The immune system is a complex network in which different cell types and soluble factors interact to efficiently eliminate various kinds of microorganisms as well as aberrant cell clones. The roots of immunologic investigations reach far into the past. In 430 BC, Thucydides reported that survivors of the plague did not present a second time with similar symptoms. The first report of a successful immu- therapy was made by Edward Jenner in 1798 who found a protective effect of cowpox vaccination against human pox. Since then, much knowledge has been accumulated; today, investigations of the...