Question
ELISA – types and applications
Answer
ELISA (Enzyme-Linked ImmunoSorbent Assay) is a serological technique that detects and quantifies a specific antigen or antibody using an enzyme-labelled reagent, with a colour change (measured spectrophotometrically as optical density) indicating and quantifying a positive reaction.
Types:
- Direct ELISA — enzyme-labelled antibody binds directly to antigen coated on the plate; simplest format but least sensitive.
- Indirect ELISA — used to detect antibody in patient serum; antigen is coated on the plate, patient serum is added (specific antibody, if present, binds), followed by an enzyme-labelled anti-human-immunoglobulin secondary antibody; commonly used for serodiagnosis (e.g., HIV screening).
- Sandwich ELISA — used to detect antigen; a capture antibody is coated on the plate, the test sample is added, followed by a second, enzyme-labelled detection antibody binding a different epitope on the same antigen — provides high specificity (e.g., detecting hepatitis B surface antigen, various microbial toxins/antigens).
- Competitive ELISA — labelled and unlabelled antigen compete for a limited amount of antibody; signal is inversely proportional to the amount of antigen in the test sample; useful for small molecules/haptens that cannot be “sandwiched.”
Applications: serodiagnosis of infections (HIV, hepatitis B/C, dengue, etc.), detection of microbial antigens/toxins, hormone assays (e.g., beta-hCG pregnancy testing), allergen-specific IgE testing, and CD4 count-related and other quantitative immunological assays — valued for high sensitivity, ability to process many samples simultaneously (96-well plate format), and suitability for automation and large-scale screening programmes.