The primary immune response patterns of young adult male and female hamsters injected with SRBC cells were studied and compared. The females were shown to produce higher numbers of PFC per spleen than did males. Similar results were obtained using aged males and females. These results were found to be independent of circadian rhythm for the males tested. Effects of pre- and post-pubertal gonadectomy on the primary immune response suggest that female advantage in terms of specific immunoglobulin synthesis depends primarily on the relative absence of male gonadal hormones.

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Ontogeny of the primary response to sheep erythrocytes of age-matched groups of male and female hamsters was studied at various chronological ages ranging from prepuberty to senescence. Selected organs were likewise weighed upon sacrifice to obtain developmental patterns. Adrenal weights were higher in the male, and pituitary weights were higher in the female; for both organs typical dimorphism was demonstrable by 36 days. Spleen weight and index favored the female by 46 days. Immunological sex dimorphism first appeared in groups injected at 53 days and autopsied at 58 days and persisted through senescence. Sexual dimorphism of antibody-mediated immunity, previously shown to favor the female in both the primary and secondary immune responses, thus follows the dimorphism of total amount of splenic lymphoid tissue and occurs shortly after realization of sexual maturity in the male. These findings support our previous suggestion of the suppressive effect of androgens on the antibody-mediated immune responsiveness of the male hamster.

Randomly bred pigs of both sexes were injected intracardially with one-half of a 50% lethal dose of Listeria monocytogenes. When infected animals were skin tested with 30 Ag of a water-soluble extract of sonically disrupted Listeria, both males and females had uniformly detectable levels of delayed hypersensitivity (DH) 4 days after infection. In males, cutaneous hypersensitivity to Listeria antigens reached a peak on day 5 or 6 of infection, and high levels of DH persisted through the 7th week. In females, DH reached a peak on day 6 or 7, remained at this level through the 4th week, and then dropped sharply. Cutaneous reactivity was usually higher for males than for females, and differences between the sexes were statistically significant 5, 6, and 7 weeks after infection. Low levels of DH were still present 41 weeks (females) or 46 weeks (males) after infection. Assays to determine the number of viable Listeria present in spleen homogenates indicated that bacterial multiplication occurred only during the first 24 h of infection. The number of Listeria declined steadily thereafter, and by day 13 no bacteria could be recovered from the spleens of infected animals. Spleen assays indicated that Listeria-infected animals of both sexes were resistant to a small challenge dose of Listeria given 48 h, 7 days, or 2 weeks after the primary infection. Resistance to re-infection was absent in females challenged at 41 weeks and in males challenged at 46 weeks.

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