A regular ELISA/immunoassay is: surface->Ab->protein<-Ab+signal. In school these are taught as "sandwich-assays". Basically you have one antibody to bind your molecule of interest to your substrate, and then another second which binds the bound molecule... which has been bound to the surface. This second antibody is decorated with an enzyme which will do something fancy (color-change) or a fluorescent protein to light-up if we shoot it with a laser. SPR is a label-free approach which results in an optical response in real-time as molecules associate with the SPR sensor. Detection is then a function of how long you need to let things bind to the sensor before your ability to detect the signal optically.

Sounds like your undergrad kid has been paying attention in class...