The health-promoting effects of lactic acid bacteria (LAB) are well recognised, making them a popular functional food ingredient. Commercially available probiotic products are often promoted as capable of improving immune defences also in healthy subjects. However, while strain-specific differences exist in the effects of LAB, conventional yoghurt bacteria have proved beneficial as well. For comparing the immunological effects of conventional and probiotic LAB, young healthy women received either a commercially available probiotic fermented milk product or a conventional yoghurt for four weeks. Both treatments showed comparable effects resulting in a stronger immunological reaction to stimuli (natural cytotoxicity against cancer cells, mitogen-induced T-lymphocyte activation and stimulated cytokine production). To study the mechanisms behind these effects, conventional (Lactobacillus delbrueckii) and probiotic (Lactobacillus rhamnosus GG), LAB were compared in vitro at the cellular level. Interestingly, L. rhamnosus GG was more potent in inducing maturation of dendritic cells (DC) that play a substantial role in directing the immune response to stimuli. In turn, L. delbrueckii provoked a higher secretion of proinflammatory cytokines as well as IL-10. These effects were, however, observed only after direct incubation of DC and LAB, not when both were separated by a layer of enterocyte-like Caco-2 cells. LAB also induced cytokine secretion in peripheral blood mononuclear cells in a similar manner and this effect was reduced in a Caco-2 cell model, suggesting a modulating influence of gut epithelial cells. While both conventional and probiotic strains modulate the immune response, specific properties may offer therapeutic options in the treatment of certain diseases.

Polycystic ovary syndrome (PCOS) is a common, chronic endocrine condition affecting young women of reproductive age. It is characterised by hyperandrogenaemia, and profound menstrual and ovulatory dysfunction with consequent sub-fertility. A clustering of metabolic aberrations is commonly associated with this condition and these include insulin resistance, disordered lipid metabolism and chronic low-grade inflammation. Overweight and obesity, as well as a degree of adipose tissue dysfunction, are present in a large proportion of women with PCOS, and where present, magnify the inherent hyperandrogenaemia characteristic of the condition, in addition to worsening the metabolic profile. Diet and lifestyle interventions are among the first-line treatments for PCOS, and weight reduction through energy restriction has been shown to exert positive influences on both metabolic and hormonal aspects of this condition. Alterations in carbohydrate amount and type have also been investigated, and more recently, dietary fatty acids, with a particular emphasis on PUFA, have been shown to have a positive impact within this population group. Although it is likely that diet is not the root cause of PCOS, it represents a modifiable variable with the potential to improve the health of women with this condition. Work to date has provided insights into the role of diet in PCOS; however, further work is required to determine the role of nutrients specifically within the context of PCOS, in order to develop more effective, evidence-based dietary guidelines for this condition.

The thymus gland, where T lymphocyte development occurs, is targeted in malnutrition secondary to protein energy deficiency. There is a severe thymic atrophy, resulting from massive thymocyte apoptosis (particularly affecting the immature CD4+CD8+ cell subset) and decrease in cell proliferation. The thymic microenvironment (the non-lymphoid compartment that drives intrathymic T-cell development) is also affected in malnutrition: morphological changes in thymic epithelial cells were found, together with a decrease of thymic hormone production, as well as an increase of intrathymic contents of extracellular proteins. Profound changes in the thymus can also be seen in deficiencies of vitamins and trace elements. Taking Zn deficiency as an example, there is a substantial thymic atrophy. Importantly, marginal Zn deficiency in AIDS subjects, children with diarrhoea and elderly persons, significantly impairs the host's immunity, resulting in an increased risk of opportunistic infections and mortality; effects that are reversed by Zn supplementation. Thymic changes also occur in acute infectious diseases, including a severe thymic atrophy, mainly due to the depletion of CD4+CD8+ thymocytes, decrease in thymocyte proliferation, in parallel to densification of the epithelial network and increase in the extracellular matrix contents, with consequent disturbances in thymocyte migration and export. In conclusion, the thymus is targeted in several conditions of malnutrition as well as in acute infections. These changes are related to the impaired peripheral immune response seen in malnourished and infected individuals. Thus, strategies inducing thymus replenishment should be considered as adjuvant therapeutics to improve immunity in malnutrition and/or acute infectious diseases.

Lactic acid bacteria are present in many foods such as yoghurt and are frequently used as probiotics to improve some biological functions of the host. Many researchers have evaluated the effects of yoghurt and lactic acid bacteria against diseases such as cancer and intestinal inflammation. The preventive effect of probiotics on intestinal carcinogenesis may be associated with changes in the intestinal microbiota, suppressing the growth of bacteria that convert procarcinogens into carcinogens. Other mechanisms could be related to the immune response modulation and have been evaluated using milks fermented with lactic acid bacteria in chemically induced colon cancer and hormone-dependent breast cancer models. We demonstrated, using a murine colon cancer model, that yoghurt consumption inhibited tumour growth by decreasing the inflammatory response by increasing IL-10-secreting cells, cellular apoptosis and diminishing procarcinogenic enzymes. Milk fermented with Lactobacillus helveticus R389 delayed breast tumour growth by decreasing IL-6 and increasing IL-10 in serum and in the mammary glands and tumour-infiltrating immune cells. Previous results obtained with yoghurt administration in a colon cancer model led us to analyse its effect on a trinitrobenzenesulfonic acid-induced intestinal inflammation model in mice. Yoghurt was able to attenuate the symptoms of acute inflammation by reducing inflammatory cytokines, and increasing regulatory cytokine IL-10-producing cells, leading to desirable changes of the intestinal microbiota. It was demonstrated, by using murine models, that the consumption of fermented milks can modulate the immune system and can maintain it in a state of surveillance, which could affront different pathologies such as cancer and intestinal inflammation.

Probiotics, defined as living micro-organisms that provide a health benefit to the host when ingested in adequate amounts, have been used traditionally as food components to help the body to recover from diarrhoea. They are commonly ingested as part of fermented foods, mostly in fresh fermented dairy products. They can interact with the host through different components of the gut defence systems. There is mounting clinical evidence that some probiotics, but not all, help the defence of the host as demonstrated by either a shorter duration of infections or a decrease in the host's susceptibility to pathogens. Different components of the gut barrier can be involved in the strengthening of the body's defences: the gut microbiota, the gut epithelial barrier and the immune system. Many studies have been conducted in normal free-living subjects or in subjects during common infections like the common cold and show that some probiotic-containing foods can improve the functioning of or strengthen the body's defence. Specific probiotic foods can be included in the usual balanced diet of consumers to help them to better cope with the daily challenges of their environment.