Use analogy to explain one object or process in terms of another. The aptness of the analogy is generally a point of subtle judgment. Some analogies are more valid than others. Comparing an apple to the space shuttle is not likely to be an effective analogy. Explaining the Internet by reference to a highway system might make a better analogy. In the following analogy, a body infection is compared with an invading army of attack organisms.
The Immunological Defenders and How They WorkThe immune response of billions of cellular defenders is carried out by different kinds of white blood cells, all of which are continuously being produced in the bone marrow. Neutrophils, one type of white blood cell, travel in the blood stream to areas of invasion, attacking and ingesting pathogens. Macrophages, or "big eaters," take up stations in tissues and act as scavengers, devouring pathogens and worn-out cells. Natural killer cells directly destroy virus-infected cells and cells that have turned cancerous. When lymph nodes are actively involved in fighting an invasion of microorganisms, they fill with lymphocytes.
--P. Insel and W. Roth, Core Concepts in Health