Antigen
Introduction
An Antigen is a molecule capable of stimulating an immune response. Each antigen has distinct surface features, or epitopes, resulting in specific responses. Antigens are usually proteins or polysaccharides. This includes parts (coats, capsules, cell walls, flagella, fimbriae, and toxins) of bacteria, viruses, and other microorganisms. Lipids and nucleic acids are antigenic only when combined with proteins and polysaccharides. Non-microbial non-self antigens can also be proteins or polysaccharides.
Antigen Recognition
Antigens are recognized by specific Antibodies or by specialized antigen-presenting cells such as B cells and dendritic cells. These cells process the antigens into short peptides that are presented on the cell surface to T cells. As a result, the immune system can mount a response against the antigen.
Types of Antigens
There are several different types of antigens, based on their origins and functions. These include exogenous antigens, endogenous antigens, and autoantigens.
Exogenous Antigens
Exogenous antigens are those that have entered the body from the outside, for example, by inhalation, ingestion, or injection. These antigens are taken up by antigen-presenting cells (APCs) and processed into smaller pieces. The APC then presents these pieces to T helper cells (CD4+ cells) by expressing them together with class II MHC molecules on its surface.
Endogenous Antigens
Endogenous antigens are generated within normal cells as a result of normal cell metabolism, or because of viral or intracellular bacterial infection. The infected cell processes the viral/bacterial proteins and presents them on its surface to T cells by the class I MHC molecules. Endogenous antigens include tumor antigens, which are present on cancer cells.
Autoantigens
Autoantigens are usually a normal part of the body, but can trigger autoimmune disease when they are recognized as foreign by the immune system. Examples of autoantigens include the proteins that make up the myelin sheath of nerves in multiple sclerosis, and the insulin-producing beta cells of the pancreas in type 1 diabetes.
Antigen Presentation
Antigen presentation is a vital immune process for T cell immune response triggering. In this process, an antigen-presenting cell (APC) ingests an antigen, then presents fragments of it on its cell surface. This is recognized by T cells, triggering an immune response.
Antigen-Antibody Interaction
The interaction between an antigen and an antibody is a key part of the body's immune response. Antibodies, produced by B cells, bind to antigens to neutralize them or mark them for destruction by other immune cells. This antigen-antibody interaction is highly specific, with each antibody binding only to a specific antigen.
Antigenic Variation
Antigenic variation refers to the mechanism that pathogens use to evade the host immune system. In antigenic variation, pathogens alter their surface proteins in order to avoid recognition by the immune system.