Background The hygiene hypothesis has been proposed to explain the pathogenesis of asthma.Allergen exposure was shown to inhibit asthma in an animal model.But the optimal timing of allergen exposure remains unclear.This study aims to explore the time effcct of allergen exposure and the possible mechanisms.Methods Neonate Wistar rats were randomly divided into asthma group,control group and day 1,day 3,day 7,and day 14 groups.The day 1,day 3,day 7 and day 14 groups were injected with ovalbumin (OVA) subcutaneously on days 1,3,7 and 14 after birth,respectively.Six weeks later,all groups,except the control group,were sensitized and stimulated with OVA to make the asthma model.We observed the pulmonary pathologic changes,detected the regulatory T cells,and CD28 expression level in thymus and spleen by flow cytometry.Results The asthmatic inflammation in the day 1,day 3 and day 7 groups,but not the day 14 group,was alleviated.The asthma group and day 14 group had lower proportions of regulatory T cells in the thymus compared with the control group,day 1,day 3,and day 7 groups.There was no significant difference in the CD28 expression levels on the regulatory and conventional T cells among groups.But the control group and the day 1,day 3,and day 7 groups had relatively higher proportions of CD28 positive regulatory T cells in the thymus than the day 14 group and the asthma group.Conclusions There is a “time-window” for early allergen exposure.The impairment of regulatory T cells may promote the development of asthma.Allergen exposure in the “time-window” can make the thymus produce normal quantity of regulatory cells.The CD28 signal on regulatory T cells may participate in the production of regulatory T cells.