A Study of the Afferent and Efferent Limbs of the Immune Response in Opossums

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Abstract
The immune response of the North American opossum (Didelphys virginiana) was investigated to define further the possibilities of using this species as a subject for in utero examinations of immune tolerance and related phenomena. Experiments were designed in an attempt to discern the proposed disparity between the immune response of the opossum and that of other laboratory animals. The ability of the opossum to clear bacterial antigens from the circulation was comparable to that of a rabbit. This ability reached levels comparable to those of adults in pouch young about 11 weeks after birth. The efferent limb was investigated using techniques for the detection of antibody-producing cells. Plaque assays revealed a distinct but decreased ability to respond to sheep red blood cells (SRBC) in terms of plaque-forming units. This ability first appeared in pouch young between 70 and 120 days. In comparison with mice the cell density in the opossum spleen was low, and there was a proportionally lower percentage of immunocompetent cells capable of responding to SRBC stimulation; this explained the observation that the levels of humoral antibody production and of plaque-forming units were lower than those seen in more conventional animals. The decrease in number of antigen-responsive cells and the slower antibody production suggest that the weakness in the immune system of this animal is centered in both the central and efferent limbs of the immune response.

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