Immunochemical quantitation of serum complement components in SFD and AFD infants.

Components of complement (protein concentrations of C1q, C3, C3-activator, C4, C5 and C9 and whole complement (hemolytic activity)) were measured in sera from full-term small-for-date (SFD) and appropriate-for-date (AFD) infants and their mothers. (1) Half the SFD infants showed lower C1q levels than the AFD infants. (2) SFD infants showed the same levels of C3 as AFD infants. (3) Although SFD infants showed slightly higher levels of C3-activator than AFD infants, there was no significant difference. (4) SFD infants and AFD infants showed the same level of C4. (5) With respect to C5, half the SFD infants showed lower levels than the AFD infants. (6) Levels of C9 in the SFD infants were essentially the same as those in the AFD infants. (7) The amount of protein in every complement component of an infant was smaller than that of the mother. In AFD infants, the infant-maternal ratios were 0.64 for C1q, 0.45 for C3, 0.29 for C3-activator, 0.43 for C4, 0.54 for C5 and 0.12 for C9. (8) The whole complement titers of umbilical cord sera from the AFD infants were approximately one half of those of the maternal sera. This is due to the smaller amount of complement protein itself in the infant sera. (9) The whole complement titers of umbilical cord serum agreeably correlate with gestational weeks and birth weight of the infants. (10) The whole complement titer in full-term SFD infants was lower than that in normal full-term AFD infants.