The immune system resists threats from outside as well as from within the body. ‘What is the immune system?’ considers the basic mechanisms of immunity and describes the specific structures in the immune system where more complex activities take place. The critical cells—‘white blood cells’ or leukocytes—are generated in the bone marrow. Leukocytes are highly diverse, each with its own specialist function, but broadly divided into the myeloid (develops in the marrow) and the lymphoid (develops in lymphoid structures including the thymus, lymph nodes, and spleen) leukocyte. We have both ‘innate’ (which we are born with) and ‘adaptive’ (which encompasses learned, very specific responses to individual infections) immunity.