Antigen
Definition
A molecule recognised by the adaptive immune system through specific binding to an antibody or T-cell receptor. Defined by immunogenicity (capacity to drive antibody production) and antigenicity (capacity to bind to antibodies). Proteins, polysaccharides, lipopolysaccharides, and nucleic acids can all act as antigens.
- Type
- Molecule recognized by antibodies
- In blood
- Usually carbohydrate or protein structures attached to red cell membranes
- Common forensic examples
- ABO, Rh, MNS, Kell, Duffy, Kidd systems
- In immunoassays
- The analyte (substance) the lab searches for in evidence
Common questions
What's an antigen in forensic testing?+
An antigen is a molecule the immune system recognizes. In forensic labs, antigens are usually the specific substance you're testing for, like a drug residue or a protein marker. Antibodies are designed to bind to antigens, which is how forensic immunoassays work.
How are antigens used in blood group testing?+
Blood group antigens sit on the surface of red blood cells. These molecules are either carbohydrates (like in the ABO and Lewis groups) or proteins (like Rh, MNS, Kell, Duffy, and Kidd). They're what antibodies target when labs test to determine a person's blood type in forensic investigations.
Why does an antigen matter for forensic identification?+
Antigens give forensic evidence a signature. Because different people have different antigens on their cells, labs can use them to link or exclude suspects.
Related terms
- Secretor Status
- A genetic trait that determines whether a person expresses their ABO blood group antigens in secreted body fluids such as saliva, semen,...
- Agglutination
- Clumping of red cells that occurs when antibodies in serum bind to incompatible antigens on the cell surface, cross-linking cells into visible...
- Lateral-Flow Immunoassay
- A point-of-care competitive immunochromatographic format on a nitrocellulose strip. In the absence of drug, colloidal gold-labelled antibody is captured at a test...
- ABO System
- The blood-group classification described by Karl Landsteiner in 1901: groups A, B, AB, and O, defined by antigens on red-cell surfaces and...
- Antibody
- An immunoglobulin (typically IgG) raised to bind a specific antigen. Source decides specificity: polyclonal antibodies recognise many epitopes, monoclonal antibodies recognise one.
- Antibody (Immunoglobulin)
- A Y-shaped glycoprotein produced by B lymphocytes that binds a specific antigen at its variable regions. Antibodies used as forensic reagents are...
- Antibody (Isohaemagglutinin)
- An immunoglobulin in plasma directed against red-cell antigens the host does not carry. Group O serum carries anti-A and anti-B; group AB...
- Bombay Phenotype (Oh, Hh)
- Y. M. Bhende, 1952, K.E.M. Hospital Mumbai. Homozygous absence of the H antigen, so neither A nor B can be expressed; types...
- Competitive vs Non-Competitive Format
- Competitive: labelled antigen and sample antigen compete for a limited number of antibody sites; signal is inversely proportional to analyte. Non-competitive (sandwich):...
- Cross-Reactivity
- The capacity of an antibody raised against one analyte to bind structurally related compounds. In RIA, cross-reactivity is the main driver of...
- ELISA
- Enzyme-Linked Immunosorbent Assay. Heterogeneous, plate-based, enzyme label (HRP, alkaline phosphatase), colour or chemiluminescent readout. Four standard formats: direct, indirect, sandwich, competitive.
- ELISA (Enzyme-Linked Immunosorbent Assay)
- A plate-based immunoassay format in which a target antigen is captured by an antibody bound to a solid surface, detected by a...
Explained in these topics
- ABO System: Biology and Forensic HistoryA surface molecule that the immune system can recognise and respond to. On red blood cells, ABO antigens are oligosaccharide chains attached to glycoproteins a...
- Antigens: Structure, Types, and Forensic RelevanceA molecule recognised by the adaptive immune system through specific binding to an antibody or T-cell receptor. Defined by immunogenicity (capacity to drive an...
- Blood Group Systems in Forensic Science: ABO, Rh, MNS and BeyondA molecule on the red-cell surface that an antibody can bind. Blood group antigens are usually carbohydrate (ABO, Lewis) or protein (Rh, MNS, Kell, Duffy, Kidd...
- Immunoassays in Forensic Science: Principle, Types, Techniques and ApplicationsA molecule (drug, protein, body-fluid marker) recognised by an antibody. In forensic immunoassays it is usually the analyte the lab is looking for.
- The Role of Immunology in Forensic ScienceAny molecule that is recognised and bound by an antibody. In forensic serology, antigens of interest include the A and B oligosaccharide antigens on red blood...