The gut microbiota, immune system, and nervous system interact in health and disease. The gut microbiota influences host physiology, including immune development and brain function. Studies show the microbiota plays a critical role in brain development and behavior, with the immune system emerging as a key regulator. Intestinal microbes affect the maturation and function of CNS immune cells and peripheral immune cells, which regulate responses to neuroinflammation, brain injury, autoimmunity, and neurogenesis. Both the gut microbiota and immune system are implicated in neurodevelopmental, psychiatric, and neurodegenerative diseases such as autism, depression, and Alzheimer's disease. Microglia, the most abundant CNS immune cells, are influenced by the microbiota. GF mice show altered microglial morphology and function, with increased immature microglia and reduced expression of genes related to immune responses. The microbiota also affects astrocytes, which regulate blood-brain barrier integrity and neuroinflammation. Gut microbes modulate astrocyte activity via microbial metabolites that activate aryl hydrocarbon receptors, influencing neuroinflammation. Innate and adaptive immune cells in the CNS, including perivascular macrophages, T cells, and mast cells, are also influenced by the microbiota. The microbiota modulates peripheral immune responses, which can affect CNS inflammation and injury. Gut bacteria influence CNS inflammation through modulation of pro- and anti-inflammatory immune responses. The microbiota also regulates the pathogenesis and resolution of CNS injury, with probiotics and immune cells playing a role in recovery. The gut microbiota influences the development of neurodevelopmental disorders such as autism, with altered microbiota composition and immune responses contributing to behavioral abnormalities. The microbiota also affects stress, anxiety, and depression, with altered gut microbiota linked to behavioral impairments and changes in immune function. The microbiota influences neurodegenerative diseases such as Alzheimer's and Parkinson's, with gut bacteria contributing to amyloidosis and neuroinflammation. The gut microbiota communicates with the CNS through various mechanisms, including immune signaling, neurotransmitter modulation, and vagus nerve stimulation. The microbiota influences neuroendocrine pathways, with the HPA axis playing a role in stress responses and intestinal physiology. The microbiota also affects neuronal signaling and neurotransmitter pathways, with gut microbes influencing the production of serotonin, GABA, and other neurotransmitters. Overall, the microbiota, immune system, and nervous system interact in complex ways to regulate brain function and behavior. Understanding these interactions is critical for developing new therapeutic strategies for neurological diseases.The gut microbiota, immune system, and nervous system interact in health and disease. The gut microbiota influences host physiology, including immune development and brain function. Studies show the microbiota plays a critical role in brain development and behavior, with the immune system emerging as a key regulator. Intestinal microbes affect the maturation and function of CNS immune cells and peripheral immune cells, which regulate responses to neuroinflammation, brain injury, autoimmunity, and neurogenesis. Both the gut microbiota and immune system are implicated in neurodevelopmental, psychiatric, and neurodegenerative diseases such as autism, depression, and Alzheimer's disease. Microglia, the most abundant CNS immune cells, are influenced by the microbiota. GF mice show altered microglial morphology and function, with increased immature microglia and reduced expression of genes related to immune responses. The microbiota also affects astrocytes, which regulate blood-brain barrier integrity and neuroinflammation. Gut microbes modulate astrocyte activity via microbial metabolites that activate aryl hydrocarbon receptors, influencing neuroinflammation. Innate and adaptive immune cells in the CNS, including perivascular macrophages, T cells, and mast cells, are also influenced by the microbiota. The microbiota modulates peripheral immune responses, which can affect CNS inflammation and injury. Gut bacteria influence CNS inflammation through modulation of pro- and anti-inflammatory immune responses. The microbiota also regulates the pathogenesis and resolution of CNS injury, with probiotics and immune cells playing a role in recovery. The gut microbiota influences the development of neurodevelopmental disorders such as autism, with altered microbiota composition and immune responses contributing to behavioral abnormalities. The microbiota also affects stress, anxiety, and depression, with altered gut microbiota linked to behavioral impairments and changes in immune function. The microbiota influences neurodegenerative diseases such as Alzheimer's and Parkinson's, with gut bacteria contributing to amyloidosis and neuroinflammation. The gut microbiota communicates with the CNS through various mechanisms, including immune signaling, neurotransmitter modulation, and vagus nerve stimulation. The microbiota influences neuroendocrine pathways, with the HPA axis playing a role in stress responses and intestinal physiology. The microbiota also affects neuronal signaling and neurotransmitter pathways, with gut microbes influencing the production of serotonin, GABA, and other neurotransmitters. Overall, the microbiota, immune system, and nervous system interact in complex ways to regulate brain function and behavior. Understanding these interactions is critical for developing new therapeutic strategies for neurological diseases.