CD4 and asthma: With the most suitable MR methods, we observed that two kinds of immune-related cells had protective causal effects on asthma (CD4+ T: OR = 0.78, p = 1.73 × 10–13; CD4+ T/CD8+ T: OR = 0.92, p = 2.42 × 10–02) (Table 4 and Fig. 4A).