Ecology and Evolution - 579 



RED BLOOD CORPUSCLES 



SPOROZOITE 



TROPHOZOITE 



SPOROZOITE 



SPOROZOITE- 



GAMETOCYTES 



MICROGAMETE 



MACROGAMETE 



SPERM 



FERTILIZATION OF EGG 



STOMACH WALL 

 PLASMODIUM ZYGOTE 1 



Fig. 30-7. Life cycle of one of the malaria parasites, a protozoan (Class, Sporozoa; Genus, Plasmodium). 

 Note the three separate reproductive stages and the several adaptations to different conditions in two 

 hosts. In man, the malarial fever flares up whenever swarms of the parasites emerge from one set of cor- 

 puscles (which are destroyed) and pass, via the plasma, into another set of corpuscles. (From Buchanan, 

 Elements of Biology. Harper and Row.) 



skin, may be phagocytized by leucocytes, 

 either locally at the site of the infection (p. 

 334), or in the lymph nodes, which filter the 

 lymph as it drains from the infected area 

 (p. 335). Or the parasite may be immobilized 

 by encasement within a tough-walled cyst, 

 which is constructed by the local connective 

 tissues in response to the irritating presence 

 of the foreign organism. But by far the most 

 important defensive adaptation of organisms 

 generally is a capacity to produce specific 

 antibodies. Each antibody is a specific chem- 

 ical compound that is formed by the tissues 

 of the host and serves to limit the growth or 

 to neutralize the toxins of some particular 

 infective agent. 



Disease Resistance: Artificial Defenses. 

 Vaccines and antisera are widely used in 

 modern medicine to augment the natural 



antibody defenses of man and other animals. 

 The vaccine with which an individual is 

 inoculated (or vaccinated) always contains a 

 specific antigen (p. 320) that stimulates the 

 subject to produce the proper antibody. The 

 vaccine may be an extract of an infectious 

 organism; or the infectious organism itself 

 may be used for the inoculation. But in the 

 latter case, the infectious organism must first 

 be killed or weakened (attenuated), or it 

 must be selected from a nonvirulent strain. 

 An antiserum, on the other hand, is an 

 antibody-rich serum obtained from an ani- 

 mal (for example, horse or cow) that pre- 

 viously was vaccinated against a particular 

 infection. 



Man has developed many other weapons 

 to combat the ravages of parasitic organisms 

 (see Table 30-1). In addition to natural 



