**4. Label-free immunoassays**

There are few immunoassays where labels are not required, for example, in one immunoassay, the antigens are measured by change in resistance in the electrode as the antigen attaches to it. In another method, the binding between unlabeled antibody and antigen is detected by resonance and the technique is called as surface plasmon resonance, and these resonance signals are produced by metal nanoparticle tags which can be measured by a microphone [23, 24].

Different techniques where the immunoassays are being used are as follows:


*3.4.1. Radioactive isotopes*

52 Protein-Protein Interaction Assays

*3.4.2. Fluorogenic reporters*

*3.4.4. DNA reporters*

labeled probes [21, 22].

*3.4.5. Enzymes*

*3.4.3. Electrochemiluminescent tags*

chemiluminescence is detected [20].

**4. Label-free immunoassays**

microphone [23, 24].

**1.** Radioimmunoassay

**2.** ELISA

which cause them to produce chemiluminescence or light.

[16, 17].

To produce a radioimmunoassay (RIA), radioactive isotopes can be added into the immunoassay reagents and the radiations emitted by bound antigen–antibody complex can be determined by the conventional methods. RIA is considered as the earliest developed immunoassay, and they are not used frequently nowadays because of the hazards of radioactivity

Many modern immunoassays are performed by the use of fluorogenic reporters, and the pro-

Electrochemical tags are the labels which emit light as a response of electric current, and the

In real-time quantitative PCR, the traditional immunoassays techniques are added and this is called as real-time immunoquantitative PCR (iqPCR). The labels used in this assay are DNA-

The most commonly used labels in immunoassays are enzymes, such immunoassays are called as enzyme-linked immunosorbent assays (ELISA) or sometimes enzyme immunoassays (EIAs). Different enzymes are used in such assays, for example, glucose oxidase, horseradish peroxidase (HRM) and alkaline phosphatase. The enzymes are exposed to the reagents

There are few immunoassays where labels are not required, for example, in one immunoassay, the antigens are measured by change in resistance in the electrode as the antigen attaches to it. In another method, the binding between unlabeled antibody and antigen is detected by resonance and the technique is called as surface plasmon resonance, and these resonance signals are produced by metal nanoparticle tags which can be measured by a

Different techniques where the immunoassays are being used are as follows:

tein microarrays are the best example where these labels are being used [18, 19].


## **4.1. Radioimmunoassay**

RIA is an extensive method in which radioactive labels are used in a stepwise manner and as it is very specific and sensitive method which require a special equipment. Another such method is called immunoradiometric assay (IRMA) in which radiolabels are used in an immediate manner rather in steps. Radioallergosorbent test (RAST) is used to determine the allergen in case of allergy. It is the cheapest method to perform immunoassay. Although it is the cheapest method to perform immunoassay, it needs special licensing and precautions as radioactive compounds are being used [25–28].
