Immunopathology of SIDS

Immunopathological studies of SIDS share the problems of all necropsy based studies of this syndrome: the extent of autolytic changes in the material under study; and the lack of appropriate controls. Despite these problems, several studies have been performed on serum, bronchoalveolar lavage, and pulmonary tissue. Many of these studies have been inspired by the modified anaphylaxis hypothesis, based on the experiments of Coombs and coworkers. Lightly anaesthetised guinea-pigs, which had been sensitised to cows' milk protein, were shown to die after intratracheal challenge. Studies of serum IgE concentrations in SIDS initially indicated raised specific IgE for Dermatophagoides pteronyssinus, Aspergillus fumigatus, and bovine beta-lactoglobulin, but subsequent studies have not sustained these findings. Raised immunoglobulin concentrations in bronchoalveolar lavage fluid have been found in association with SIDS but this probably reflects plasma leakage rather than local secretion. Immunocytochemical analysis of lavage cells performed by the same group revealed no major difference between SIDS cases and controls, although these were limited to four cases. To date, there have been no comprehensive studies of the inflammatory cell content of the pulmonary parenchyma in SIDS. In our own studies, we have examined the mast cell and eosinophil populations in the lungs of 49 cases of infants with SIDS and in 33 infants dying of non-pulmonary causes in the first 18 months of life. We found no difference in mast cell numbers between the groups but there was a striking excess of eosinophils in the lungs of infants dying of SIDS. Because eosinophils can secrete oxygen free radicals and cytotoxic cationic proteins, we regard this as evidence of a potential mechanism for the pulmonary oedema that is characteristic of SIDS. A viral infection which might otherwise have been trivial could therefore prove fatal.