This is a study to evaluate the effects of a prebiotic, AHCC, a standardized extract of cultured Lentinula edodes mycelia, on immunologic markers and the incidence of respiratory tract infections in children.

A three-month, randomized, double-blind, placebo-controlled, pilot study will be performed in Indonesian healthy children aged 4 to 6 years in Nursery Schools. Subjects will be administered dextrin alone (placebo) or dextrin including AHCC. The incidence of RTI will be the primary outcome, and changes in serum Interferon Gamma (IFN-γ) as the crucial cytokine that acts as anti infective in the immune system will be the secondary outcome. Following completion of the 3 months intervention, participants will enter a 5 months follow up period (Months 4-8). New onset adverse events, ongoing adverse events, serious adverse events, and investigator assessed intervention related adverse events will be collected and evaluated.

Inclusion Criteria:

1. Children aged 4-6 years old at the time of enrolment
2. Good nutritional status with weight-for-height > -2, and height-for-age z-score > -2 based on WHO growth standards
3. Infants in a group setting (daycare centers).
4. No congenital disorder
5. Not suffering from severe coughing or diarrhoea at the time of recruitment
6. Written informed consent will be obtained from parents or legal guardians before enrollment.

Exclusion Criteria:

• Presence of chronic illnesses or conditions (congenital disorder, genetic syndromes, congenital heart disease, chronic renal failure, cystic fibrosis, etc)
• Children who have participated in another clinical intervention study within the past 30 days.
• Children who are likely to be transferred to another facility that is not part of the study before completing the follow-up period