There is a tricky balance between an effective immune system and one that's too gungho and you get auto-immune problems. Frankly it's amazing it works at all -

There is a tricky balance between an effective immune system and one that's too gungho and you get auto-immune problems. Frankly it's amazing it works at all - not only does the immune system have to be able to react to stuff it's never seen before - it needs to be able to distinguish between self and non-self - and self is pretty damn complex. So to make an analogy - cancers like a bunch of terrorists in a busy shopping centre and the immune system is the government response. Only one side has to worry about bystanders.