It is known to the authors of this review that one of these antibodies, a neutrophil agglutinin directed against HNA-3a, induced acute dyspnoea even in a healthy human volunteer after injection of a small volume of donor plasma. Further details of the mechanism were studied in an ex vivo rabbit lung model (Seeger et al, 1990). Severe vascular leakage was reproduced in isolated rabbit lungs by application of HNA-3a antibodies. In the presence of HNA-3a positive neutrophils and complement, severe lung oedema occurred after a latent period of 3–6 h. In contrast, no such reaction was noted in the absence of HNA-3a antibodies, HNA-3a positive neutrophils, or a complement source. From these experiments it was concluded that leucoagglutinating antibodies and concomitant complement activation are key players in the initiation of TRALI. Complement activation is not thought to occur as a result of neutrophil-antigen interaction. In addition, it was not found to be a prerequisite for TRALI induction in other ex vivo experiments, where the induction of TRALI because of CD177-specific antibodies was dependent on the density of the cognate antigen (see below), but occurred in a complement-free environment (Sachs et al, 2006). It seems reasonable to speculate that antibodies to HNA-3a were capable of priming and activating neutrophils in the model published by Seeger et al (1990). Indeed, it was demonstrated recently that HNA-3a antibodies are able to prime neutrophils in vitro (Kopko et al, 2004; Silliman et al, 2006), as are antibodies to HNA-4a and HNA-2a (Sachs et al, 2004; Sachs et al, 2006). Antibodies directed against the neutrophil-specific surface marker CD177 (HNA-2a) were recently used by ourselves to demonstrate that TRALI induction in an ex vivo rat lung model is dependent on the density of the cognate antigen and can be enhanced efficiently by the addition of fMLP, a substance that mimics the activity of bacterially derived peptides and is used to approximate active infection (Sachs et al, 2006). HNA-2a expression is heterogeneous in man, with HNA-2a being expressed on either a large subpopulation (70%) or a small subpopulation (30%) of neutrophils, varying from individual to individual. We used neutrophils from each group of healthy donors in an ex vivo rat lung model, and application of the corresponding antibody induced TRALI only if CD177 was present on the majority of neutrophils. Obviously, under these conditions, neutrophil-antibody interaction was capable of fully activating the cells. In contrast, if CD177 was present on the minority of cells, no TRALI reaction could be induced by antibody addition. However, if fMLP was added to the buffer in the circuit, TRALI induction was promoted in the presence of neutrophils from individuals expressing CD177 on more than 70% as well as those expressing CD177 on <30% of their cells. These findings indicate that additional stimuli (fMLP) can overcome the inability of a stimulus to activate neutrophils. Thus, it might be deduced from these findings that, if antibodies are present in a blood component, they may be able to induce TRALI in an otherwise healthy individual, whenever the antigen/antibody ratio allows appropriate neutrophil activation. This finding is also in accordance with reports from healthy volunteers developing TRALI upon antibody infusion (Brittingham, 1957; Dooren et al, 1998). However, if the individuals’ neutrophils encounter additional stimuli, as outlined in the previous section (and mimicked by fMLP in our experiments), it appears that TRALI can develop more readily, a finding that is in accordance with the fact that most TRALI patients are not ‘healthy’, but suffer from an active infection or had recent surgery.