The purpose of an ELISA is to determine if a particular protein is present in a sample and if so, how much. There are two main variations on this method: you can determine how much antibody is in a sample, or you can determine how much protein is bound by an antibody. The distinction is whether you are trying to quantify an antibody or some other protein. In this example, we will use an ELISA to determine how much of a particular antibody is present in an individuals blood.
ELISAs are performed in 96-well plates which permits high throughput results. The bottom of each well is coated with a protein to which will bind the antibody you want to measure. Whole blood is allowed to clot and the cells are centrifuged out to obtain the clear serum with antibodies (called primary antibodies). The serum is incubated in a well, and each well contains a different serum (see figure below). A positive control serum and a negative control serum would be included among the 96 samples being tested.