Avidity of IgA antibody to Escherichia coli polysaccharide and diphtheria toxin in breast milk from Swedish and Pakistani mothers.

The avidity of breast milk IgA antibody was studied with the aid of thiocyanate elution of antibody from solid-phase bound E.coli polysaccharides and diphtheria toxoid. The relative avidity index for each sample was determined by the molarity of thiocyanate required to elute 50% of the bound IgA antibody under conditions of antigen excess. Milk samples collected from Pakistani mothers during early lactation (2-4 weeks after delivery; n = 12) had a significantly lower median relative avidity index of IgA antibody to E.coli antigens than did early lactation samples from Swedish mothers (n = 11; avidity indices 1.78 M and 2.65 M; P less than 0.02). Samples collected from Pakistani mothers in mid-lactation showed a significant rise in the relative avidity index to a median of 2.50 M (P less than 0.01), with a subsequent fall in late lactation (28-36 weeks after delivery) to 1.75 M (P less than 0.01). Milk samples from Pakistani mothers in mid-lactation (n = 12) also had a lower median relative avidity index of IgA antibody to diphtheria toxoid than did samples from Swedish mothers (n = 14; avidity indices 2.35 M and 4.30 M; P less than 0.002). The lower avidity of breast milk IgA in Pakistani mothers in comparison with Swedish mothers may arise from differences in antigen exposure or nutritional status or could possibly be genetically determined.