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Figure 2: Factors involved in postnatal intestinal innate immune adaptation. The colonizing microflora and the intestinal mucosa interact in a two-way street to establish life-time tolerance and mutualism between each other. While bacteria activate innate immune pathways in host epithelial and immune cells inducing immune maturation and tolerance, mucosal cells produce factors (antimicrobial peptides AMPs, mucins, immunoglobulin A IgA, etc.) to control the number and the composition of bacteria. Microflora is also influenced by environmental factors (hygiene, lifestyle, etc.). Maternal factors, such as IgA, mucins, oligosaccharides, or other soluble factors, can modulate the microflora, and contribute to improve host immune defense and maturation (maternal immune cells, soluble factors, etc.).