# Immunomodulatory Properties of Probiotics and Their Derived Bioactive Compounds
## 1 Introduction  
The intestine provides a fruitful environment for the establishment of balanced relationships between commensal bacteria and immune cells [1]. The evolution of this diverse ecosystem over time has occurred through consecutive exposures to multiple antigens. The cellular and molecular components comprising the gut immunity are constantly under intimate interaction with intestinal commensal microbes, thus contributing to the modulation of immune responses as well as the maintenance of gut homeostasis [2]. Alterations in gut microbiota composition may lead to gut dysbiosis, thereby activating various signaling cascades that are implicated in the maturation, differentiation, and proliferation of gut immune cells. Disturbance of this complex microbial community has been directly associated with the development of severe health issues [3], including metabolic disorders [4], chronic inflammation [5], cancer [6], cardiovascular diseases through the gut-heart axis [7], and neurodegenerative diseases through the gut-brain axis [8]. In this context, probiotics have been suggested as a promising treatment strategy for the restoration of the intestinal microbial diversity [9].  
Probiotics are microorganisms that confer health benefits to the host when administered in adequate amounts [10]. Bacterial strains with accredited probiotic properties, which include tolerance to intestinal conditions, ability of adherence to epithelial cells, and cell-surface hydrophobicity [11], were found to counteract infections through modulation of gut immunity [12]. Probiotics exert their immunomodulatory properties on the host in several ways, including colonization of the perturbed mucosal barrier, competitive exclusionand inhibition of harmful invading pathogens, production of bioactive compounds [13], enhancement of anti-inflammatory cytokine production [14], detoxification of virulent substances [15], and recruitment of various immune cells [16].  
Bioactive probiotic molecules include amino acids, vitamins, exopolysaccharideacharides, enzymes, short chain fatty acids (SCFAs), and bacterioxins [16; 17]. SCFAs are metabolic by-products of non-digestible fiber fermentation [18], while bacterioxins comprise a heterogeneous group of peptides with antimicrobial activity against pathogens [13]. The profound effect of SCFAs, tryptophan catabolic products, and secondary bile acids originating from bacterial metabolism on the immune system-gut microbiota crosstalk has recently been reviewed [1; 3]. Although studies on bacterioxins mainly focus on particular commercially available forms and their effects on host health [19], their contribution in the modulation of the immune system has also been reported [20; 21]. Therefore, the utilization of bioactive compounds for therapeutic purposes comprises a new field of research investigations.  
This review focuses on the underlying mechanisms that configure the intricate interaction between probiotics, bacterioxins, and SCFAs with the host immune cells. Understanding the impact of probiotics and probiotic derived compounds on gut immunity is challenging as well as extremely significant in order to develop efficient therapeutic strategies for the amelioration of life-threatening diseases.