Sheep red blood cells which have been washed free of sheep complement proteins, but were not coated with hemolysin antibodies. These cells have traditionally been used as negative controls for assays of the human classical and alternative complement pathways in serum samples (Morgan, B.P. (2000); Dodds, A.W. and Sim, R.B. (1997)). Most human serum samples have small amounts of natural antibodies (usually IgG) to sheep antigens and at high concentrations of serum will agglutinate Es and activate the classical pathway leading to lysis. This rarely occurs at dilutions used in the CH50 titration assay where serum is diluted more than 1/100.
Es are supplied at assay-ready concentrations in the traditional buffer used in CH50 assays (GVB++). They are prepared fresh every Monday morning and need to be ordered the previous Friday in order to receive them the next week. They are shipped Monday afternoon by overnight courier for delivery on Tuesday (or Wednesday for most international shipments). They can usually be used for 2 weeks after preparation. They are shipped cold, but are not harmed by extended periods at room temperature (note that they circulate 60-90 days a 37oC in vivo). They should be washed once before each use (5 min at 500 to 1000 x g at 4oC) and resuspended in GVB++ to reduce background. This procedure may also be used to concentrate the cells.
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