Major Depressive Disorder (MDD) is a complex disorder with multiple interacting pathogenic factors including genetic predisposition, stress, and comorbidities. The review explores several hypotheses related to MDD, such as the monoamine hypothesis, HPA axis dysfunction, cytokine involvement, and neuroinflammation, highlighting the complex interplay between the brain and other organ systems like the gut and liver, in addition to outlining potential biomarkers and treatments.

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This is a comprehensive review covering a wide range of topics relevant to Major Depressive Disorder. While it serves as a good overview, it lacks critical analysis and synthesis of the literature. The figures could be improved.