The primary reason we evolved immune systems was to protect us from small creatures of the microbial world that wanted to feast on our flesh. Bacteria and fungi like to grow on our body surfaces, where they cause no harm, but some microbes like to invade deeper into our tissues, slurping up the nutrient-rich juices inside and between our body cells, while viruses hijack our cells and use them as factories to create more viruses.
These microbes cause sickness and death, ranging in severity from the common cold to the bubonic plague, but most of the time our immune system is waiting to hit back with antibodies to knock out the invaders, or with killer white blood cells to destroy infected host cells.
A surprising additional function of the immune system is its ability to destroy our own cells should they become cancerous. In a lifetime, our immune system can destroy potentially cancerous cells millions of times. The reason for this is that cancer cells can overproduce antigens that make them look different to other harmless cells. In this way cancerous cells may be detected and destroyed by the immune system as soon as they arise.
One of the best strategies is to vaccinate people against cancer before it arises so that the immune system is primed to attack a small mass of cells that could turn into tumours. Vaccination induces an adaptive (memory)...
