Hapten
Definition
A small molecule with antigenicity but no immunogenicity on its own. Haptens must be conjugated to a carrier protein to raise antibodies. Common forensic examples include drug metabolites (morphine, benzoylecgonine), some pesticide residues, and certain explosives residues detected by immunoassay.
- Property
- Antigenic but not immunogenic alone
- Requirement
- Must conjugate to a carrier protein to raise antibodies
- Forensic examples
- Morphine, benzoylecgonine
- Also includes
- Some pesticide and explosives residues
- Detection
- Immunoassay
Common questions
Why can't a hapten trigger an immune response on its own?+
It is too small to be recognised effectively by the immune system's antigen-presenting machinery, so it needs to be chemically linked to a larger carrier protein before an antibody response is raised against it.
How does this property translate into a forensic immunoassay test?+
Antibodies are raised in the lab against the hapten-carrier conjugate, then used in test kits such as ELISA or lateral-flow devices to detect the free hapten, like a drug metabolite, in a sample without needing to conjugate the actual evidence.
Why do drug screening immunoassays sometimes cross-react with unrelated compounds?+
The antibody recognises a structural shape rather than the exact molecule, so compounds with a similar hapten structure to the target can bind the same antibody, producing a presumptive positive that a confirmatory method like mass spectrometry must resolve.
Related terms
- Antigen
- A molecule recognised by the adaptive immune system through specific binding to an antibody or T-cell receptor. Defined by immunogenicity (capacity to...
- Cross-Reactivity
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- Epitope
- The specific region of an antigen that makes physical contact with an antibody's binding site (paratope). Also called the antigenic determinant. A...
- Immunogen
- A molecule that is both immunogenic (triggers antibody production) and antigenic (binds those antibodies). Used interchangeably with 'complete antigen'. Most protein antigens...
- Secretor Status
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