You are holding on to the central pole of a subway car, or talking on your phone, or opening a door. Your hand brushes metal, plastic, wood. And there, where you got a paper cut yesterday, germs bypass the barrier of your skin and enter your body.
As soon as the germs reach the bloodstream, though, your body senses their foreignness. Immediately, cells filled with anti-bacterial poisons swarm to the site of the trespassers, engulfing and killing them. Thousands of times a day, these assassinations of potentially harmful cells occur without our even noticing.
This immediate response is part of the innate immune system- the body's first line of defence against infection. For 99.9 per cent of our interactions with the world's microbes, the innate immune system is sufficient for our protection.
But if the germs you picked up happen to be a particularly virulent strain of flu virus, for example, the innate immune system may not be able to hold off the pathogen for more than a few days. Then the body's second line of defence, the adaptive immune system, kicks in.
The adaptive immune system is the one we know and acknowledge. In the course of its more aggressive battles, it causes the symptoms of sickness we know all too well: fever, headaches, swelling. It is because of these symptoms that we notice the immune system is working at all.
So while the adaptive immune system gets credit for its attack on particularly virulent diseases, the innate immune system, which conquers everyday microbe attacks, is often the unsung hero.