Objective:To explore the influence of short-time pasteurization (62.5±0.5℃ for 5 s) on the main bioactive components and immune cells in human breast milk.Methods:Fresh breast milk was collected from 53 women whose premature infants were admitted to the neonatal intensive care unit of the Children's Hospital of Fudan University from May 2020 to October 2020. Each sample (20 ml) was divided into unsterilized, Holder pasteurized (62.5 ℃ for 30 min), or short-time pasteurized groups. The concentration of secretory immunoglobulin A (sIgA), lactoferrin (LTF), lysozyme (LZM), and insulin-like growth factor binding protein-3 (IGFBP-3) in breastmilk whey were detected by enzyme-linked immunosorbent assay and the number of viable immune cells (leukocytes, monocytes, T cells, and B cells) in breastmilk by flow cytometry.Results:(1) A total of 87 breast milk samples were collected. The levels of sIgA, LTF, and LZM were the highest in the unsterilized group, followed by the short-time and Holder pasteurized group [0.42 mg/ml (0.33-0.65 mg/ml) vs 0.40 mg/ml (0.28-0.62 mg/ml) and 0.25 mg/ml (0.17-0.37 mg/ml); (3.57±1.06) vs (3.53±1.11) and (0.85±0.58) mg/ml; 128.60 μg/ml (77.18-203.00 μg/ml) vs 121.70 μg/ml (68.66-188.20 μg/ml) and 83.40 μg/ml (47.40-151.40 μg/ml); all P<0.05]. There was no significant difference in the level of IGFBP-3 among the groups. The median retention rates of sIgA, LTF, and LZM in the Holder pasteurized group were all lower than those in the short-time pasteurized group [55.87% (46.01%-71.41%) vs 96.93% (83.03%-115.90%); 21.72% (12.54%-29.42%) vs 97.88% (88.98%-104.30%); 69.26% (49.42%-89.08%) vs 93.80% (74.85%-110.20%); all P<0.05]. No significant difference in the level of preserved IGFBP-3 was observed between the three groups ( P>0.05). (2) The number of viable leukocytes, monocytes, T cells, and B cells in the Holder pasteurized group were lower than those in the unsterilized group [leukocytes: 185.50 (87.00-356.50) vs 1 271.00 (540.50-2 283.00); monocytes: 12.00 (6.00-16.75) vs 266.00 (137.30-518.80); T cells: 1.00 (0.00-2.00) vs 47.50 (28.50-116.00); B cells: 1.00 (0.00-1.75) vs 21.00(10.00-41.50); all P<0.05]. The percentage of viable leukocyte to the total leukocyte and the viable monocytes, T cells, and B cells to the viable leukocytes were lower in the Holder pasteurized group than those in the unsterilized group [24.80%(16.00%-36.80%) vs 74.20%(63.55%-86.45%); 5.91%(4.09%-8.77%) vs 21.90%(17.40%-29.30%); 0.31%(0.00%-1.31%) vs 4.00%(2.69%-6.43%); 0.30%(0.00%-0.86%) vs 1.27%(0.57%-2.85%); all P<0.05]. A similar trend was observed between short-time pasteurization and unsterilized groups (all P<0.05). (3) The percentages of viable monocytes, T cells, and B cells in their subsets were lower in both Holder and short-time pasteurized groups than those in the unsterilized group [2.94%(1.33%-7.14%) vs 9.72%(5.77%-16.00%) and 52.60%(31.35%-68.75%); 0.00%(0.00%-1.61%) vs 0.49%(0.00%-2.53%) and 28.10%(10.55%-57.00%); 0.00%(0.00%-0.83%) vs 0.24%(0.00%-2.47%) and 13.80%(3.27%-41.00%); all P<0.05].The number and percentage of viable leukocytes in total leukocytes and viable monocytes in total monocytes [leukocytes: 279.50(116.80-548.50), 32.20%(20.70%-45.75%); monocytes: 32.00(21.00- 83.75),15.60%(11.10%-19.15%)] were higher than those in the pasteurized group (all P>0.05). The short-time pasteurized group was noted only for a higher percentage of the viable monocytes to viable leukocytes than the Holder pasteurized group (all P<0.05). Conclusions:Compared with the Holder pasteurization, sIgA, LTF, LZM level, and monocyte activity in breast milk can be better preserved by short-time pasteurization.