Enzyme Immunoassays (EIA) exploit the specificity of antibodies to detect and quantify antigens or antibodies in a sample by utilizing an enzymatic reaction. The most commonly used form of EIA is the Enzyme-Linked Immunosorbent Assay (ELISA). In an ELISA, an enzyme is conjugated to an antibody or antigen, which then interacts with a colorless substrate to produce a colored product. This substrate, known as a chromogenic substrate, allows for the visual or spectrophotometric detection of the immune reaction.

Various enzymes are employed in ELISA due to their ability to catalyze reactions efficiently and produce detectable signals. Commonly used enzymes include:

• Alkaline Phosphatase (ALP)
• Horseradish Peroxidase (HRP)
• β-Galactosidase
• Urease

ELISA is mainly grouped into four categories:

1. Direct ELISA: Detects an antigen directly with an enzyme-labeled antibody.
2. Indirect ELISA: Detects an antigen using a primary antibody followed by an enzyme-labeled secondary antibody.
3. Sandwich ELISA: Captures the antigen between two antibodies, one of which is enzyme-labeled.
4. Competitive ELISA: Measures the amount of antigen by its ability to compete with a labeled antigen for binding to an antibody.

Principle:

The basic principle of ELISA involves interaction between a specific antigen and a corresponding immobilized antibody. Once the antigen is bound to the immobilized antibody, the antigen-antibody complex is detected using an enzyme-conjugated secondary antibody.