Bronchus-associated and gut-associated lymphoid tissues (BALT and GALT) have both functional and morphologic similarities and are involved in seeding lung, gut, and other mucosal sites with predominantly IgA-containing B cells. Both types of lymphoid tissue are engaged in the regulation and the controlled amplification of immune responses, which vary from positive mucosal responses in both mucosae and peripheral tissues to local mucosal responses and systemic tolerance. Their further involvement in provision of cells destined to reside in the epithelial compartment of the body appears likely but requires further investigation. Their role in the provision of precursors of mucosal mast cells must also be explored further, but some participation in this event appears likely. The mucosa-associated lymphoid tissue (MALT) system appears to be integrated with the systemic immune system but may be considered as separate from it in several functional ways.

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Methods originally employed to demonstrate the heterogeneity of mast cells in the rat and more recently in the human intestine were used to study mast cell heterogeneity in the human bronchial mucosa and lung parenchyma. Thus, a quantitative survey of pulmonary mast cells using different fixation and staining procedures indicated that two distinct mast cell subpopulations (formalin-sensitive and formalin-resistant) are present and have distinct patterns of distribution and abundance. The findings are of potential clinical importance because histochemical heterogeneity may be a marker of functional mast cell differences in humans including differences in responsiveness to antiallergic drugs, as occurs in rats. The findings also indicate that conventional methods of fixation are likely to lead to a gross underestimation of total mast cell numbers.