1. (Science: gynaecology) Colloquial abbreviation for papanicolaou's stain.
In the first stage the material, for example a section, is reacted with a specific antiserum (say rat) against the antigen. In the next stage a large excess of say rabbit antirat immunoglobulin is applied so that only one of the binding sites is bound to the first antibody.
Then a rat antiperoxidase antiserum is bound to the second antibody unfilled sites and finally peroxidase is added and binds to the third antiserum before the peroxidase is used to develop a colour reaction.