 The analyte is also called the ligand because it will specifically bind or ligate to a detection reagent, thus ELISA falls under the bigger category of ligand binding assays.The ligand-specific binding reagent is "immobilized," i.e., usually coated and dried onto the transparent bottom and sometimes also side wall of a well (the stationary "solid phase"/"solid substrate" here as opposed to solid microparticle/beads that can be washed away), which is usually constructed as a multiple-well plate known as the "ELISA plate.