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The authors describe three accidental deaths resulting from occupational hazards involving environmental gas alterations. One involved the displacement of oxygen caused by leakage of liquid nitrogen during the installation of a magnetic resonance imaging system. Two involved elevated environmental carbon dioxide concentrations: dry ice sublimation in a walk-in refrigerator in a research laboratory, and activation of a carbon dioxide fire alarm-extinguisher system by a woman locked in a bank vault. The autopsy findings, scene investigations, and certifications of these deaths, as related to the mechanisms of death, are discussed.

S U M M A R YDuring the initiation of an immune response, antigen-presenting cells employ MHC class II antigens as key molecules to present small peptides to CD4-positive lymphocytes. The invariant chain (Ii; CD74) plays a critical role in this process by influencing the expression and peptide loading of the MHC class II molecules. Therefore, coordinate expression of these molecules is believed to play an important role in antigen presentation. This study explores the expression of these molecules in fetal tissues. Formalin-fixed, paraffinembedded multi-organ tissue blocks from aborted fetuses (age range 7-22 weeks) were immunostained for Ii/CD74 and MHC class II antigens using commercially available monoclonal antibodies for Ii/CD74 (LN2) and MHC class II antigens (LN3), respectively. Coordinate staining for Ii/CD74 and MHC class II antigens was seen in the skin, proximal renal tubules, tips of small intestinal mucosa, and cells of the reticuloendothelial system, including the spleen and thymus. Expression of Ii/CD74, but not of MHC class II antigens, was seen in pulmonary alveolar epithelium in all cases and in testicular Leydig cells (11 of 11 testes examined). The distribution and intensity of staining did not change significantly with age. In conclusion, this study describes distribution of Ii/CD74 and MHC class II antigens in human fetal tissues. Coordinate expression of Ii/CD74 and MHC class II antigens was identified in most fetal tissues, but there were also notable exceptions. In all cases this took the form of expression of Ii/ CD74 in the absence of MHC class II expression. Discordance was particularly striking in pulmonary alveolar epithelium and testicular Leydig cells. This suggests that the Ii/CD74 molecule has functional roles in addition to its role in antigen presentation.

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