immunoglobulin or T-cell receptor binds. Many immunogens, including all microbial pathogens, are complex assemblages of several different types of molecules, not all of which are antigenic. Not all antigens are immunogenic, meaning that not every chemical substance that can be bound by an immunoglobulin can elicit an immune response.

Antigenic molecules can be multivalent, with multiple epitopes, or monovalent, with only one epitope. In general, multivalent antigens elicit a stronger immune response than monovalent antigens because large arrays of antibody molecules are produced against multiple antigens.


The response to an immunogen is often enhanced if it is administered as a mixture with substances called adjuvants. Adjuvants work in one or more of the following ways: – by increasing the effective size of the immunogen, – by stimulating the local influx of macrophages and/or other immune cell types to the injection site and promoting their subsequent activities.

Alum precipitate, a suspension of aluminum hydroxide on which the immunogen is adsorbed, is the most commonly used adjuvant in humans. This adjuvant increases the immunogen’s effective size, promoting its ingestion and presentation by macrophages.

 Conditions must be met for a substance or molecule to be immunogenic.

These conditions include the following:

Chemical composition

Large, macromolecular proteins are the most potent immunogens. Polysaccharides and short polypeptides can also be immunogenic under certain conditions. Because pure lipids and nucleic acids have not been shown to be immunogenic, they can be considered antigenic but not immunogenic molecules.

Molecular Dimension

Extremely small molecules, such as amino acids or monosaccharides, are typically not immunogenic, implying that immunogenicity requires a certain minimum size. A few substances with molecular weights less than 1000 have been shown to be immunogenic, but molecules with molecular weights less than 10,000 are either weakly or completely immunogenic. Proteins with a molecular weight of more than 100,000 are the most potent immunogens.

Chemical Complexity

To be immunogenic, a molecule must possess a certain level of chemical complexity. Simple polypeptides containing tyrosine, for example, are better immunogens than comparable polypeptides lacking tyrosine, and the immunogenicity of such polymers is proportional to their tyrosine content.


Normally, the immune system distinguishes between self and nonself, so only molecules that are foreign to the host are immunogenic. As a result, albumin isolated from rabbit serum and injected back into the same or another rabbit will not elicit an immune response. This endogenous protein is tolerated by all rabbits. However, if the same protein is injected into other vertebrate species, it is likely to elicit significant antibody responses.

Administration method

The elicitation of an immune response by a substance is also affected by the dose and mode of administration. A substance that has no effect when injected intravenously may elicit an antibody response when injected subcutaneously, especially if an adjuvant is present.

Factors influencing Antigen and  Antibody Reaction


The optimal temperature required to achieve equilibrium in an antibody-antigen reaction varies depending on the antibody. IgM antibodies have a thermal range of 4-22 degrees Celsius and IgG antibodies have a thermal range of 37 degrees Celsius.


Specificity refers to an antibody’s ability to combine with one antigen rather than another.

This property is determined by an immunoglobulin molecule’s antigen-binding fragment. Antigen-antibody reactions can be extremely specific.


Although the optimum pH for all reactions has not been determined, a pH of 7.0 is commonly used in laboratory testing.

The ionic strength

The amount of salt present in the reaction medium influences antibody uptake by membrane-bound erythrocyte antigens. In solution, sodium and chloride ions have an inhibiting effect.

These ions congregate around antigen and antibody molecules with opposite charges, partially neutralizing them. This prevents antibodies from binding to antigen. Increasing the ionic strength of a reaction medium, such as low-ionic strength salt, can boost antibody uptake.


The concentration of antigen and antibody should be optimal under normal conditions, but this is not always the case. Excess antibody or antigen concentration will result in a false reaction, also known as a zonal reaction. Excess antigen concentration will result in a post zone reaction.

A prozone reaction occurs when there is an excess of antibody. This phenomenon can be avoided by performing serial dilutions until the optimal amount of antigen and antibody is present.

Cross reactivity

Antigens with similar antigenic determinants can be found in unrelated molecules. This means that a portion of the antibodies directed against one antigen will also react with the other. This is known as cross reactivity.

Cross reactivity occurs when antibodies directed against a protein in one species react in a detectable manner with the protein’s homologue in another species.

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