Immune system

What does it actually mean to be immune to something, and how does the body defend itself against external invaders?

The immune system protects us from disease-causing pathogens, but at the same time maintains immune tolerance towards the body's own tissues, i.e. it differentiates between innate and foreign structures, meaning between self and non-self. Disease-causing pathogens are microbes such as bacteria, viruses, fungi and parasites. The immune system is made up of different cells and molecules, which have many different ways of acting against penetrating pathogens.