An oft-used, but valuable, analogy compares the human system with an army. The defending troops are thewhite blood cells called lymphocytes, born in the bone marrow, billeted in the lymph nodes and spleen, and onexercise in the blood and lymph systems. A body can muster some 200m cells, making the immune systemcomparable in mass to the liver or brain.
      The lymphocytes are called to action when the enemy makes itself known. They attack anything foreign. Theirjob is to recognize the enemy for what it is, and then destroy it. One of the key features of the immune system is itsspecificity. Inoculation with smallpox provokes an attack on any smallpox virus, but on nothing else. This specificityof response depends on the lymphocyte’s ability to identify the enemy correctly by the molecules on its surface,called antigens.
     An antigen is an enemy uniform. It cat be a protein on the surface. of a cold virus, or it can be a protein on thesurface of a pollen grain, in which case the immune response takes the form of an allergy. An antigen can also be aprotein on the surface of a transplanted organ, in which case the immune response “rejects” the transplant. Organscan therefore be transplanted only between closely related people-- in whom the antigens are the same -- or intopeople treated with a drug that suppresses the immune system, such as cyclosporin.
46.The author’s primary purpose in the passage is to do which of the following?
(A)Merge two differing views of a bodily process.
(B)Compare the immune system to the brain.
(C)Clarify the workings of the body’s defense system
(D)Demonstrate the inadequacy of an analogy.