ANTIGENIC STRUCTURE 933 



1944). In serum broth at pH 6-0-8-5 rickettsial suspensions may be stored for 

 months at — 77° C. with little alteration in potency (Elford and van den Ende 1944). 



Antigenic Structure. — The difficulty of obtaining suspensions of rickettsiae free 

 from admixture with cells and other bacteria has rendered the study of the antigenic 

 structure of these organisms peculiarly difficult. Ledingham (1920) and others 

 have shown that inoculation of infective material or of rickettsial suspensions into 

 rabbits is followed by the appearance of specific agglutinins. Advantage has been 

 taken of this circumstance to study the relationship of the pathogenic rickettsise 

 to Proteus X strains. Without entering here into the practical performance of the 

 Weil-Felix test (see Chapter 83), it may be mentioned that the serum of patients 

 suffering from typhus and typhus-like diseases frequently agglutinates Proteus OX 19 

 or one of its variant strains, OX 2 or OX K. Castaneda and Zia (1933), studying 

 E. prowazeki and Proteus X 19 by the agglutination and absorption of agglutinins 

 technique, found that these organisms behaved as if each possessed a specific and 

 a group somatic antigen. White (1933), using in addition the precipitation test, 

 obtained evidence of the existence in X 19 of two distinct heat-stable somatic 

 receptors : (1) an alkali-labile receptor (Castaneda's P factor), which is mainly 

 responsible for the agglutination of this organism by its own antiserum ; (2) an 

 alkali-stable receptor (Castaneda's X factor), which is responsible for the reaction 

 of this organism with the sera of typhus patients. White's conclusion received 

 confirmation from the further work of Castaneda (1934, 1935), who was successful 

 in extracting specific soluble substances of polysaccharide nature from X 19 and 

 R. proivazeki. These substances have already been referred to as P and X. It 

 appears, therefore, as if Proteus X 19 and R. prowazeki possess a common alkali- 

 stable antigenic factor (X), of polysaccharide nature, which is responsible for the 

 Weil-Felix reaction. In addition, Proteus X\% contains a specific alkali-labile 

 receptor, also apparently of polysaccharide nature, which plays no part in this 

 reaction (see Chapter 27). Whether R. prowazeki contains a specific receptor of 

 its own similar to the P factor of Proteus Z 19 is not yet clear, but there seems 

 little doubt from Castaneda and Zia's work that it possesses a heat-labile antigen, 

 behaving in much the same way towards the heat-stable antigen as the Vi antigen 

 of the typhoid bacillus does to the antigen. Topping (1944) has reported the 

 existence of a " soluble " substance present in the supernatant fluid after ether 

 extraction and centrifugation of yolk sac cultures of R. prowazeki possessing appar- 

 ently the same antigenic properties as those of the intact organisms. 



The relationship of the different types of typhus virus to each other, and to the 

 viruses of Rocky Mountain spotted fever, tsutsugamushi fever, fievre boutonneuse, 

 tick-bite fever and Q fever has been studied partly by serological methods and 

 partly by cross-protection tests in living animals. The interpretation of the 

 results is so closely bound up with the Weil-Felix reaction and with the clinical 

 and epidemiological characteristics of these diseases that it is proposed to defer 

 further discussion of this subject to Chapter 83. Suffice it to say that there appear 

 to be at least three major receptors in Proteus X strains, represented by the OX 19, 

 OX 2, and OX K types, which correspond to similar receptors in rickettsial strains 

 isolated from different typhus and typhus-like diseases. Ciuca and his colleagues 

 (1938) have shown that all three receptors are of glycolipoid nature and can be 

 extracted from the bodies of the bacilli by the trichloracetic acid method. 



Pathogenicity. — As already mentioned, there are five or six known pathogenic 

 species for man and one for cattle. Leaving aside this last species, R. ruminantium, 



