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Inflammation is an important component of numerous cancers and chronic diseases and many inflammatory mediators have been shown to have potential prognostic roles. Tumor-infiltrating mast cells can promote tumor growth and angiogenesis, but the mechanism of mast cell activation is unclear. Early studies have shown that immunoglobulin free light chains (FLC) can trigger mast cell activation in an antigen-specific manner. Increased expression of FLC is observed within the stroma of many human cancers including those of breast, colon, lung, pancreas, kidney, and skin. These overexpressed FLCs are co-localized to areas of mast cell infiltration. Importantly, FLC expression is associated with basal-like cancers with an aggressive phenotype. Moreover, FLC is expressed in areas of inflammatory cell infiltration and its expression is significantly associated with poor clinical outcome. In addition, serum and bronchoalveolar fluid FLC concentrations are increased in patients with idiopathic pulmonary fibrosis (IPF) and hypersensitivity pneumonitis (HP) compared to control subjects. In this review, we provide an update on the role of FLC in the pathogenesis of several lung disorders and indicate how this may contribute to new therapeutic opportunities.