Major depressive disorder (MDD) affects approximately 8.4% of the U.S. population and is a significant global health issue. The monoamine hypothesis, which posits that MDD is caused by the depletion of monoamine neurotransmitters like serotonin and norepinephrine, has been widely accepted but is now challenged by evidence showing that only one-third of patients respond to monoamine-based antidepressants. Recent research highlights the role of inflammation and cytokines in MDD, suggesting that the condition involves more complex pathways than previously thought. Inflammation, including chronic stress, is a risk factor for MDD, and elevated levels of pro-inflammatory cytokines such as IL-6, TNF-α, and CRP are associated with MDD. Inflammation can affect serotonin production through the kynurenine pathway, leading to depressive symptoms. Toll-like receptors (TLRs) also play a role in the inflammatory response to pathogens and stress, contributing to MDD. Anti-inflammatory treatments, such as those targeting IL-6 or using anti-inflammatory drugs like statins and infliximab, show promise in treating MDD, particularly in patients with treatment-resistant depression. The role of the gut microbiome and the gut-brain axis in MDD is also being explored, with findings indicating that alterations in the gut microbiota can influence mental health. Future research aims to develop more personalized treatment strategies for MDD by integrating inflammatory markers and immune pathways into clinical practice.Major depressive disorder (MDD) affects approximately 8.4% of the U.S. population and is a significant global health issue. The monoamine hypothesis, which posits that MDD is caused by the depletion of monoamine neurotransmitters like serotonin and norepinephrine, has been widely accepted but is now challenged by evidence showing that only one-third of patients respond to monoamine-based antidepressants. Recent research highlights the role of inflammation and cytokines in MDD, suggesting that the condition involves more complex pathways than previously thought. Inflammation, including chronic stress, is a risk factor for MDD, and elevated levels of pro-inflammatory cytokines such as IL-6, TNF-α, and CRP are associated with MDD. Inflammation can affect serotonin production through the kynurenine pathway, leading to depressive symptoms. Toll-like receptors (TLRs) also play a role in the inflammatory response to pathogens and stress, contributing to MDD. Anti-inflammatory treatments, such as those targeting IL-6 or using anti-inflammatory drugs like statins and infliximab, show promise in treating MDD, particularly in patients with treatment-resistant depression. The role of the gut microbiome and the gut-brain axis in MDD is also being explored, with findings indicating that alterations in the gut microbiota can influence mental health. Future research aims to develop more personalized treatment strategies for MDD by integrating inflammatory markers and immune pathways into clinical practice.