Epitope and Hapten
Definition
Epitope is the small surface patch on an antigen the antibody actually binds. A hapten is a small molecule (most drugs of abuse) that is too small to be immunogenic on its own and must be conjugated to a carrier protein to raise antibodies.
- Hapten size
- Typically under about 1000 Da
- Hapten immunogenicity
- Cannot raise antibodies alone
- Carrier examples
- BSA, keyhole limpet hemocyanin (KLH)
- Forensic use
- Basis of drug-of-abuse immunoassay antibodies
Common questions
Why must a hapten be attached to a carrier protein before it can be used to raise antibodies?+
A hapten is too small to be recognised by the immune system on its own. Conjugation to a large carrier protein creates a combined molecule big enough to trigger an antibody response, with the antibodies then binding the hapten portion.
Does an antibody made against a hapten-carrier conjugate always distinguish the drug from close relatives?+
Not necessarily. Because the antibody was raised against a small conjugated fragment, it can cross-react with structurally similar compounds, which is why immunoassay positives for drugs of abuse require confirmatory testing.
Related terms
- Antibody
- An immunoglobulin (typically IgG) raised to bind a specific antigen. Source decides specificity: polyclonal antibodies recognise many epitopes, monoclonal antibodies recognise one.
- Antigen
- A molecule recognised by the adaptive immune system through specific binding to an antibody or T-cell receptor. Defined by immunogenicity (capacity to...
- 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):...
- ELISA
- Enzyme-Linked Immunosorbent Assay. Heterogeneous, plate-based, enzyme label (HRP, alkaline phosphatase), colour or chemiluminescent readout. Four standard formats: direct, indirect, sandwich, competitive.
- Homogeneous vs Heterogeneous
- Homogeneous assays (EMIT, FPIA) need no wash step, so they run fast on clinical analysers. Heterogeneous assays (ELISA, RIA) require a wash...
- Label
- The reporter coupled to either the antigen or the antibody so the binding event becomes measurable. Three families dominate: radioisotopes (RIA), enzymes...
- 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...
- RIA
- Radioimmunoassay. Berson and Yalow, 1959. Heterogeneous, competitive, radioisotope label (most often 125-I). Highest sensitivity historically, now displaced in most Indian labs by...