Lactobacillus acidophilus and Bifidobacterium longum are key strains of probiotics which are found in the human gastrointestinal tract (or gut). Probiotics have been shown in research to influence the activity of the immune system. A healthy balance of microflora (probiotics) is needed for good gut function. With good gut function you can absorb your nutrients better, so you are better primed to fight off winter colds and other nasties.