Antigens are big protein molecules that are found on the surface of pathogens like bacteria, fungi, viruses, and other foreign particles. When dangerous chemicals enter the body, the immune system responds by producing antibodies. Common types of antigens are Endogenous antigens, exogenous antigens, autoantigens, tumor antigens & native antigens.
Characteristics of Antigens
Antigens have the following Properties:
- To elicit an immunological response, the antigen should be a foreign material.
- The antigens’ molecular weight ranges from 14,000 to 6,00,000 Da.
- Proteins and carbohydrates make up the majority of them.
- They’ll be more immunogenic if they’re more chemically complicated.
- Antigens are unique to each species.
- Immunogenicity is influenced by age. Immunogenicity is quite low in very young and very old humans.
Structure of Antigens
- The ability of an antigen to bind to the antigen-binding site of an antibody defines its molecular structure.
- Antibodies distinguish between distinct antigens based on the unique molecular structures found on the antigen’s surface.
- Proteins and polysaccharides make up the majority of antigens. Coats, capsules, flagella, poisons, and fimbriae of bacteria, viruses, and other microbes are examples. In addition, secretions and other similar substances can behave as antigens.
- These microbes’ lipids and nucleic acids are only antigenic when coupled with proteins or polysaccharides.
Antigens can have a variety of structures based on the antigen’s composition, size, and immunogenicity.
- Various antigens have different numbers of epitopes, which dictates how many antibodies a particular antigen can bind to.
- Antigens have diverse structural components of interaction, which determines which antibody classes they bind to.
- A paratope is the part of an antibody that interacts with antigens. The structure of an epitope and a paratope can be described using a lock and key metaphor because the structures are distinct and fit together.
Types of Antigens
Antigens can be classified into many categories based on a variety of variables. The origin of the antigen and its immunogenicity are two of the most popular classifications.
1. Antigen types according to their source
On the basis of their origin, antigens can be divided into two categories:
a: Exogenous Antigens
Exogenous antigens are antigens that come from somewhere other than the host’s body and are thus foreign to the host.
These antigens can enter the body through inhalation, ingestion, or injection, and then circulate through bodily fluids.
Antigen Processing Cells (APCs) such as macrophages, dendritic cells, and others are principally responsible for the acquisition of foreign antigens through phagocytosis.
Many antigens, such as intracellular viruses, may start out as foreign and then transform into endogenous antigens.
Examples of exogenous antigens
- rat feces
b: Endogenous Antigens
Antigens that arise within the body of the host during metabolism or as a result of intracellular viral or bacterial infection are known as endogenous antigens.
The cells of the body, as well as fragments, chemicals, and antigenic products of metabolism, are examples of endogenous antigens.
These are normally handled by macrophages before being identified by immune system cytotoxic T-cells.
Antigens that are xenogenic or heterologous, autologous, and idiotype or allogenic are all examples of endogenous antigens.
Because the host immune system recognizes its own cells and particles as immunogenic, endogenous antigens may cause autoimmune disorders.
Examples of endogenous antigens
- pathogenic fungi
- unicellular parasites
- multicellular parasites
Autoantigens are proteins or protein complexes that are attacked by the immune system of the host, causing autoimmune illness.
Autoantigens can be fatal to the host because the immune system should not attack the body’s own cells.
Due to hereditary and environmental factors, immune tolerance to such antigens is lost.
Antigens of Tumors
Major Histocompatibility Complex (MHC) I and II present tumor antigens or neoantigens on the surface of tumor cells.
During the malignant transformation of normal cells, antigens are created as a result of a tumor-specific mutation.
Because tumor cells evolve mechanisms to circumvent antigen presentation and immune defense, these antigens seldom elicit an immunological response.
Native antigens are antigens that have not been processed by any antigen-presenting cells (APC), hence immune cells such as T-cells are unable to bind to them. B-cells, on the other hand, can be triggered by such antigens without any processing.
2. Antigen types according to immunological response
Antigens are divided into two categories based on their immune response:
a. complete antigens/immunogens
Antigens that trigger a specific immune response are known as complete antigens or immunogens.
Without any carrier particles, these antigens can elicit an immunological response.
These are usually high-molecular-weight proteins, peptides, or polysaccharides (greater than 10,000 Da).
b. Haptens/incomplete antigens
Antigens that are incomplete or haptens are antigens that cannot elicit an immune response on their own.
These are usually non-protein compounds that require the presence of a carrier molecule in order to function as a full antigen.
Haptens have low molecular weight (often less than 10,000 Da) and fewer antigenic determinant sites than other proteins.
The non-antigenic component of the hapten is the carrier molecule, which is either a protein or a polysaccharide molecule.
Antigens and their Applications
- Because antigens are usually quite specific, the detection of different antigens in distinct species can be used to distinguish between bacterium species.
- Antigens can also be used to detect the presence of antibodies in a sample for diagnostic purposes.
- Antigens are necessary components of antigen-antibody complexes, which are used in forensics to identify human blood and other samples.
- Immunoassays for the quantification of various chemical and biological substances also utilize these.
- Autoantigens are the cause of autoimmune disorders, which can be fatal in some situations.
- Inactivated enzymes are employed in vaccines to provide passive immunity against a variety of diseases.
Pathogens Vs Antigens