Antigen and Its types

 

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An antigen is a substance, usually a protein in nature but sometimes a polysaccharide, that triggers a specific immune response when introduced into a living animal. This immune response can involve the production of specific antibodies, the development of specially sensitized T cells, or both.

Crucially, antigens can react specifically with the corresponding antibodies in a detectable way.

 Types of Antigens

Antigens are broadly classified into two main types :

Complete Antigens (Immunogens): These substances can stimulate an immune response on their own. They generally have a high molecular weight (more than 10,000 Daltons). While mostly proteins, some can be polysaccharides. Complete antigens possess two important properties :

·       Immunogenicity: This is the capacity to induce the formation of corresponding antibodies. When B cells encounter an antigen, they can differentiate into plasma B cells (which secrete antibodies) and memory B cells.

·    Specificity: This refers to the ability of the antigen to react specifically with the antibodies produced against it. When B cells are activated, they secrete antibodies that will bind to these specific antigens.

Incomplete Antigens (Haptens): These are substances with a low molecular weight (less than 10,000 Daltons). They are usually non-protein substances. Haptens cannot induce an immune response by themselves. However, they can become immunogenic when they are covalently linked to proteins called carrier proteins, either in a laboratory setting (in vitro) or within a living organism (in vivo). When a hapten binds to a carrier protein, it increases the overall molecular weight, making the complex large enough to trigger an immune response. This hapten-carrier protein complex is then considered an immunogen.

Importantly, even though haptens cannot initially trigger an immune response alone, they can still react specifically with their corresponding antibodies once those antibodies have been produced.

Examples of haptens include drugs like penicillin, bacterial capsular polysaccharides (like those of pneumococci), lipids (like Forssman and cardiolipin), and chemicals that can cause allergic contact dermatitis and drug hypersensitivity.

 Heterophil Antigens (Cross-reactive Antigens):

These are antigens that are found in unrelated plants or animals but are either identical or so closely related that antibodies produced against one will cross-react with the other. This means antibodies to these closely related antigens produced by one species can react with antigens of other species or even with certain antigens present in different tissues of more than one species.

Examples of heterophil antigens include

• ·     Forssman antigen: Found in most guinea pig tissues, it's a lipoprotein polysaccharide complex that, when injected into rabbits, stimulates the production of hemolysin for sheep red blood cells.
• ·       Cross-reacting microbial antigens: For instance, certain antigens present in Rickettsiae (causing typhus fever) are also shared by some strains of Proteus bacteria (OX19, OX2). This forms the basis of the Weil-Felix reaction, a diagnostic test where serum from patients with typhus fever agglutinates with these Proteus strains.
• ·       Human RBC and E. coli: Human red blood cells of blood group B share an antigen with E. coli, illustrating another instance of heterophil antigen characteristics.

 Determinants of Antigenicity

Several factors influence the ability of a substance to act as an antigen (its antigenicity):

 Foreignness: The immune system typically distinguishes between self and non-self molecules, with only foreign molecules being immunogenic. Generally, the greater the difference between the substance and the host's molecules, the stronger the immune response.. For example, injecting serum from one rabbit into another usually doesn't induce antibody production, but injecting it into a different animal species will.

 Size: Generally, larger molecules are more likely to be immunogenic. A molecule with a molecular weight of 10,000 Daltons or more is considered a macromolecule in terms of immunogenicity. Very large molecules like hemocyanin and others in the 14,000-60,000 dalton range (e.g., tetanus toxoid, egg albumin, thyroglobulin) are highly antigenic. Substances with a molecular weight less than 10,000 daltons are often not antigenic or only weakly so. Penicillin and aspirin, with molecular weights below 1000 Daltons, are poor antigens on their own.

 Chemical Composition:

More complex substances chemically tend to be more immunogenic. The antigenic determinants (the specific parts of the antigen that antibodies bind to) are determined by the primary sequence of residues in the polymer and/or the molecule's secondary, tertiary, or quaternary structure. Proteins are generally more antigenic than lipids and carbohydrates. However, not all proteins are antigenic (e.g., gelatin, histones), while bacterial exotoxins and egg albumin are strong antigens. More complex carbohydrates, especially when bound to proteins (e.g., cell wall antigens of Gram-negative bacteria), can also be immunogenic.

 Physical Form:

Particulate antigens are generally more immunogenic than soluble ones, and denatured antigens are often more immunogenic than their native forms.

 Degradability: Antigens that are easily phagocytosed (engulfed by immune cells) are generally more immunogenic. This is because for most antigens, the development of an immune response requires the antigen to be phagocytosed, processed, and presented to helper T cells by antigen-presenting cells.