Initially in a direct ELISA test which is considered to be the simplest type of ELISA the antigen is adsorbed to a plastic plate, then an excess of another protein (normally bovine serum albumin) is added to block all the other binding sites. While an enzyme is linked to an antibody in a separate reaction, the enzyme-antibody complex is applied to adsorb to the antigen. After excess enzyme-antibody complex is washed off, enzyme-antibody bound to antigen is left. By adding in the enzyme's substrate, the enzyme is detected illustrating the signal of the antigen.
Direct ELISA, when compared to other forms of ELISA testing, is performed faster because only one antibody is being used and fewer steps are required. This can be used to test specific antibody-to-antigen reactions, and helps to eliminate cross-reactivity between other antibodies.