Abstract
Under ordinary conditions, C3 and factor B interact constantly in order to generate C3b and prime the APC. This interaction is closely regulated by the activity of two other serum proteins, β1H and C3bINA. In the older child and adult, this regulation results in a serum ratio of C3 + B/β1H + C3bINA (determined by RID) which is surprisingly constant at 1.06 + 0.08 (mean + S.D.; n=153). In the newborn, serum levels of all 4 of these proteins are normally reduced. Despite this physiologic hypocomplementemia, umbilical cord blood shows a much higher ratio of these components with nearly 70% (n=200) of neonates being greater than 1.3. These data suggest that the activity of β1H and C3bINA in regulating the interaction of C3 and factor B is more potent in the newborn. Measurement of specific functional β1H activity revealed little difference between the adult and newborn in the number of β1H molecules required to inactivate 1 effective C3b molecule. By contrast, specific C3bINA functional assays demonstrate that the newborn requires 50-75% fewer C3bINA molecules than the adult in order to inactivate 1 effective C3b molecule. Thus, the neonate appears to have an altered form of C3bINA which is 2-3 times more potent than its adult counterpart. This altered molecule could allow for less C3b deposition on an offending agent and, therefore, less efficient opsonization.
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Spitzer, R., Stitzel, A., Blinder, E. et al. 969 PHYSIOLOGIC MODIFICATION OF THE ACTIVITY OF THE ALTERNATIVE PATHWAY OF COMPLEMENT (APC) IN THE NEONATE: A POTENTIAL MECHANISM FOR DEFECTIVE OPSONIZATION. Pediatr Res 15 (Suppl 4), 604 (1981). https://doi.org/10.1203/00006450-198104001-00994
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DOI: https://doi.org/10.1203/00006450-198104001-00994