The Response to the Intracutaneous Inoculation of BCG as an Index of Native Resistance to Tuberculosis

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Abstract
Summary and Conclusions: The response of rabbits to the intracutaneous inoculation of BCG may serve as an index of native resistance and is useful as a method of selecting breeders of the desired resistance or susceptibility to tuberculosis. There is as yet no clear evidence that the response of human beings to the intracutaneous inoculation of BCG can serve as an index of their native resistance. The nodule at the site of intracutaneous injection in resistant rabbits grows rapidly, reaches its peak quickly, tends to ulcerate and heals soon. In the susceptible rabbit, this nodule grows slowly, reaches its peak tardily, does not usually ulcerate and heals much later. The BCG multiplies in the skin of the resistant rabbit for a shorter time and is subsequently more rapidly destroyed than in the susceptible rabbit. The associated histological host responses are accelerated in the resistant animal. Consequently, the development of allergic sensitivity to tuberculin and antibodies are enhanced in the resistant animal. As a result of the increased rate of development of allergic sensitivity in the resistant animal, its BCG nodule is at first larger than in the susceptible rabbit. Variations in native resistance are essentially variations in the rate of development of an increased capacity by the phagocytes to destroy tubercle bacilli. The degree of heightened resistance conferred by a given BCG inoculation is superimposed on, and determined by, the innate resistance of the individual vaccinated, so that the resistance to the genesis of primary pulmonary tubercles in the natively resistant rabbit may be increased several fold while that of the highly susceptible rabbit is hardly affected.