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Objective: To explore the expression characteristics of complement C3 in childhood asthma and its mechanism of regulating macrophage migration and polarization in immune inflammatory responses. Methods: (1) Clinical research section:a total of 200 children hospitalized and diagnosed with acute asthma exacerbation at Children's Hospital of Soochow University from January 2020 to December 2023 were retrospectively enrolled, including 135 males and 65 females with a mean age of (7.0±3.0) years. According to clinical symptoms on admission, patients were divided into mild group (n=89), moderate group (n=71) and severe group (n=40). Meanwhile, 100 healthy children receiving physical examination during the same period were recruited as healthy control group, consisting of 64 males and 36 females with a mean age of (6.6±3.5) years. Clinical characteristics were compared between the asthma group and healthy control group as well as among asthma subgroups of different severities. Multivariate logistic regression model was applied to analyze relevant factors associated with asthma severity. (2) Animal experiment section:Female C3 knockout (C3^-/-) mice aged 6 to 8 weeks and weighing (20±2) g were selected. Female wild-type C57BL/6J mice aged 6 to 8 weeks with a body weight of (20±2) g served as the control group. There were 10 mice in each group. All mice were randomly divided into 4 groups using a random number table method: wild-type control group, wild-type asthma group, C3^-/- control group and C3^-/- asthma group, with 5 mice in each group. Asthma mouse models were established by airway instillation sensitization and challenge with cockroach extract, while phosphate buffered saline (PBS) was administered in control groups. All mice were anesthetized and sacrificed on day 14. Bronchoalveolar lavage fluid (BALF) was collected for classification and counting of inflammatory cells. Enzyme-linked immunosorbent assay was used to detect levels of Th2 cytokines including interleukin-4 (IL-4) and interleukin-5 (IL-5) in BALF, as well as serum total immunoglobulin E (IgE) and immunoglobulin G1 (IgG1). Lung tissues were harvested, and quantitative real-time polymerase chain reaction (qRT-PCR) was performed to assess the distribution and polarization of M1-type (iNOS⁺) and M2-type (Arg-1⁺) macrophages. (3) Cell experiment section: murine macrophage cell line RAW264.7 was cultured in vitro and treated with different concentrations of recombinant C3 protein to observe macrophage migration ability. Cells were divided into PBS group, recombinant C3 protein group, and recombinant C3 protein combined with SB290157 (C3a receptor antagonist) group according to interventions. Cells were stimulated with PBS, IL-4 and lipopolysaccharide (LPS), respectively. The polarization of M1 (iNOS⁺) and M2 (Arg-1⁺) macrophages was detected via qRT-PCR. Results: (1) Clinical sample results: the level of complement C3 in the asthma group was significantly higher than that in the healthy control group [(1.20±0.20) vs (0.98±0.18) g/L, P<0.001]. Children with moderate and severe asthma had elevated complement C3 levels compared with those with mild asthma [(1.27±0.19) vs (1.13±0.20) g/L, (1.22±0.16) vs (1.13±0.20) g/L, both P<0.05]. Multivariate logistic regression analysis indicated that complement C3 was positively correlated with moderate-severe asthma (OR=12.373, 95%CI: 1.756-87.153). (2) Animal experimental results: the total number of inflammatory cells, counts of neutrophils, eosinophils, macrophages and lymphocytes, as well as levels of IL-4 and IL-5 in BALF were lower in the C3^-/- asthma group than those in the wild-type asthma group. Serum total IgE and IgG1 levels were also decreased (all P<0.05). qRT-PCR results revealed that the relative mRNA expression of iNOS and Arg-1, together with the M2/M1 ratio in lung tissues were lower in the C3^-/- asthma group (all P<0.05). (3) Cell experimental results: compared with the PBS group, recombinant C3 protein significantly enhanced macrophage migration ability, and the absorbance (A) increased with the elevation of recombinant C3 protein concentration (all P<0.05). After LPS stimulation, macrophage differentiation toward M1 phenotype was markedly higher in the recombinant C3 protein+SB290157 group than in the PBS group and recombinant C3 protein group (all P<0.001). Following treatment with PBS, IL-4 or LPS, M2 polarization level was higher in the recombinant C3 protein group relative to the PBS group, while the recombinant C3 protein+SB290157 group presented lower M2 polarization than the other two groups (all P<0.01). Conclusions: Complement C3 is markedly elevated in children with asthma and positively correlated with moderate and severe asthma. Complement C3 may enhance macrophage migration and promote M2 polarization, thereby participating in and aggravating immune inflammatory responses in asthma.