C3/C4 concentration ratio reverses between colostrum and mature milk in human lactation.

The levels of complement fractions C3 and C4 were assayed in human milk in a classic nephelometric assay adapted to this secretion. Concentrations of these molecules were measured in 667 milk samples obtained sequentially from 76 volunteer lactating mothers during the first 12 weeks of lactation. Immunonephelometry was performed using skimmed milk samples diluted 10 times and yielded reproducible (coefficients of variation in within- and between-run precision lower than 9% for C3 and than 14% for C4) and accurate (linear recovery in dilution-overloading assay) data. High concentrations (mean +/- SE) were found for C3 (199.32+/-16.35 mg/L) and C4 (113.42+/-11.16 mg/L) in colostrum samples (n = 159; days 1-5). A significant (P<0.001) and rapid decrease was observed in transitional milk samples (n = 198; days 6-14), containing 57.71+/-5.18 and 72.39+/-4.98 mg/L of C3 and C4, respectively. Stable lower levels were noted in mature milk samples (n = 310; days 15-84) at 30.36+/-1.57 mg/L for C3 (P<0.001) and 53.38+/-3.61 mg/L for C4 (P<0.05). The decrease rate was different for C3 and C4, yielding a reversal of the C3/C4 ratio between colostrum and more mature milk.