A recent study out of Vanderbilt University's School of Medicine synthesized and evaluated a number of randomized controlled trials to review the relationship between probiotics and the effects of hay fever, or allergic rhinitis.
The conclusion? Moderately hopeful, in the sense that some of the studies did show a benefit over a placebo in treating allergy symptoms. (Others, however, did not.)
Additional studies are needed before more definitive conclusions can reasonably be drawn.
("Probiotics may be beneficial in improving symptoms and quality of life in patients with allergic rhinitis; however, current evidence remains limited due to study heterogeneity and variable outcome measures. Additional high-quality studies are needed to establish appropriate recommendations.")