Receptor Molecules

http://www.britannica.com/eb/article-215558/immune-system


Britannica has a pretty good explanation of their function.


Good luck!

If you need more than that, try this article on how cells work:

http://science.howstuffworks.com/cell.htm

http://en.wikipedia.org/wiki/Signal_transduction

http://www.vivo.colostate.edu/hbooks/pathphys/endocrine/moaction/surface.html

http://web.mit.edu/esgbio/www/cb/membranes/gf.html




Basically, a receptor is just a molecule with a bit that sticks out from the surface of a cell and a bit that sticks down into the cell. The bit that sticks out has an affinity for molecules with a certain shape (basically), and when that molecule bumps into it, it sticks for a tiny part of a second. The interaction between the two changes the shape of the receptor molecule such that the a chemical reaction is initiated at the bit of the receptor that's inside the cell. This begins what's called a signal cascade -- a series of biochemical reaction that has some biological effect on the cell. It may induce an action potential (in a neuron), muscular contraction (in a myocyte), or induce any of a number of changes in the cell's genome.

Course, it's all a hell of a lot more complicated than that.