The immune system is a complex system of specialised proteins (such as antibodies), immune cells and organs. It fends off foreign substances and beings by distinguishing between the body’s own structures and those of foreign entities. Foreign bodies trigger immune responses that enable the body to repel the intruders. The system is even able to reject the body’s own cells if they are working correctly. Humans, animals and plants all have an immune system. The Latin term immunis, meaning “free”, reveals the function of the immune system: when firing on all cylinders, it frees the body from troublesome germs.