Human psychoneuroimmunology research has demonstrated that stress, depression, and close relationships reliably shape immune and endocrine function in ways that matter for health. Across studies of examination stress, laboratory stressors, marital discord, cancer survivorship, and dementia caregiving, psychosocial adversity predicts more infections, weaker vaccine responses and faster erosion of vaccine protection, slower wound healing, heightened inflammation, and accelerated cellular aging. Depression also sensitizes immune function, producing larger inflammatory responses when individuals encounter stressors. Loneliness, low support, and distressed relationships can amplify stress reactivity and are linked to greater inflammatory and metabolic vulnerability, including postprandial inflammatory and endothelial responses. More recent work has extended these pathways to the gut microbiome and intestinal permeability (leaky gut), integrating microbial, neuroendocrine, and immune mechanisms. Collectively, the evidence supports a biobehavioral model in which social stress accelerates immune aging and increases risk for inflammation-related disease, while behavioral and nutritional interventions can modify these trajectories.