  Whipworms in the intestine  Whipworms in the intestine. Click
to enlarge.   The hygiene hypothesis was first proposed to
explain observations like these:    The immune system is an
incredibly complicated mesh of many different components that
interact with each other. The hygiene controversy focuses on Th1
and Th2. To grossly oversimplify, Th1 responds to intracellular
pathogens (bacteria and some viruses) and Th2 responds to
multicellular parasites like intestinal worms. They produce
different kinds of cytokines, but there is a confusing overlap. 
 Th1 produces inflammation and Th2 suppresses some of that Th1
inflammation and contributes to a pregnant woman’s ability to
tolerate the fetus, but it is also involved in producing
allergic reactions. Over-activation of either pathway can cause
disease, and either pathway can down-regulate the other.  Th1
and Th2 were originally thought to act like a seesaw where one
goes up as the other goes down; but it turned out not to be that
simple. An early hypothesis was