Probiotics, prebiotics, and phytobiotics could be a possible solution as immunostimulants in monogastric animal nutrition. Beneficial effects of application thereof in animals are determined by many factors, e.g. the type of the probiotic strain, probiotic compounds, or plant species used as a supplement. A significant role is also played by the animal species, dosage, and the time and method of administration. The activity of these compounds is primarily focused on prevention of pathogen infections and, consequently, improvement of animal welfare. Probiotics compete with pathogenic bacteria by covering the intestinal epithelium mucosa, thereby interrupting pathogen colonization in the gastrointestinal tract. Supplementation with probiotics, prebiotics, and phytobiotics can also induce positive changes in the intestinal morphology, e.g. elongation of villi or deepening of intestinal crypts. In a majority of cases, they also modulate the immune response of the host. They mobilise the cellular components of the innate immune system (macrophages and heterophils), which defend the animal organism against gastrointestinal infection. Another possibility is the synthesis and release of pro-inflammatory cytokines that modulate adaptive immunity or stabilization of the intestinal microbiome. The main target of immunomodulatory feed additives is reduction of local inflammation, enhancement of the function of the immune system, a substantial impact on the health status of livestock animals, and improvement of their health status and production performance.