Antigen
An antigen is any molecule — typically a protein, polysaccharide, or glycolipid — that is recognised by the adaptive immune system and capable of inducing a specific immune response. The term encompasses both immunogens (which trigger antibody or T-cell responses) and haptens (small molecules that are antigenic only when conjugated to a carrier protein). Antigens can be exogenous (from pathogens, toxins, or transplanted tissue) or endogenous (e.g., tumour-associated antigens or self-antigens involved in autoimmunity), and they are the molecular basis of diagnostic tests, vaccine design, and blood-group typing.
Types of Antigens and Examples
| Type | Origin | Example | Immune Response Triggered |
|---|---|---|---|
| Exogenous antigen | Bacteria, virus, fungi | Influenza haemagglutinin | Antibody + CD4+ T cells |
| Endogenous antigen | Infected/tumour cells | MHC-I peptide complex | CD8+ cytotoxic T cells |
| Autoantigen | Self tissue | Thyroglobulin | Autoimmune response |
| Hapten | Small chemical (conjugated) | Penicillin + carrier | Allergic antibody response |
| Superantigen | Bacterial toxin | Staphylococcal enterotoxin B | Massive non-specific T-cell activation |
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Related Terms
Antibody
An antibody (immunoglobulin) is a Y-shaped glycoprotein produced by plasma cells (differentiated B lymphocytes) in response to a specific antigen. Each antibody molecule consists of two identical heavy chains and two identical light chains linked by disulfide bonds, with highly variable antigen-binding regions at the tips of the Y that confer exquisite specificity. Antibodies neutralise pathogens, opsonise targets for phagocytosis, activate the complement cascade, and mediate antibody-dependent cell cytotoxicity — making them central to both natural immunity and therapeutic interventions such as monoclonal antibody drugs.
Immune System
The immune system is a complex network of cells, tissues, organs, and soluble molecules that defends the body against pathogens, abnormal cells, and foreign substances. It comprises two principal arms: the innate immune system, which provides rapid, non-specific responses such as inflammation and phagocytosis, and the adaptive immune system, which mounts pathogen-specific responses mediated by B lymphocytes (antibody production) and T lymphocytes (cell-mediated immunity). Immunological memory generated by the adaptive arm enables faster and stronger responses upon re-exposure to the same pathogen, forming the basis of vaccination.
Hormone
A hormone is a chemical signalling molecule produced by endocrine glands or specialised cells and secreted directly into the bloodstream to regulate the physiology and behaviour of distant target organs. Hormones act at very low concentrations by binding to specific receptors on or inside target cells, triggering a cascade of biochemical responses. They coordinate processes such as growth, metabolism, reproduction, mood, and homeostasis across the entire organism.
Contracted from "antibody generator" — a term formalised in the early 20th century. The prefix "anti-" is from Greek "anti" (against), and "gen" from Greek "genos" (origin, producing). The concept of substances that generate antibodies was developed by Paul Ehrlich and colleagues around 1900.