Tests of antigen-antibody interactions fall into two categories, indirect and direct measurements.
Precipitation reactions are dependent on the presence of multivalent antibodies and antigens.
They are inherently relatively insensitive since there must be enough reactants present to form a viable precipitate. Complement fixation is more sensitive than precipitation reactions but is seldom used in clinical labs anymore. Thus, they are not dependent on formation of a visible precipitate. The machines can detect amounts far smaller than one can see. The direct assays include radioimmunoassay, enzyme linked immunosorbant assay, and fluorescent antibody techniques.