Fig. 4.

A. Solid phase PLA. (A) Capture antibody immobilised in a microtitre well binds target antigen, with unbound particles and other sample components removed by washes. Proximity probes are then added to the well and a PLA is carried out. (B) First generation PEA. The 3′-oligonucleotide is double stranded with a 3′-overhang. In the presence of target antigen, the probe oligonucleotides can hybridise to each other, leading to the extension of one oligonucleotide into a DNA template that can be detected and quantified. (C) Second generation PEA. Single stranded oligonucleotides hybridise directly to each other, with one becoming extended by a DNA polymerase to generate an amplifiable DNA template.