The microbiota plays a fundamental role in the induction, training, and function of the host immune system. The immune system has evolved to maintain a symbiotic relationship with the microbiota. When functioning optimally, the immune system-microbiota alliance allows the induction of protective responses to pathogens and the maintenance of regulatory pathways for tolerance to innocuous antigens. However, in high-income countries, overuse of antibiotics, changes in diet, and elimination of constitutive partners like nematodes may have selected for a microbiota that lacks resilience and diversity, contributing to the rise of autoimmune and inflammatory disorders. The human body is a meta-organism composed of the host and its symbiotic microbiota. The microbiota, which includes bacteria, fungi, viruses, and other microbes, plays a critical role in host physiology. The immune system, composed of innate and adaptive components, has evolved to maintain symbiosis with the microbiota. The microbiota promotes and calibrates multiple aspects of the immune system. The immune system-microbiota alliance interweaves innate and adaptive immunity in a dialog that selects, calibrates, and terminates responses. However, the acquisition of a complex immune system and its reliance on the microbiota have led to pathologies such as allergies, autoimmune, and inflammatory disorders. Alteration of the microbiota due to antibiotic use, diet, and elimination of constitutive partners has transformed microbial allies into potential liabilities. The microbiota contributes to the postnatal development of the immune system, which in turn helps contain it. Studies in germ-free animals show that the microbiota is crucial for secondary lymphoid structure development. The microbiota also contributes to the fortification of the intestinal barrier through mechanisms like epithelial cell maturation and angiogenesis. The microbiota plays a critical role in maintaining the balance between the host and its commensals. The "mucosal firewall" includes the epithelial barrier, mucus layer, IgA, and DCs and T cells, which limit the passage of commensals to the gut-associated lymphoid tissue. The microbiota induces regulatory T cells (Tregs) and Th17 cells, which help maintain homeostasis. Commensals can control pathogenic microbes through competition for nutrients, production of antimicrobial molecules, and modulation of immune cells. The microbiota also promotes protective immunity by enhancing the host's ability to respond to pathogens. The microbiota's role in immune regulation is evident in the induction of regulatory responses and the maintenance of tissue homeostasis. Commensals can influence the immune system through various mechanisms, including the production of antimicrobial peptides and the modulation of immune cell function. The microbiota is essential for the host's effector response. Changes in the microbiota can affect local and systemic immune responses. The microbiota's ability to calibrate systemic immunity has significant implications for immunotherapy. The microbiota can enhance the effectiveness of treatments by modulating immune responses. The microbiThe microbiota plays a fundamental role in the induction, training, and function of the host immune system. The immune system has evolved to maintain a symbiotic relationship with the microbiota. When functioning optimally, the immune system-microbiota alliance allows the induction of protective responses to pathogens and the maintenance of regulatory pathways for tolerance to innocuous antigens. However, in high-income countries, overuse of antibiotics, changes in diet, and elimination of constitutive partners like nematodes may have selected for a microbiota that lacks resilience and diversity, contributing to the rise of autoimmune and inflammatory disorders. The human body is a meta-organism composed of the host and its symbiotic microbiota. The microbiota, which includes bacteria, fungi, viruses, and other microbes, plays a critical role in host physiology. The immune system, composed of innate and adaptive components, has evolved to maintain symbiosis with the microbiota. The microbiota promotes and calibrates multiple aspects of the immune system. The immune system-microbiota alliance interweaves innate and adaptive immunity in a dialog that selects, calibrates, and terminates responses. However, the acquisition of a complex immune system and its reliance on the microbiota have led to pathologies such as allergies, autoimmune, and inflammatory disorders. Alteration of the microbiota due to antibiotic use, diet, and elimination of constitutive partners has transformed microbial allies into potential liabilities. The microbiota contributes to the postnatal development of the immune system, which in turn helps contain it. Studies in germ-free animals show that the microbiota is crucial for secondary lymphoid structure development. The microbiota also contributes to the fortification of the intestinal barrier through mechanisms like epithelial cell maturation and angiogenesis. The microbiota plays a critical role in maintaining the balance between the host and its commensals. The "mucosal firewall" includes the epithelial barrier, mucus layer, IgA, and DCs and T cells, which limit the passage of commensals to the gut-associated lymphoid tissue. The microbiota induces regulatory T cells (Tregs) and Th17 cells, which help maintain homeostasis. Commensals can control pathogenic microbes through competition for nutrients, production of antimicrobial molecules, and modulation of immune cells. The microbiota also promotes protective immunity by enhancing the host's ability to respond to pathogens. The microbiota's role in immune regulation is evident in the induction of regulatory responses and the maintenance of tissue homeostasis. Commensals can influence the immune system through various mechanisms, including the production of antimicrobial peptides and the modulation of immune cell function. The microbiota is essential for the host's effector response. Changes in the microbiota can affect local and systemic immune responses. The microbiota's ability to calibrate systemic immunity has significant implications for immunotherapy. The microbiota can enhance the effectiveness of treatments by modulating immune responses. The microbi