Macromolecules do not react like ions in a chemical reaction giving a color, or any other sign that shows this reaction, so it is hard to recognize their presence or find their concentration.
Immunoassay depends on the features of antibodies that each one is able to detect one type of molecule and binds to it.
This antibody is previously produced using the monoclonal antibody technique, and then it is conjugated to a tag, which is a material that gives color in a solution.
When having a sample that needs to be analyzed for the presence of the suspected macromolecule, the tagged antibody is added to it.
The excess antibody is washed off and the coloring material is added.
If the macromolecule we are testing is present, then the antibody will be sticking to it, and hence the coloring material will stick to the antibody and give this color.
Sometimes the tagging is done through other ways than coloring like UV fluorescence or radiation.