Antigen - Definition and Types

Introduction

An antigen is a body marker that indicates the body's immune system responds against them by producing antibodies. This indicates that your immune system is attempting to combat the chemical because it does not recognize it. Antigens are present on the surfaces of viruses, bacteria, toxins, chemicals, and other normal cells of the body. It activates the lymphocytes, which are the component of white blood cells and fight against infections. Antigens can also be found on cancer cells and evoke an immune response. Antigens are used as biochemical markers for the identification of the cell and tissues which are infected, cancerous, or have tumors. Antigen testing is also conducted for the identification of viral infections and to establish that the donor is a healthy match for the recipient. An antigen can be made up of proteins, peptides, and polysaccharides.

What Are the Types of Antigens?

Antigens are classified on the basis of their origin:

• Exogenous Antigens - These are the antigens that make entry into our body through various external routes like ingestion (via mouth), inhalation (via (nose), or through cuts present in the skin. They include bacteria, viruses, chemicals, toxins, fungi, and allergens.

• Endogenous Antigens - These antigens are present in the cells of the body. These antigens convey to the immune system if they are harmful or friendly antigens. The antigens include the cells that are infected by the microbes and indexed by the immune system for destruction. Red blood cells and human leucocyte antigens are the friendly endogenous antigens of the body.

• Autoantigens - These antigens are the protein components of the normal cells that are attacked by the immune system by mistake. Autoantigens are the main reason for autoimmune disease in a person. The genetic or environmental condition of the body due to an impaired immune system causes a normal protein of the body to become an autoantigen.

• Tumor Antigens - During the neoplastic transformation of normal cells into neoplastic cells, tumor-specific mutations occur, producing tumor antigens. These antigens are present on the surface of the cancer cells and evoke an immune reaction. These antigens may sometimes be part of a normal cell that enacts abnormally in the tumor cells, or, at times, it may be due to a result of mutations in the genes or due to the presence of a virus in the tumor cells. In spite of cell surface antigens' expression on the cell surface of the cancer cells, the cells escape the elimination process by the immune system.

What Are Antigen-Presenting Cells (APCs)?

Antigen-presenting cells can boost the immune system by detecting and engulfing pathogens in response to an infection. They are also known as phagocytic cells of the immune system. The T-cells can identify these antigenic portions and initiate an immune response.

What Is the Mechanism of Action of Antigen-Presenting Cells?

The immune system of our body is driven by epitope-paratope interaction. An epitope is an antigen; the antibody recognizes this site on the antigen and binds to the antigens. The paratope site is present on the antibody that attaches to the antigens.

• Conformational Epitope - The amino acid sequence of the antigen is discontinuous.

• Linear Epitope - The amino acid sequence of the antigen is continuous.

Once the antigen enters the body, it stimulates the B and T lymphocytes, the specialized immune cells of the adaptive immune system. Adaptive immunity can be either antibody-mediated or cell-mediated immune responses:

• The Antibody-Mediated Immune Response - The response produces antibodies against the antigens in the bloodstream, not inside the cells. The antibodies present on the surface of the B-cell identify the epitope part of the antigen and binds to it and thus are identified by the helper T cells. The activated B cells divide and give rise to the plasma cells (antibody production) and memory B-cells (provide future protection).

• The Cell-Mediated Immune Response - These immune responses are directed by antigen-presenting cells like the dendritic cells, macrophages, and B-cells. These cells engulf the antigens. The fragmented antigenic portion is expressed on the cell surface of the antigen-presenting cells via the major histocompatibility complex (MHC).

There Are Two Classes of MHC Molecules:

• Class 1 Major Histocompatibility Complex (MHC) Molecules - They present the antigenic parts to the cytotoxic T cells.

• Class 2 Major Histocompatibility Complex (MHC) Molecules - They present the antigenic parts to the helper T cells.

Once the antigenic fragments bind to the MHC, the immune system recognizes the MHC-antigen complex and the complement system is activated. The cytokines are secreted, which causes T cells' maturation. T cells convert into helper T cells and secrete more cytokines that activate and attract the macrophages, lymphocytes, and neutrophils and thus causing phagocytosis. T cells that convert into cytotoxic cells act upon pathogen-infected cells, causing their destruction.

MHC molecules pass on the antigenic fragments to the T cell either by endogenous or exogenous pathways:

• Endogenous Pathway - Endogenous antigens are the antigens obtained from the proteins produced by the pathogens within the infected cells. MHC class 1 molecule presents these endogenous antigens to the cytotoxic T cells.

• Exogenous Pathway - The antigens are produced by the pathogens outside the infected cells, and the MHC class 2 molecules present these antigens to the helper T cells.

What Are Haptens?

Haptens are tiny molecules that start an immune response in an individual after combining with a carrier protein. The complex formed by the hapten-carrier complex is called an adduct. For example, Urushiol is an irritant of poison ivy, causing contact dermatitis. Quinone interacts with the skin protein and forms a hapten adduct.

What Is the Importance of Antigens in Medical Science?

• Diagnostic Markers - Antigens specific to pathogens are used as laboratory diagnostic markers to diagnose the infection present in an individual.

• Vaccine Production - Vaccines contain inactive or weak parts of the pathogenic antigen. The pathogen-specific antigens are refined to obtain the desired immune response rather than causing disease.

• Rapid Antigen Test - Rapid antigen tests are conducted to detect proteins specific to the pathogens in biological samples. These are immunoassay tests.

• Tumor Vaccines - Tumor-specific antigens trigger immune cells that can target and destroy cancer cells.

Conclusion

Antigens are usually pathogenic and activate our immune system to produce antibodies. B-lymphocytes are the first cell that responds to the antigens and produces antibodies that stimulate immune responses against these antigens. Antigens are vital in detecting various pathogens as they act as diagnostic markers.