Further Observations on Acquired Immunity to the Tick Dermacentor variabilis Say

Abstract

It has been shown (Trager, 1939) that guinea pigs and rabbits once infested with larvae of the tick Derlziacentor variabilis acquire an effective immunity against these larvae. This immunity can be produced artificially by the inoculation of an extract of larval ticks and can be passively transferred. It depends to a large extent on the presence of a circulating antibody which accelerates a local cellular reaction which in turn walls off the tick larva from its normal food supply. Some additional information has now been obtained concerning two of the points left open in the previous paper. I. Immunization with various tick extracts. Guinea pigs were inoculated intracutaneously on the left side with various tick tissue extracts, or with 0.85 per cent salt solution alone. By means of the pill box method previously described, larval ticks from a single batch of eggs were then applied to both sides and to the left ear of all the guinea pigs. Table 1 shows the results of such an experiment. It is clear that antigenic substance is present in the digestive tract and the cephalic glands of partly engorged ticks, as well as in the salivary glands of both fed and unfed ticks and in whole larval ticks. The cephalic gland extract, however, is not nearly as potent as the salivary gland extracts. It is worthy of note that during the course of the inoculations guinea pigs 4 and 6 showed marked reddening and hardening of the skin at the site of injection; guinea pigs 5, 9 and 10 showed a similar but milder reaction; while the other animals showed no local reaction whatever. Nevertheless, guinea pig 2 showed more immunity than guinea pig 4. II. The coimnplenment fixation reaction. In the previous paper it was noted that guinea pig serum could not be used to demonstrate specific complement fixation because normal guinea pig serum reacted with tick extract antigen to fix complement. This difficulty has been overcome by using rabbit serum. Table 2 shows a typical result. The complement fixation technic recommended by Taliaferro (1929) was followed. The reagents used were: Antigen: Approximately 0.2 gm of larval ticks ground up in 2 cc of 0.85 per cent saline solution. Mixture centrifuged and supernatant diluted with saline to 25 cc.