In a nutshell, an antigen is a molecule or molecular structure that can cause an immune response in the body. Antigens are typically foreign particulate matter (like pollen grains or other particles) that bind to a specific antibody or T-cell receptor. When these proteins are found in a person’s body, they can cause an immune response.
Non-microbial non-self antigens
Antigens are substances that our immune system recognizes as non-self. These substances can come from the body or from an external environment. When our body is in a state of homeostasis, the immune system usually does not react to these substances. However, during times of distress, our immune system is supposed to respond in a specific way.
The innate immune system was designed to distinguish between self and non-self, and recognizes non-microbial antigens by using different recognition mechanisms. The recognition mechanisms of the innate immune system are based on germ-line encoded receptors, which recognize the non-self molecules in microbes. In addition, the innate immune system recognizes changes in cells due to infection. By contrast, the adaptive immune system produces a massive repertory of receptors. To eliminate a pathogen, the adaptive immune system must learn to distinguish between self and non-self antigens in order to kill the pathogen.
In the case of an autoimmune disease, an antigen is a molecule that triggers the production of antibodies. The antigen may be a protein, peptide, or nucleic acid, or can be a molecule on the surface of a virus or bacterium. The purpose of an antigen is to stimulate the immune system to create antibodies, and then combines them with other proteins to create antibodies.
Mimetic microbial antigens can cause an autoimmune disease by mimicking the host antigens. Moreover, such infectious agents may trigger cross-reactive responses of a host’s T cells against other antigens. This could tip the balance towards autoimmune disease.
Pathogen-specific antigens
Pathogen-specific antigens (PAs) are molecules that the body recognizes to kill a particular pathogen. Antibodies to HIV-1 and influenza virus are two examples of PAs. Both are highly immunogenic and target particular germ line lineages. Epitope mapping and binding affinity studies have been used to identify novel epitopes and identify their immunogenicity and mechanisms of action. However, most antigen responses in humans are directed against epitopes that have been well-characterized by researchers.
Antigens can be polypeptides, proteins, peptides, or a mixture of these. When an antigen is recognized by the T cells, it activates the cytotoxic T cells. The cytotoxic T cells are then activated to attack pathogen-infected cells. Antigens are also used in vaccine production, where they are processed to provoke a particular immune response.
Pathogen-specific antigens are acquired over a lifetime through repeated exposures to a pathogen. Exposure to pathogens triggers the development of an immune memory, which improves the body’s response in subsequent encounters. This process is called “adaptive immunity” and is an essential first line of defence for human health.
The humoral immune response, defends the body from pathogens present in the blood. These cells react to pathogens by producing antibodies against the pathogen-specific antigens. The antigens are proteins or macromolecules that trigger the immune response. B cells, called B cells, express antigen-specific receptors (BCRs) on their membranes. These antibodies can detect and coat key sites on pathogens that can cause infection. The antibodies will then trigger the complement cascade against the antigen-bound pathogen.
In contrast, phagocytes help fight off pathogens that get past the body’s outer defenses. They surround the invading pathogen and neutralize it. Healthy phagocytes are essential for the body’s immune system.
Tolerogens
Antigens are biological substances of various origins that carry markers of genetic foreignness. These substances trigger an immune response and initiate cellular immunity, resulting in specific antibodies. The properties of antigens are governed by an intricate set of features, including immunogenicity, specificity, and immunological memory.
Antigens may be free or contained in cells. A patient’s immune system can respond to certain antigens while ignoring the others. This phenomenon is known as tolerance. It differs from immune deficiency and non-specific immunosuppression, as tolerance is an active process. Tolerance occurs in both B and T cells, and at varying levels. Inducing tolerance in T cells is easier than that of B cells. The antigen must be persistent in the body for the patient to develop tolerance. Tolerance can be broken naturally or artificially. Certain drug treatments and exposure to cross-reactive antigens may break tolerance.
Infections caused by microbes are a result of antigens. Microbes share similar antigens, and this causes cross-immunological reactions. These interactions occur through a process called antigen mimicry. This mimicry makes a specific antigen more difficult to detect and inhibit.
Antigens are proteins, nucleic acids, polysaccharides, and lipids. Antigens carry signals that tell our immune system to react against them. Proteins are the most common antigens, and they are also the most immunogenic. For example, the viral neuraminidase and bacterial exotoxins are protein antigens. However, other antigens can trigger an immune response, such as viruses and bacteria.
Several strategies have been developed to induce tolerance to specific antigens. These methods include the use of soluble peptides, altered-peptide ligands, and DNA vaccines.
Exogenous antigens
Antigens that enter the body from the outside are known as exogenous antigens. These include bacteria and pathogens, as well as pollen and food particles. Exogenous antigens exist outside of cells, where they are degraded by digestive enzymes in lysosomes. The immune system recognizes these antigens and responds to them by triggering an immune response.
When exogenous antigens reach the cytoplasm, they must cross the membrane. This translocation requires a distinct mechanism for protein antigens. In vivo, peptides derived from particulate antigens are cross-presented more efficiently than soluble antigens. The mechanism of cross-presentation is not clear, but evidence suggests that it is enhanced when unconjugated particles are mixed with soluble antigen particles.
An antigen is a large biological polymer that contains surface features called epitopes. Antigens usually bind to several antibodies. Each antibody recognizes different epitopes on the antigen, creating a string-like structure. Each antibody has a different complementarity determining region.
Antigens are proteins or polysaccharides that the immune system recognizes as non-self. They trigger an immune response by binding to the receptors on lymphocytes called epitopes. These epitopes are composed of five to fifteen amino acids or three to four sugar residues. Some types of non-self antigens are pollen, egg white, or proteins from transplanted tissues or transfused blood cells.
In vivo studies, immune cells responded to exogenous antigens induced by MSCs. It is believed that MSCs cross-present exogenous antigens to CD8+ T cells and induce an effective cytotoxic immune response. The findings suggest that MSCs can be used as therapeutic biopharmaceuticals.
Non-self antigens
Antigens are proteins that bind to a specific antibody or T cell receptor, which results in an immune response. There are two main types of antigens: self-antigens and non-self antigens. The former type is found on the body’s own cells, while the latter is not. Both types of antigens can cause an autoimmune response.
Non-self antigens are those antigens that enter the body from outside. This may occur through ingestion, inhalation, or injection. They are then taken up by antigen-presenting cells and processed to release fragments, which are then presented to T helper cells with class II histocompatibility molecules. These T cells then become activated and secrete cytokines, which then trigger a response by the immune system.
The mechanism by which these cells become activated is not clear. Various hypotheses have been proposed, including loss of regulatory T cells, sequestered self-antigens, and the escape of auto-reactive clones. Another possible cause of autoimmunity is the presence of determinants of pathogens that cross-react with self antigens. This response then induces effector cells and antibodies against tissue antigens.
In addition, the mechanisms by which microbial antigens protect against autoimmune diseases are unknown. Recent research suggests that the immune response to exogenous antigens may result in tolerance, as in the case of mice protected by mycobacterial hsp60. Immunizing against non-self antigens may inhibit the development of autoimmune diseases by altering the cytokine environment and cytokine profile.
Tolerance is an important process that helps the immune system distinguish between a self and non-self antigen. The immune system has a critical role in maintaining normal physiological functions. Several viruses and bacteria have evolved clever ways to induce tolerance. For example, patients with leprosy do not mount an immune response against Mycobacterium leprae.