Tear fluid participates in systemic immunity.

Tear fluid, also referred to as tears or tear film, is an important biological fluid that plays a key role in maintaining ocular surface health and immune homeostasis. Recent studies have found that tear fluid not only participates in the occurrence and development of ocular diseases, but also exerts profound effects in the immune pathological mechanisms of systemic diseases, breaking through the inherent understanding previously held by the scientific community. Immune cells in tear fluid (such as T cells, neutrophils, natural killer cells, macrophages), cytokines, and immunoglobulins can specifically participate in autoimmune diseases (such as Sjögren's syndrome, rheumatoid arthritis, systemic lupus erythematosus, multiple sclerosis, Graves' ophthalmopathy) and systemic diseases (such as Alzheimer's disease, diabetes mellitus, graft-versus-host disease). The dynamic changes in tear fluid components can reflect systemic immune homeostasis imbalance. Tear fluid biomarkers, such as exosomal microRNA (miR)-204, miR-200b-5p, and the protein marker β2-microglobulin, have shown great potential in early disease screening, diagnostic stratification, and therapeutic target discovery. Tear fluid immune component analysis may provide innovative diagnostic tools and therapeutic targets for systemic diseases. Future research should focus on promoting the standardization and clinical transformation of tear fluid testing technologies and their clinical application.