294 TUBERCULOSIS 



agglutinating power of the serum with the degree of immunity 

 possessed by the individual from which . it was derived. The 

 method has been used by some as a means of diagnosis, but its 

 value is doubtful and it is certainly inferior to the methods 

 depending on supersensitiveness. 



(3) Opsonins. — The serum of most normal men and of several 

 species of animals contains opsonins to the tubercle bacillus. In 

 tubercular subjects these are frequently diminished, and to 

 obtain a standard of comparison between infected and healthy 

 subjects samples of serum from a number of persons presenting 

 no signs of tuberculosis are taken and mixed. While the 

 technique of the opsonic method presents great difficulties, it 

 may be taken that with the use of such a standard an opsonic 

 index below - 8 indicates a deficiency in opsonins and an index 

 above 1 '2 indicates an excess. In strictly localised tuberculosis, 

 indices from - 1 to '8 are frequently found, while in tuberculosis 

 with general disturbance the index fluctuates greatly from day 

 to day, being sometimes below, sometimes above unity. 



While there is thus evidence that, when the tubercle bacillus 

 gains entrance into the body, reactions similar in nature to 

 those observed in other infections are developed, the processes 

 underlying recovery from tuberculosis are exceedingly obscure, 

 and certain factors have to be taken into consideration which 

 perhaps play a greater part in this disease than in other infec- 

 tions. One of these is the great chronicity so often observed. 

 It is possible that in many cases this is due to wide variations 

 in individual susceptibility and to differences in susceptibility 

 at different age periods. Thus on the whole the most acute 

 cases of tuberculosis are found in childhood. In view of the 

 widespread opportunities for infection which occur, especially 

 in city life, it is probable that the great mass of the adult 

 population is on the border line between complete resistance and 

 a susceptibility of varying degree. On the other hand, there is 

 some evidence that variations exist in the virulence of different 

 strains of the tubercle bacilli. As has been pointed out, it is 

 probable that the bovine variety is less pathogenic for man than 

 the human, but it is probable that even amongst human strains 

 variations in virulence occur, as has recently been insisted upon 

 by Burnet. It has been supposed by many that a cause of insus- 

 ceptibility in the adult is found in the fact that infection has 

 previously occurred in childhood whereby an immunity is 

 established. The evidence for this at present is rather of an 

 academic nature, and it is certainly extremely difficult to 

 immunise animals against infection with virulent bacilli. It 



