Probiotics comprise one of the most popular yet least understood ingredient categories in the dietary supplement industry. They are live bacterial species designed to support health in the same way as the beneficial indigenous bacteria that inhabit our skin, oral cavity and gastrointestinal tract. Probiotics that are derived from the same strains found in the human body can often colonize after supplementation, crowding out more undesirable strains. Those that are not derived from human strains are more transient, requiring constant supplementation to gain temporary benefits, since they won’t typically colonize. Most probiotics are taken for their benefits to immune functioning and/or digestion.
Probiotic immune health products support the resident bacteria that are an important part of the portion of the immune system that resides in the gut, mostly in the colon (lower section of the large intestine). In the colon, beneficial bacteria interact with antigens. Antigens are foreign substances, usually proteins, that cause an immune response in the body. They often enter the body via the mouth, harmlessly through the diet or detrimentally on the surface of pathogens. As a result, the gut encounters more antigens than any other part of the body, and it is crucial that the resident gut bacteria function effectively alongside other parts of the gut-mediated immune system or GALT – (Gut-associated lymphoid tissue).
These important bacteria in the gut produce compounds through their metabolism that directly interact with the GALT response; and when functioning optimally, the GALT system and microbiota together ensure the initiation of protective responses to pathogens and the tolerance to innocuous antigens. Supplementation with a well-formulated probiotic product can help guarantee that the gut microbiota remain intact and functioning well, which is especially critical after taking antibiotics that destroy many of the resident bacteria along with their targeted pathogens. Immune support probiotics are often derived from species, such as Lactobacillus acidophilus, L casei, L salivarius, L lactis, Bifidobacterium bifidum and B lactis, known to naturally inhabit the intestines of healthy individuals.
Most of the current probiotic research in the area of immune health centers around gaining a better understanding of the role of probiotics in immunomodulation; focusing on the role of probiotics in humoral, cellular and nonspecific immunity modulation, as well as their role in promoting the immunological barrier. It has been found that specific strains of Lactobacillus acidophilus, L rhamnosus, L delbrueckii, Bifidobacterium lactis, B bifidum, Streptococcus thermophiles, among others, are particularly beneficial. These probiotics can play vital roles in innate immunity, by enhancing the functioning of certain immune cells such as natural killer cells (a specific type of white blood cell), as well as adaptive immunity through their interaction with other immune cells, such as enterocytes (the important absorptive cells that line the inner surface of the small and large intestines). Probiotics can also provide immunomodulatory effects on proinflammatory and anti-inflammatory cytokine production, thus helping to guide the immune system to a more beneficial inflammatory response. This influence on intestinal inflammation can be particularly helpful for those who have intestinal challenges that are caused by a dysfunctional inflammatory response, such as colitis or IBD.
The importance of the microbiome has expanded to include areas of the body outside the gastrointestinal tract. Notably, the oral cavity has become a focus of study within the last decade or so, resulting in the development of probiotics designed to work in the mouth and regions of the upper respiratory tract. We now recognize that the oral cavity is home to some 700 species of bacteria, and maintaining a healthy environment of bacterial species in the mouth is crucial to overall health. In addition to probiotics designed to support dental health, oral cavity probiotics can also be extremely beneficial to immune health, particularly in the upper respiratory tract. Ideally these probiotics are derived from species that are indigenous to the human oral cavity. Most function predominantly through competitive inhibition, crowding out detrimental species that enter the body through the nose and mouth. Others produce protective substances, called bacteriocin-like inhibitory substances (BLIS) that target unfavorable strains and limit their entry into the body.
Streptococcus salivarius is one of the most common species of bacteria in the oral cavity of healthy individuals and several strains do produce favorable BLIS. There is extensive background research establishing the safety and efficacy of these strains. One particularly beneficial S. salivarius strain is the K12 strain. This is a rare human-derived strain that is particularly beneficial in inhibiting such species as S. pyogenes and Moraxella catarrhalis. The breadth of human studies in young children make it a particularly desirable probiotic for supporting immune health in the very young. 
As the study of the human microbiome expands along with our knowledge of the myriad of ways in which it impacts our overall health, so will the desire for appropriate probiotics to support our own body’s microbiota.
 Belkaid Y and Hand T. (2014) Role of the microbiota in immunity and inflammation. Cell 157(1):121-141.
 Azad MAK, S M and Wan D. (2018) Immunomodulatory effects of probiotics on cytokine profiles. Biomed Res Int doi:10.1155/2018/8063647
 Aas JA, Paster BJ, Stokes LN, Olsen I and Dewhirst FE. (2005) Defining the normal bacterial flora of the oral cavity. J Clin Microbiol 43(11):5721-5732.
 Burton JP, Wescombe PA, Cadieux PA and Tagg JR (2011) Beneficial microbes for the oral cavity: time to harness the oral streptococci? Benef Microbes 2(2):93-101.
 Di Pierro F et al. (2018) Use of Streptococcus salivarius K12 to reduce the incidence of pharyngo-tonsillitis and acute otitis media in children: a retrospective analysis in not-recurrent pediatric subjects. Minerva Pediatr 70(3):240-245.
 Marini G et al. (2019) Pilot study to explore the prophylactic efficacy of oral probiotic Streptococcus salivarius K12 in preventing recurrent pharyngo-tonsillar episodes in pediatric patients. Int J Gen Med 12:213-217.