Antigen- Properties, Types and Determinants of Antigenicity

Antigen is a substances usually protein in nature and sometimes polysaccharide, that generates a specific immune response and induces the formation of a specific antibody or specially sensitized T cells or both.

Although all antigens are recognized by specific lymphocytes or by antibodies, only some antigens are capable of activating lymphocytes. Molecules that stimulate immune responses are called Immunogens.

Epitope is immunologically active regions of an immunogen (or antigen) that binds to antigen-specific membrane receptors on lymphocytes or to secreted antibodies. It is also called antigenic determinants.

Auto-antigens, for example, are a person’s own self antigens. Examples: Thymoglobulin, DNA, Corneal tissue, etc.

Alloantigens are antigens found in different members of the same species (the red blood cell antigens A and B are examples).

Heterophile antigens are identical antigens found in the cells of different species. Examples: Forssmann antigen, Cross-reacting microbial antigens, etc.

Adjuvants are substances that are non-immunogenic alone but enhance the immunogenicity of any added immunogen.

Chemical nature of Antigens (Immunogens) –

A. Proteins –

The vast majority of immunogens are proteins. These may be pure proteins or they may be glycoprotein or lipoproteins. In general, proteins are usually very good immunogens.

B. Polysaccharides –

Pure polysaccharides and lipo-polysaccharides are good immunogens.

C. Nucleic Acids –

Nucleic acids are usually poorly immunogenic. However, they may become immunogenic when single stranded or when complexed with proteins.

D. Lipids –

In general lipids are non-immunogenic, although they may be haptens.

Types of Antigen on the basis of order of their class (Origin) –

1. Exogenous antigens –

These antigens enter the body or system and start circulating in the body fluids and trapped by the APCs (Antigen processing cells such as macrophages, dendritic cells, etc.)

The uptakes of these exogenous antigens by APCs are mainly mediated by the phagocytosis. Examples: bacteria, viruses, fungi etc. Some antigens start out as exogenous antigens, and later become endogenous (for examples, intracellular viruses).

2. Endogenous antigens –

These are body’s own cells or sub fragments or compounds or the antigenic products that are produced. The endogenous antigens are processed by the macrophages which are later accepted by the cytotoxic T-cells. Endogenous antigens include xenogenic (heterologous), autologous and idiotypic or allogenic (homologous) antigens.

Examples: Blood group antigens, HLA (Histocompatibility Leukocyte Antigens), etc.

3. Autoantigens –

An autoantigen is usually a normal protein or complex of proteins and sometimes DNA or RNA that is recognized by the immune system of patients suffering from a specific autoimmune disease. These antigens should not be, under normal conditions, the target of the immune system, but due mainly to genetic and environmental factors, the normal immunological tolerance for such an antigen has been lost in these patients.

Examples: Nucleoproteins, Nucleic acids, etc.

On the basis of immune response –

1. Complete antigen or immunogen –

Posses antigenic properties denovo, i.e. they are able to generate an immune response by themselves. It has high molecular weight (more than 10,000). It may be proteins or polysaccharides.

2. Incomplete antigen or Hapten –

These are the foreign substance, usually non-protein substances. Unable to induce an immune response by itself, they require carrier molecule to act as a complete antigen. The carrier molecule is a non-antigenic component and helps in provoking the immune response. Example: Serum Protein such as Albumin or Globulin. It has low molecular weight (less than 10,000). Haptens can react specifically with its corresponding antibody. Examples: capsular polysaccharide of pneumococcus, polysaccharide ‘C’ of beta hemolytic streptococci, cardiolipin antigens, etc.

Determinants of Antigenicity –

The whole antigen does not evoke immune response and only a small part of it induces B and T cell response. The small area of chemical grouping on the antigen molecule that determines specific immune response and reacts specifically with antibody is called an antigenic determinant.