Pulmonary responses to particulate materials capable of activating the alternative pathway of complement

The cellular sequence of events evoked by the intratracheal injection of both respirable mouldy hay dust and zymosan in unsensitized rats and rabbits was markedly similar and contained many features seen in lung biopsies from farmer's lung cases. It is considered that a combination of ability to activate the alternative pathway of complement and the particulate nature of mouldy hay dust and zymosan produced the pulmonary response seen and as such, considerable insight has been gained into mechanisms by which unsensitized individuals respond to dust inhalation. It is suggested that alternative pathway activation may produce an attack of farmer's lung in the absence of precipitins or other forms of hypersensitivity to the dust or its components and that cases designated as 'pulmonary mycotoxicosis' are more rationally explained by this mechanism.