**1. Introduction**

Probiotics are defined by the World Health Organization as "live microorganisms, which when administered in adequate amounts, confer a health benefit upon the host." The main benefit of probiotics is that they help restore balance in the intestinal microbiota. Probiotics play a role in defining and maintaining the delicate balance between necessary and excessive defence

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mechanisms including innate and adaptive immune responses. The immunological mecha‐ nisms supporting probiotics and prebiotics effects continue to be better defined with novel mechanisms being described for dendritic cells, epithelial cells, T regulatory cells, effector lymphocytes, natural killer T cells, and B cells [1].

Looking to probiotics to support immune health is nothing new; the idea has existed for more than 100 years [2]. For millennia, humans have consumed microorganisms via fermented foods. Human beings and gut microbiota are in a symbiotic relationship, and the hypothesis of a "super organism" composed of the human organism and microbes has been recently proposed. The gut microbiota performs important metabolic and immunological tasks, and the impairment of its composition might alter homeostasis and lead to the development of microbiota-related diseases [3].

The most common illnesses associated with alterations of the gut microbiota include inflam‐ matory bowel disease, gastrointestinal infections, irritable bowel syndrome and other gastro‐ intestinal functional diseases, colorectal cancer, metabolic syndrome and obesity, liver disorders, allergy, and neurological diseases [4,5].

Neural pathways and central nervous system (CNS) signalling systems, according to new studies, can be activated by bacteria in the gastrointestinal (GI) tract, including commensal, probiotic, and pathogenic bacteria. Novel approaches for prevention and treatment of mental illness, including anxiety and depression [6], may be provided by actual and future animal and clinical studies, aimed at understanding the microbiota–gut–brain axis. In theory, every disease associated with the impairment of intestinal microflora might benefit from the therapeutic modulation of the gut microbiota.

The purpose of this review is to address the most recent findings regarding probiotic regulation of immune health. Probiotic genes and probiotic-derived factors involved in the regulation of host immunity, molecular targets of probiotic action responsible for the host immune respons‐ es, and roles and mechanisms of probiotics in prevention and treatment of diseases [7], which are included in clinical applications and mechanisms of action, are of special interest.

The role of specific microorganisms and the overall diversity of the microbiota in many human diseases can be understood to a great extent owing to the rapid growth of metagenomics strategies. Therapies focused on specific effects of different probiotics and prebiotics on the gut microbiota [8] can be helped by the development of this knowledge.
