Competitive ELISA

The fourth type of ELISA is competitive ELISA. As the name suggest there is going to be a competition between someone. Now let’s see between whom is this competition exactly going to be.

Competitive ELISA
Let’s take an example wherein we have to detect HIV antibodies from a serum sample.

For detection of HIV antibodies, the wells of micro titre plate are coated with HIV antigen followed by incubation of antibodies from the sample. After incubation detection antibodies specific to HIV antigens are added.

Now, if the sample contains HIV antibodies then antigen-antibody complexes are formed. No antigen  is left for detection antibody to bind.

Hence, these detection antibodies remain free and are washed off.  Addition of substrate to such a well gives no colour change indicating a positive result.

On the other hand, if the sample does not contain HIV antibodies then there is no antigen-antibody complex formed. Thus, the detection antibodies bind to the antigen which will show a colour change on addition of substrate indicating a negative result.

Thus, competitive ELISA involves a competition between detection antibody and sample antibody to bind with the sample antigen.

Also, in other types of ELISA colour change detected a positive result and no colour change detected a negative result whereas in competitive ELISA it’s exactly opposite!

Written by Komal M Kadam
Illustration by Immense Immunology Insight

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