A number of studies have been performed examining the influence of various probiotic organisms, either alone or in combination, on immune parameters, infectious outcomes, and inflammatory conditions in humans. Some components of the immune response, including phagocytosis, natural killer cell activity and mucosal immunoglobulin A production (especially in children), can be improved by some probiotic bacteria. Other components, including lymphocyte proliferation, the production of cytokines and of antibodies other than immunoglobulin A appear less sensitive to probiotics. Probiotics, including lactobacilli and bifidobacteria, administered to children can reduce incidence and duration of diarrhoea, but the precise effects depend upon the nature of the condition. Probiotic supplementation can reduce the risk of travellers' diarrhoea in adults, but does not affect duration. The effect of probiotics on other infectious outcomes is less clear. Probiotics may benefit children and adults with irritable bowel syndrome and adults with ulcerative colitis; studies in Crohn's Disease are less clear. Probiotics have little effect in rheumatoid arthritis. Probiotic supplementation, especially with lactobacilli and bifidobacteria, can reduce risk and severity of allergic disease, particular atopic dermatitis; early supplementation appears to be effective. Overall, the picture that emerges from studies of probiotics on immune, infectious and inflammatory outcomes in humans is mixed and there appear to be large species and strain differences in effects seen. Other reasons for differences in effects seen will include dose of probiotic organism used, duration of supplementation, characteristics of the subjects studied, sample size, and technical differences in how the measurements were made.