Effect of exercise on milk immunoglobulin A. Med. Sci. Sports Exerc., Vol. 29, No. 12, pp. 1596-1601, 1997. The major immunoglobulin (Ig) in human secretions is IgA. The immune properties of breast milk are well documented; however, the immunological influence of maximal exercise has not been established. The objective of this study was to investigate the role that exercise has on breast milk IgA and IgA subclasses. Breast milk was collected from 17 lactating women (4.6 ± 2.3 months postpartum) before and after randomized exercise and control periods. The exercise treatment was a maximal graded treadmill test (˙VO2max = 30.3 ± 5.7 mL·min-1·kg-1). Milk was collected at rest, the breasts were emptied, and samples obtained 10, 30, and 60 min following either exercise or 30-min control rest periods. IgA concentrations were established by enzyme-linked immunosorbent assay. The results indicated that samples taken 10 and 30 min after the exercise period had significantly lower (P≤ 0.05) milk IgA concentrations (21.0 ± 1.8 and 21.8 ± 1.4μg mL-1, respectively) than the corresponding control resting samples (52.8 ± 3.5 and 79.3 ± 7.7 μg·mL-1). The exercise samples were similar to the control samples at 60 min (134.0± 24.6 and 116.0 ± 15.4 μg·mL-1, respectively), indicating that by 1 h, milk IgA production had recovered. The IgA1 data showed a similar significant decrease (P ≤ 0.05) at 10 min postexercise, which also returned to control concentrations by the 30- and 60-min collection intervals. There was no significant change in the milk IgA2 concentrations at any of the time points studied. Milk IgA concentrations increased significantly in both exercise and resting control groups after the breasts were emptied, suggesting that breast emptying stimulated milk IgA synthesis. The results provide evidence that exercise alters milk IgA and IgA1 concentrations for 10-30 min after exhaustive exercise, but recovers by 1 h, and provide additional support for exercise effects on the mucosal immune system.