OK, I use a mechanism called “surface plasmon resonance” (SPR). This occurs on the surface of metals. I have a glass prism coated with a layer of gold. I shine light at an angle through the prism at the gold. The light bounces of the gold and hits a camera. This will give me a spectrum. Now I couple an antibody to the surface of the gold. I shine light through the prism again. The extra layer of antibodies causes the angle of light that bounces back to the camera to change slightly. I get a shift in the spectrum. Now I assay serum. If the disease is present, the antigen specific to the antibody will bind and add an extra layer, resulting in another shift of the spectrum. If the disease is not present, I won’t get the shift in the spectrum and I know you’re OK. 🙂
It is not really my area but I did read a very interesting article recently about a group in Australia who made a laser that emitted light that certain gases could absorb. As there are certain diseases that affect the gases in your breath, for example kidney disease, you could use this laser to detect these gases and diagnose a disease!