Use analogy to explain one object or process in terms of another. The aptness of the analogy isgenerally a point of subtle judgment. Some analogies are more valid than others. Comparing anapple to the space shuttle is not likely to be an effective analogy. Explaining the Internet byreference to a highway system might make a better analogy. In the following analogy, a bodyinfection 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 bloodcells, all of which are continuously being produced in the bone marrow. Neutrophils, one type ofwhite 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 pathogensand worn-out cells. Natural killer cells directly destroy virus-infected cells and cells that have turnedcancerous. When lymph nodes are actively involved in fighting an invasion of microorganisms, theyfill with lymphocytes.
--P. Insel and W. Roth, Core Concepts in Health
The McGraw-Hill Companies, Inc..