No, it is not a misprint. And no, that command is certainly not cited from religious scripture or a guide to ethical living. It is a top imperative that the human body, like every other organism, must obey while dealing with the rough and tumble world, where rolling out the welcome mat to microbial or other visitors and invaders is a risky proposition. Not that all visitors are bad. There is strong suspicion that our mitochondria, which manage the energy needs of every single cell, were once free-living microbes. Also, it is not quite clear when and how it happened, but over 40 percent of our very own DNA has its origin in so-called retrotransposons, which most probably were parts of viruses that became integrated into our genome and gave us genes that we did not possess before but that are apparently useful. Finally, the body has learned not only to tolerate but to benefit greatly from complex, diverse, and distinct microbial communities that live on the skin, in the gut, and in all cavities that are connected to the outside world: the mouth, nose, ears, anus, urinary tract, and vagina.

Notwithstanding these beneficial exceptions, most encounters with foreign invaders are potentially a matter of life and death, and every higher organism must therefore take this threat very seriously. And because of what's at stake, the immune system defending our body could hardly be more complicated in its structure and function. It is truly a multi-faceted system of systems that covers every level of biological functionality, has lots of inbuilt redundancy, and is very robust, amazingly adaptive, and able to respond to threats never before encountered. It is a paradigm of what systems biology must learn to address.

In fact, while anthropocentric comparisons often fall flat, it does not take much imagination to identify a number of parallels between our body's immune system and a typical military defense apparatus or police force.