Elisa Technique

All ELISA tests rely upon the final color change to detect a positive result, no matter the method or specific test.

ELISA stands for enzyme-linked immunosorbent assay and is used to test for many diseases by detecting either the antibody or the antigen. Two main types of ELISA are direct and indirect.

Antigens

An antigen is any foreign substance that the immune system recognizes and attacks. For example, the HIV virus is an antigen that attacks the immune system directly.

Antibodies

Antibodies come in many forms, many of which are manufactured in response to specific antigens to assist in the attack against the antigen. For example, antibodies to HIV are made against the HIV virus in exposed individuals.

Direct ELISA--Test for antigens

In this test, a well is coated with antibodies that will recognize the antigen in the sample (patient's serum); any antigen in the sample will bind to the antibodies that coat the well. After washing, another sample is added that contains antibodies carrying a reporter enzyme; these antibodies will bind to the antigen already bound to the first antibody so that when a substrate to the enzyme is added a color change will be noted (positive result).

Indirect ELISA - Test for Antibodies

The indirect ELISA is similar to the direct ELISA in method, the main difference being that the well is coated with antigen and the sample added is tested for antibodies. This type of ELISA is commonly used in HIV tests.

Control

In both tests, positive and negative controls are used, both of which are used to compare to the test well. The positive control has a strong color change; the negative control has little or no color change.

References

• biotest: ELISA--How does the test work?
• Sumanas Inc.: ELISA--Animations
• Biology @ Davidson: ELISA method