Background: Myristica fragrans is an evergreen aromatic tree cultivated in many tropical countries. Nutmeg, the dried ripe seed of M. fragrans, is a popular spice used in sweet and savory cooking, and a variety of drinks.

Objective: Nutmeg has a variety of pharmaceutical effects and in this study its effects on immune responses were evaluated.

Methods: Six groups (each group 8) of Wistar rats were treated as follows: Even groups received Nutmeg extract at dose 300 mg per kg of body weight intraperitoneally (IP), daily up to 12 days. Odd groups received PBS instead of nutmeg extract. The rats of groups 1 and 2 were immunized IP with 1.35×10⁹ Sheep RBC (SRBC) in days 1 and 6. The rats of groups 3 and 4, were immunized IP with 1.35×10⁹ SRBC in days 1 and subcutaneously in right foot pad with 2.7×10⁷ SRBC in a volume of 0.1 ml on the day 9.  Titer of anti-SRBC antibody in the groups 1 and 2 and lysozyme and alternative complement pathway activity of groups 5 and 6 were evaluated on day 13. The swelling of footpads in groups 3 and 4 were measured at 24, 48, and 72 hours after challenge with SRBC and these rats euthanized on day 13 and theirs foot were examined histopathologically for infiltration of inflammatory cells.

Results: The result showed that nutmeg extract significantly increase anti SRBC titer (P=0.005), inhibit inflammatory cells infiltration (P< 0.001) but has not any effect on serum lysozyme or complement activity (P=0.4).

Conclusion: In conclusion nutmeg extract shows a significant suppression on cell mediated immunity and stimulatory effect on humeral immune response to SRBC in Wistar rats.