['Mast cells participate in immune defense and allergic disease. At baseline, serum tryptase levels primarily reflect mast cell burden, while mast cell degranulation leads to granule tryptase release, which may be detectable as a transitory elevation of serum tryptase levels. Thus, mast cell burden and mast cell activity are reflected by serum tryptase levels, but reports are scarce in infants under 1 yr. We aimed at defining levels of total serum tryptase levels in this population.', 'Total serum tryptase levels (ImmunoCAP; Phadia) were measured in 372 sera from infants younger than 1 yr. Two hundred and forty-two sera came from non-atopic, non-allergic infants in good condition, who had blood drawn for routine follow-up or diagnosis of illnesses that are not known to induce changes in serum tryptase levels. Seventy-two sera were from atopic and/or allergic infants, and 58 sera were from non-atopic, non-allergic infants requiring intensive care.', "Median serum tryptase levels were highest in infant2s under 3 months (6.12 ± 3.47 μg/l) and gradually decreased before reaching levels similar to those described in adults and older children (3.85 ± 1.8 μg/l between 9 and 12 months). Atopic/allergic status was associated with even higher tryptase levels (14.20 ± 10.22 μg/l in infants younger than 3 months). Intensive care patients had lower levels of serum tryptase (4.12 ± 3.38 μg/l in infants younger than 3 months). Longitudinal follow-up was performed in 27 patients and showed tryptase levels decrease over time in individual patients. Infants'sex was not found to interfere with serum tryptase levels."] Are total serum tryptase levels higher in young infants?