The Stranger's "Ask Science" column offers a detailed explanation of just what, exactly, boogers are. It is simultaneously gross and mesmerizing:
Mucus, chemically, is quite fascinating. Sugar chains are attached to a protein backbone in mucus cells, with the contraption released out into the open. These glycoprotein molecules rapidly and aggressively suck up water until they are plump, slick, and slimy. To an invader, this is a nightmare to navigate: tangled chains of protein and sugar, with every nook and cranny crammed with water molecules. (Boogers are when these chains become ever more tangled, finally resulting in a rubbery ball of partially dried-out snot. Neat!) The body adds antimicrobial enzymes to this mix, which digest the invading organisms as they slowly attempt to chew through this barrier and reach the thin underlying lining of cells.
Which reminds me: Over the years, I've stumbled across some interesting discussions about whether picking your nose and/or eating boogers is a psychological or biological phenomenon. That is, when people do this, does it reflect some kind of psychological or socio-cultural issue; or is there a biological reason why booger-eating could be beneficial?
The truth is, there's not been a great deal of research done on this subject, at least from the biology angle. We know about booger-eating as a function of human behavioral development. There's been some research into it from the perspective of evolutionary psychology (i.e., why do people think this is gross?). But analysis of whether or not there is a biological reason people engage in booger eating has been lacking. Perhaps unsurprisingly. It would be interesting to see the responses you'd get if you tried to recruit volunteers for that study. Especially considering the fact that, as I think about it, you'd probably want your test subjects to eat both their own boogers, and those of other people, to see whether that had any impact on any presumed immune system response.
But I digress. If you are not totally grossed out yet, I'd recommend reading "Eating Snot – Socially Unacceptable but Common: Why?", a chapter in the book Consuming the Inedible: Neglected Dimensions of Food Choice. You can read most of that chapter on Google Books. Author Maria Jesus Portalatin focuses mainly on the better documented socio-cultural implications, but gets into a bit of the biology. One thing she points out, nasal mucus is about 95% water, so there's a possibility that you might expect more mucus eating in arid places. But nobody has ever done the studies necessary to test that hypothesis out. Her main hypothesis—also untested—is that eating mucus might help prime the body's immune system, allowing it to have more contact with weakened forms of potential pathogens so it can better detect and destroy those pathogens later. In other words, she thinks that eating your boogers is sort of like self-immunization.
Blame Tim Lloyd for sending me down this train of thought.