Friday, February 8, 2013

Immune system "remembers" pathogens it has never seen before

My son likes to eat snow.

Actually, he's somewhat of a snow connoisseur. He steers clear of the freshly fallen snow and heads right for the muddy, full of rocks, been-walked-through-for-a-week snow. New research from Stanford University suggests this habit could help make him a healthier adult.

Memory cells
Our immune system has several ways of fighting off disease, and memory cells play an important part in keeping us well. The first time your immune system comes up against a pathogen it may take weeks for you body to mount its response. After this first exposure, though, memory cells are able to recognize the pathogen and the next time you're exposed your immune system is able to respond much quicker.

This recent study found an abundance of CD4+ memory cells present in adults that had never had the initial exposure. These results are unexpected - it has been generally thought that until first exposure memory cells remained in an inactive "naive" state. In this study nearly all of the adults had active CD4+ cells in their blood for infections to which they had never been exposed. In contrast, the same memory cells were not found in umbilical blood of newborns - meaning we're not born with the memory cells but we don't always develop them from direct exposure either.

Low specificity memory cells
Chemical reactions are often described by a "lock and key" mechanism - if you want a reaction to happen the chemicals need to fit together just right. Specificity is the ability for one chemical to "recognize" another. Without high specificity our bodies would be a giant mess of uncontrolled chemical reactions instead of the orderly, almost sentient protein reactions that keep us alive.

Another surprise from this study is the low specificity of CD4+ cells. These cells were once thought to have a very high specificity. So high, in fact, that they circulated in the blood searching for a single pathogen. That doesn't appear to be the case. A large number of CD4+ cells were found to be reactive to harmless environmental microbes. This may mean that exposure to a high number of harmless environmental pathogens creates memory cells that will also be activated when exposed to a more dangerous pathogen - being exposed to a few harmless germs prepares you for the really nasty ones. It may also explain why the measles vaccines has been shown to give health benefits unrelated to the measles - the inoculation may be preparing your body for infections other than the measles.

Mark Davis, the principle investigator for the study, said:
"It may even provide an evolutionary clue about why kids eat dirt. The pre-existing immune memory of dangerous pathogens our immune systems have never seen before might stem from our constant exposure to ubiquitous, mostly harmless micro-organisms in soil and food and on our skin, our doorknobs, our telephones and our iPod earbuds."
So maybe it's ok if my son eats some muddy snow. Those harmless microbes are preparing his immune system for a bigger battle some day in the future.