Abstract: Objective To primarily evaluate the effects of ulinastatin on immune function of patients with severe burn injury. Methods Forty patients with severe burn admitted to our ward from March 2013 to October 2015, conforming to the study criteria, were divided into conventional treatment group (CT, n=20) and ulinastatin treatment group (UT, n=20) according to the random number table and patient′s consent. After admission, patients in group CT received antishock treatment, antibiotic treatment, debridement, skin grafting, and nutrition support, etc. On the basis of the above-mentioned treatment, patients in group UT received intravenous drip of ulinastatin from first day after admission twice a day, with a dosage of 8×10⁵ U every time, for 7 days in addition. Peripheral venous blood samples were collected from patients in groups CT and UT on post treatment day (PTD) 1, 3, 5 and 7, respectively. Twenty healthy volunteer were selected as health control group (HC), and peripheral venous blood samples were collected on the first day of the study. Percentage of CD4⁺ CD25⁺ regulatory T lymphocytes (Tregs) was determined by flow cytometer. The proliferative activity of T lymphocytes was detected by microplate reader (denoted as absorbance value). Content of interleukin 2 (IL-2) in culture supernatant of T lymphocytes, and content of IL-4 and γ interferon (IFN-γ) in serum were detected by enzyme-linked immunosorbent assay. Expression of human leukocyte antigen-DR (HLA-DR) on CD14⁺ monocytes was determined by flow cytometer. Data were processed with analysis of variance for repeated measurement, chi-square test, and LSD-t test. Results (1) Compared with that of volunteer in group HC, the percentage of CD4⁺ CD25⁺ Tregs of patients in group CT was significantly increased from PTD 1 to 7 (with t values from 13.303 to 26.043, P values below 0.01). Compared with that in group CT, the percentage of CD4⁺ CD25⁺ Tregs of patients in group UT was significantly decreased on PTD 5 and 7 (with t values respectively 8.317 and 15.071, P values below 0.01). (2) The proliferative activity of T lymphocytes of patients in group CT on PTD 1, 3, 5, and 7 was respectively 0.71±0.11, 0.61±0.15, 0.54±0.12, and 0.67±0.17, which was significantly lower than that in group HC (1.21±0.22, with t values from 8.686 to 11.957, P values below 0.01). The proliferative activity of T lymphocytes of patients in group UT on PTD 3, 5, and 7 were respectively 0.81±0.11, 0.85±0.14, and 1.08±0.13, which was significantly higher than that in group CT (with t values from 4.808 to 8.568, P values below 0.01). (3) Compared with those of volunteer in group HC, content of IL-2 in culture supernatant of T lymphocytes of patients in group CT was significantly decreased from PTD 1 to 7 (with t values from 8.073 to 9.288, P values below 0.01), content of IL-4 in serum of patients in group CT was significantly increased from PTD 1 to 7 (with t values from 18.926 to 41.451, P values below 0.01), and content of IFN-γ in serum of patients in group CT was significantly decreased from PTD 1 to 7 (with t values from 4.543 to 27.659, P values below 0.01). Compared with those in group CT, content of IL-2 in culture supernatant of T lymphocytes of patients in group UT was significantly increased from PTD 3 to 7 (with t values from 6.507 to 8.869, P values below 0.01), content of IL-4 in serum of patients in group UT was significantly decreased from PTD 3 to 7 (with t values from 6.922 to 8.843, P values below 0.01), and content of IFN-γ in serum of patients in group UT was significantly increased on PTD 5 and 7 (with t values respectively 5.369 and 13.521, P values below 0.01). (4) The percentages of CD14⁺ monocytes with positive expression of HLA-DR of patients in group CT on PTD 1, 3, 5, and 7 were respectively (28±6)%, (25±7)%, (25±7)%, and (39±10)%, which were significantly lower than the percentage of volunteer in group HC [(87±8)%, with t values from 16.323 to 25.645, P values below 0.01]. The percentages of CD14⁺ monocytes with positive expression of HLA-DR of patients in group UT on PTD 3, 5, and 7 were respectively (40±6)%, (42±9)%, and (49±10)%, which were significantly higher than those in group CT (with t values from 3.071 to 7.324, P values below 0.01). Conclusions On the basis of CT, additional ulinastatin intervention can decrease CD4⁺ CD25⁺ Tregs percentage, improve the immune function of T lymphocytes and T helper cells, and increase expression of HLA-DR on CD14⁺ monocytes of patients with severe burn injury, thus improve the immune function of patients.