Apart from their well-established presence in the conducting airways, mast cells in the human respiratory system are also abundant in other key compartments such as the alveolar parenchyma or, as illustrated here, respiratory lymphoid tissue. Apart from their well-established presence in the conducting airways, mast cells in the human respiratory system are also abundant in other key compartments such as the alveolar parenchyma or, as illustrated here, respiratory lymphoid tissue. Immunohistochemical staining of lymphoid mast cells (blue stained tryptase) can be seen together with abundant structural cells containing the potent mast cell activator interleukin-33 (brown stain). HEV: high endothelial venule. Scale bar=20 μm. Image courtesy of J.S. Erjefält and P. Siddhuraj (Unit of Airway Inflammation, Lund University, Lund, Sweden). Jonas S. Erjefält Eur Respir Rev 2014;23:299-307 ©2014 by European Respiratory Society