Complement haemolytic activity (classical and alternative pathways), C3, C4 and factor B titres in healthy children

Values of complement lytic activity of classical and alternative pathways, assessed by measuring the time required to lyse 50% of target red blood cel ls, and the concentration of complement components C3, C4 and factor B were estimated in the sera of 103 healthy children aged 3 to 14 y. Age-dependen t variations were seen in the C3 and factor B concentrations, but not in C4 , with the highest values found among 5-6-y-old children. Variations in cla ssical and alternative lytic activity were not detected in this group of ch ildren, although the values are significantly different from our previously published data on adults, using the same kinetic assay (1). We also evalua ted the relationship between the lytic activity of the classical (CPT) and alternative pathways (APT) and the levels of complement components. There w ere significant correlations between: APT and factor B, APT and C3, C3 and C4, C3 and factor B, and C4 and factor B concentrations. The normal ranges measured here can be used in the initial screening of Brazilian children pr esenting diseases involving the complement system. This study also contribu tes to a better understanding of the complement system ontogeny.