Fig. 4. Enzyme-linked immunosorbant assay (ELISA). Red indicates purified pathogen antigen, which is fixed to the floor of the microtiter dish. The green antibody represents the host IgG, whereas the purple antibody is an enzyme-conjugated antibody that recognizes the fixed Ig portion of host antibody. After washing, the presence of the antibody sandwich is assayed by addition of a chromogenic substrate that changes color (here, clear → blue) via the enzyme linked to the antibody. The final colored substrate is measured with a densitometer or spectrophotometer.