Infrared LED and Photodiodes
To what's already been said here, I want to add a bit more.


Human body temperature is about 310 K. The blackbody spectrum at 310 K peaks at about 10 um:

(source)
The photodiodes used in this kind of circuit is typically a silicon device, with a bandgap energy corresponding to about 1.1 um. The responsivity falls off strongly at longer wavelengths:

(source)

So this detector will not have any practically useful (or confounding) response from the radiation from the human body. In fact, if it did, it would also be detecting radiation from other nearby objects of similar temperature, such as the PCB it's mounted on, the metal or plastic housing around the actual photodiode, etc. 


The reflectance of human skin depends somewhat on the color of the skin in question, but at the near-IR wavelengths that will most affect your photodiode, we're all about the same. We are not great reflectors, but probably good enough to have a measurable effect on your sensor:

(source)