Non-clonal mast cell activation disorder (syndrome) (NC-MCAD) and autoimmune/inflammatory syndrome induced by adjuvants (ASIA) are complex syndromic and spectral clinical entities which have contemporaneously garnered considerable attention and rightfully so. NC-MCAD presents with a predominantly allergic profile, and causative clinical variables may or may not be identified. Manifest NC-MCAD illnesses can be complex and involve several organ systems. Such diversity may be in part explained by the ubiquity and plasticity of the mast cell. ASIA, due to its broad definition, continues to expand as a clinical entity, especially with the greater recognition of what might constitute adjuvancy. Such diversity has the potential to overly stretch the boundaries for scientific investigation. For both NC-MCAD and ASIA, the inciting molecular mechanisms require elucidation. Future research would benefit from greater precision for clinical diagnosis and risk factor identification, and the latter would be potentially facilitated by the discovery of reliable laboratory markers. Some patients with ASIA may present with clinical features that may be identified as NC-MCAD. This review examines the potential spectral nexus of NC-MCAD and ASIA and considers the role of the mast cell in such a possible interface.