Learning Objective: At the end of this lesson, the learner will be able to explain the principle, types, and clinical applications of ELISA and interpret results for antigen or antibody detection in patient samples.
ELISA (Enzyme-Linked Immunosorbent Assay) is an immunologic test used to detect:
- Specific antigens in a patient’s sample (direct ELISA)
- Specific antibodies in a patient’s sample (indirect ELISA)
The method is widely used for screening infectious diseases, monitoring immune responses, and in research applications.
Principle
- The target antigen or antibody is immobilized on a solid surface (usually a microtiter plate).
- A detection antibody linked to an enzyme is added, which binds to the target.
- A substrate is added, which reacts with the enzyme to produce a detectable signal (color change, fluorescence, or chemiluminescence).
- The signal intensity is proportional to the amount of antigen or antibody in the sample.
Types of ELISA
| Type | Target Detected | Description / Use |
|---|---|---|
| Direct ELISA | Antigen | Enzyme-linked antibody binds directly to antigen; simpler but less flexible |
| Indirect ELISA | Antibody | Unlabeled primary antibody binds to antigen; enzyme-linked secondary antibody detects primary antibody; higher sensitivity |
Clinical Applications
- HIV screening: initial detection of anti-HIV antibodies (confirmatory testing done by Western blot)
- Hepatitis B virus (HBV) infection: detection of HBsAg or anti-HBs antibodies
- Autoimmune disorders: detection of autoantibodies
- Allergy testing: detection of allergen-specific IgE
Key Point: ELISA has high sensitivity but is less specific than Western blot, so positive results are often confirmed by more specific assays.
