Background: Mycoplasma pneumoniae pneumonia (MPP) is a common cause of community-acquired pneumonia in children. Immune dysregulation, particularly involving T-cell subsets, plays a key role in its pathogenesis, but the dose-response relationship between the peripheral blood CD4+/CD8+ T cell ratio and MPP severity remains unclear. This study aimed to investigate this correlation and dose-response relationship in pediatric MPP patients.

Methods: A cohort of 237 pediatric patients diagnosed with MPP was enrolled from Zhongshan Hospital, Xiamen University between June 2022 and June 2025. Based on disease severity assessed using clinical criteria, the participants were stratified into a mild group (n = 181) and a severe group (n = 56). General information, laboratory indicators, and immune cell levels of the children were collected. Stratified regression analysis was used to analyze the relationship between the CD4+/CD8+ T cell ratio and the clinical characteristics of the children. A generalized linear model (GLM) was used to assess the association between the CD4+/CD8+ T cell ratio and the severity of the children's condition. The dose-response relationship between the two was analyzed using restricted cubic splines (RCS).

Results: The CD4+/CD8+ T cell ratio in the severe group was significantly lower than that in the mild group (0.83 ± 0.29 vs 1.36 ± 0.41, p < 0.001). Stratified regression analysis revealed that duration of fever, Pediatric Early Warning Score (PEWS), interleukin-10 (IL-10), lactate dehydrogenase (LDH), and ferritin all had independent negative effects on the CD4+/CD8+ T cell ratio (all p < 0.05). GLM analysis indicated that a lower CD4+/CD8+ T cell ratio was associated with a higher risk of disease exacerbation. After adjusting for confounding factors, children with a ratio <0.80 exhibited a 6.195-fold higher risk of disease exacerbation compared to the control group (>1.65) (95% CI: 2.380–16.125). RCS analysis further revealed a significant nonlinear negative dose-response relationship between the CD4+/CD8+ T cell ratio and disease severity (nonlinear p = 0.008).

Conclusions: The peripheral blood CD4+/CD8+ T cell ratio in children with MPP was negatively correlated with the severity of the disease, and a significant dose-response relationship was observed. This ratio can be used as a potential immunological indicator for assessing the severity of the disease in children.