immunity, suppression of immunity (both innate and adaptive immunity), promotion of intestinal epithelial cell development and migration, alteration of microbiome composition and function, enhanced recovery from infection, and antimicrobial functions.
Stimulation of Immunity
Probiotics can directly stimulate both adaptive and innate immunity (Thomas and Versalovic, 2010). They can also indirectly impact host immunity by enhancing host ability to digest and absorb nutrients that have an impact on the immune system. Both the direct and the indirect effects have been described in detail in a number of studies published over the past couple of decades. For example, Yamanaka et al. (2003) reported that gut bacteria drive Peyer’s patch8 development in rats, with germ-free rats having defective and immature lymphoid follicles and conventionalized rats having very mature gut lymphoid tissue. Indeed, it has been proposed that a key function of the gut microbiome might be to serve as a “treadmill” for the host immune system (Madara, 2004). By “tickling” Toll-like receptors and other receptors and signaling pathways that build host immunity, gut microbes might be keeping the host immune system “finely tuned and fit” and preparing the immune system for new challenges (e.g., antibiotic exposure, changes in diet). Evidence supports this hypothesis. As just one example, Prescott et al. (2008) reported that pregnant women who took either a Bifidobacterium or Lactobacillus probiotic had significantly elevated levels of interferon-gamma levels in their cord blood and that maternal probiotics may enhance interferon-gamma production in neonates. Interferon-gamma production has been associated with protection against allergic disease early in life (Macaubas et al., 2003). Prescott et al. (2008) also showed a significant effect of maternal probiotics on antibody production in breast milk. Breast milk immunoglobulin A (IgA) levels were significantly elevated at both 1 week and 3 months of age in the children of women who took either a B. lactis or L. rhamnosus probiotic. Although the difference in IgA levels between the children of the experimental and control women disappeared by 6 months of age, infants fed breast milk with elevated IgA levels early on may be receiving a critical head start in gaining passive immunity.
Suppression of Immunity
Evidence suggests that probiotics don’t just stimulate immunity, they also suppress it. Versalovic’s research group has conducted mouse model
8 Peyer’s patches are aggregates of lymphoid follicles located in the epithelium of the small intestine.