188 pathogenicity; chemotherapy 



moderately virulent organism (Case 2), there is the usual small 

 decrease, followed by increasing numbers — during which phase 

 clinical symptoms appear — then a period during which the 

 organisms are being countered by the defence mechanisms 

 of the host, and finally a clearing of the blood-stream as 

 phagocytosis is successful. 



Virulent organisms differ from avirulent forms in two main 

 respects. In the first place, virulent bacteria often possess 

 a capsule which protects them against the action of the 

 phagocytes. Virulent pneumococci possess a capsule com- 

 posed of polysaccharide and if this capsule is removed by 

 enzymatic means, the organism, though viable, is no longer 

 virulent and is rapidly removed in vivo by the cells of the 

 reticulo-endothelial system. The protective capsule is not 

 always polysaccharide in nature as the capsule of B. 

 anthracis, for example, consists of a polypeptide of the 

 unnatural D -glutamic acid. 



A second property which often distinguishes a virulent 

 organism from an avirulent one is the power of the former to 

 produce a " toxin." A toxin can be described as a substance 

 which is secreted specifically by an organism, which produces 

 general toxic reactions in the host and which gives rise to the 

 production of specific antibodies in the host's blood-stream. 

 When virulent organisms are injected, after an initial phase of 

 establishment, they proceed to multiply and produce toxin. 

 At the same time, the phagocytes mobilise around the site 

 of entry, and one of the actions of the toxin is often to 

 antagonise the phagocytic action and so impair the defence 

 mechanism. The circulation of toxin in the host's blood- 

 stream exerts the toxic action with the production of clinical 

 disease. A further effect of the toxin circulating in the blood 

 is to stimulate the cells of the bone-marrow to produce specific 

 antitoxin, the chief function of which is to render the toxin 

 ineffective. If the bone-marrow response is sufficiently rapid, 

 then antitoxin is poured into the blood, the toxin neutralised, 

 and the reticulo-endothelial cells once more enabled to attack 



