*4.2.5. Applications of ELISA*

**9.** The absorbance or fluorescence or electrochemical signal (e.g., current) of samples is meas-

Indirect ELISA is the same as that of the direct ELISA, only primary antibody has been unlabeled, which is very specific to the antigen and the labeled secondary antibody is added,

**Figure 2.** Sandwich ELISA. This figure shows the antigen is the analyte which is sandwiched between two antibodies,

**Figure 3.** Indirect ELISA. This figure shows the antigen is the analyte to be detected, the primary antibody specific to the antigen is added, secondary antibody is added then, which is labelled with the enzyme and after that chromogenic

ured to determine the presence of antigen and to quantify it (**Figure 2**).

There is a competition of analyte in this ELISA. The procedure is as follows:

**2.** This antigen–antibody complex is then added into the antigen-coated well.

**1.** First sample having antigen is incubated in the presence of antibody.

which is labeled withe enzyme or any other label (**Figure 3**).

the antibody can be sandwiched between two antigens in the same way.

substrate is added, which reacts with the enzyme and give fluorescence.

*4.2.3. Indirect ELISA*

56 Protein-Protein Interaction Assays

*4.2.4. Competitive ELISA*

To determine the immune response in the body enzyme-linked immunosorbent assay (ELISA) is being used, which have different methods. These methods are being extensively used to determine analyte in the biological samples of whole blood, serum, urine and other biological fluids. These assays have wide applications in agriculture, industrial, environmental, athletic and legal/forensic fields [34–38].

This assay can be used to determine:

