ELISA is Enzyme-linked immunosorbent assay. It is a bioassay used to detect the level of antigens, antibodies, etc. in a sample for diagnostic purposes. The four main types of ELISA are:

Types of ELISA:

1. Direct ELISA

1. The antigen is immobilized onto the surface of the plate and, after that, washed to remove unbound antigens.
2. The enzyme-bound primary antibodies are added to the plate, which conjugates with the antigens.
3. The color change is observed upon adding the substrate, and the spectrophotometer notes the colorimetric output.
4. Direct ELISA is used for antibody screening and antigen detection.

2. Indirect ELISA

1. The immobilized antigen initially reacts with the primary antibody.
2. After that, the plate is incubated with an enzyme-conjugated secondary antibody complementary to the primary antibody.
3. The indirect ELISA may have a greater sensitivity, but there is a higher chance of cross-reactivity.

3. Competitive ELISA

1. This technique is used to estimate the presence of antibodies in a sample.
2. The two types of antibodies that compete for the antigen are the antibodies present in the test serum and the enzyme-bound antibody.
3. A negative result occurs if there is a color change due to the binding of the enzyme-bound antibodies with the antigen.
4. No change in color implies a positive result as the test serum antibodies bind with the antigen

4. Sandwich ELISA

1. The antigen layer is sandwiched between capture and primary antibodies layers.
2. After that, the enzyme-bound secondary antibodies are added, followed by the substrate to detect any color change.
3. The sandwich ELISA is the most sensitive among the four ELISA tests.