The steps of direct ELISA follows the mechanism below:
A buffered solution of the antigen to be tested for is added to each well (usually 96-well plates) of a microtiter plate, where it is given time to adhere to the plastic through charge interactions.
A solution of nonreacting protein, such as bovine serum albumin or casein, is added to each well in order to cover any plastic surface in the well which remains uncoated by the antigen.
The primary antibody with an attached (conjugated) enzyme is added, which binds specifically to the test antigen coating the well.
A substrate for this enzyme is then added. Often, this substrate changes color upon reaction with the enzyme.
The higher the concentration of the primary antibody present in the serum, the stronger the color change. Often, a spectrometer is used to give quantitative values for color strength.
The use and meaning of the names “indirect ELISA” and “direct ELISA” differs in the literature and on web sites depending on the context of the experiment. When the presence of an antigen is analyzed, the name “direct ELISA” refers to an ELISA in which only a labelled primary antibody is used, and the term “indirect ELISA” refers to an ELISA in which the antigen is bound by the primary antibody which then is detected by a labeled secondary antibody. In the latter case a sandwich ELISA is clearly distinct from an indirect ELISA. When the “primary” antibody is of interest, e.g. in the case of immunization analyses, this antibody is directly detected by the secondary antibody and the term “indirect ELISA” applies to a setting with two antibodies.