Early colonization with gut microbes may be associated with fewer allergies

Among sensitized infants, those with high salivary secretory IgA (SIgA) are less likely to develop allergic symptoms.

Early colonization with certain gut microbiota, e.g. Lactobacilli and Bifidobacterium species, might be associated with less allergy development.

Infants colonized early with high amounts of Bacteroides fragilis expressed lower levels of Toll-like receptor 4 (TLR4) mRNA. Bifidobacterial diversity may enhance the maturation of the mucosal SIgA system.

This 2012 study found beneficial effects: Protective effect of Lactobacillus probiotic against eczema in infants persists up to age 4, including for rhinitis.

References:

Influence of early gut microbiota on the maturation of childhood mucosal and systemic immune responses. Y. M. Sjögren, S. Tomicic, A. Lundberg, M. F. Böttcher, B. Björkstén, E. Sverremark-Ekström, M. C. Jenmalm. Clinical & Experimental Allergy.
Toll-like receptors (TLRs)

Probiotics may down modulate key parameters of allergic rhinitis. Clin Exp Allergy 2011.
Image source: Bifidobacterium adolescentis. Wikipedia, GNU Free Documentation License.