This vast network of cells and tissues is constantly on the lookout for invaders, and once an enemy is spotted, a complex attack is mounted. The immune system is spread throughout the body and involves many types of cells, organs, proteins, and tissues.

Crucially, it can distinguish our tissue from foreign tissue — self from non-self. Dead and faulty cells are also recognized and cleared away by the immune system. If the immune system encounters a pathogen, for instance, a bacterium, virus, or parasite, it mounts a so-called immune response.

White blood cells are also called leukocytes. They circulate in the body in blood vessels and the lymphatic vessels that parallel the veins and arteries. White blood cells are on constant patrol and looking for pathogens.

When they find a target, they begin to multiply and send signals out to other cell types to do the same. Our white blood cells are stored in different places in the body, which are referred to as lymphoid organs.