Enzyme-Linked Immunosorbent Assay

Enzyme-linked immunosorbent assay or ELISA is a labeled immunoassay with wide applications in detecting and quantifying analytes like antibodies, peptides, glycoproteins, hormones, and viruses.

The detection antibodies that bind to the antigen or target molecule are conjugated with specific enzymes which, upon substrate addition, produce measurable signals.

Depending on the capture and detection methods, there are four major types of ELISA.

Direct ELISA uses an enzyme-conjugated primary antibody for detecting the sample antigen coated on the plate.

While for indirect ELISA, the attached antigen captures the  sample primary antibody, to which the conjugated secondary antibody binds.

Sandwich ELISA involves two antibodies, one coated on the plate to capture the sample antigen, while the other enzyme-conjugated antibody binds to the specific antigen-epitope forming the sandwich.

In competitive ELISA, the antigen and the primary antibody are pre-incubated and added to the plate that is coated with the same antigen. 

The higher the sample antigen concentration, the more antigen-antibody complexes are formed, leaving fewer free antibodies to bind to the competing antigen from the plate, therefore generating weaker signals.