How To - ELISA - Methods in Biochemistry

That's a beautiful name, not the one of a young lady but rather stands for Enzyme Linked Immunosorbent Assay.
This is a method very commonly used in labs and the mandatory practical class for Biochemistry. 

It is based on an enzymatic interaction between antibodies and the kind of substance you want to detect;
almost everything is possible - proteins like antigenes and viruses, hormones, and other organic material can be traced with this method.

Below with the green color, you see what a finished ELISA looks like. Some of the s.c. Eppendorf tubes are heavier on the green, while others don't show any kind of color reachtion at all:

In the following picture you can see how the process works as ELISA sandwhich method (the indirect identification of a substance):

In picture 1 you see the green 'Y's (representing protein-specific antibodies), they are fixed to a to a surface. Above them, the substance we are oh-so-interested in comes flying by.

Next you see that the proteins/substance was bound by our immobilised 'Y's, so we can tell that the protein is really the one we were looking for.

But they are fucking small. Which is why those other 'Y's with the grey bobbles come flying in (again, the same type of specific antibody but this time with an enzyme connected to it).

In the 3. pic, all antibodies with grey bobbles connected as much as they could. Once an antibody connected, it cannot break loose, so it is safe to wash out the redundant ones.

Then a solution which contains a substrate is introduced (blue stuff), which will interact with the enzyme (grey bobble)

To produce the vibrant color in our test tubes, in this examples case it appears to be yellow (rather than the above shown green^^)

Hope this was helpful for exams and stuff :)