PKESIDENT.S ADDRESS — SECTION L. 265 



leucocytes of man remains viable, capable of reproduction, and of 

 infection. Submitted to the leucocytes of rabbit or guinea pig 

 it is quickly ingested and destroyed, and in these animals it is 

 aviruleut. The tubercle bacillus and that ai leprosy are also 

 highly resistant, 2>artly becaues of their acid-fast cloak and partly 

 because of their power of destroying (by necrosis) the enveloping 

 cells. 



Tempting as is the theme and the occasion, I must pass on; but 

 before leaving the subject, and bearing in mind the two-sidedness 

 of tha problein of immunity, twoi factors related tO' vii'ulenoe must 

 be alluded to. The first may be described as " the virulence of 

 position," in that not all parts of the body are capable of 

 equally strong defence. Bacteria located in certain situations are 

 much more difficult tO' dislodge, and much less susceptible to the 

 offensives of the defending forces of the body, whether cells or 

 serum, than in other situations. As an example: may be mentioned 

 typhoid bacilli in experimentally infected rabbits in which anti- 

 typhoid vaccination fails to dislodge the bacilli from the bile ducts. 

 Human '' carriers " of typhoid infection, following recovery from 

 an attack of the disease, often have an anti-typhoid serum rich in 

 antibodies, but ap>parently unable to- reach the locus of the per- 

 sisting bacteria. Diphtheria shows a similar case where the bacilli 

 may persist in the mucous membranes of recovered persons. 

 Johne's disease, a chronic bacillary enteritis of cattle, is another 

 example. Contagions abortion of bovines, infecting the udder, and 

 streptococcic mastitis afford further important types of the same 

 virulence of position. , 



Then, again, an old cbservation by Roux, Bordet, and others, 

 showed that not all the body fluids have the same antibody con- 

 tent. The aqueous humour and the cerebro -spinal fluid are both 

 notably poor in antibodies, even in animals strongly immunized. 

 This explains the successful production of tetanus in animals 

 strongly immunized against the toxin if the dose of toxin is intro- 

 duced inside the meninges. 



The other factor is the capacity for '' lying low," provoking no 

 reaction, but continuing to multiply at the point of infection, 

 insidiously, persistently, until in sufficient numbers toi overwhelm 

 the surprised defence, often when some special circumstance 

 renders it more vulnerable. It may be that offence and defence 

 are practically equal. At first the infecting germs, having only 

 natural resistance to overcome, multiplv and establish a nidus of 

 infection, the weaker ones being killed off, but the more vigorous 

 ones continuing and reproducing a more resistant type. At the 

 same time antibody formation goes on, opsonins, agglutinins, 

 bacteriolysins, and others, but offence and defence develop equally, 

 and the issue may be for a long time in doubt. Often the infection 

 is held up'. and remains for months or years circumscribed, as, for 

 example, in leprosy, or in bovine tuberculosis, or, again, in 



