Quantitative studies with [125I]IgM anti-Lea

One of two volunteers with anti-Le^a in their plasma developed rises in titre following the intramuscular injection of Le^a glycoprotein.

With partially purified [¹²⁵I]IgM anti-Le^a, it was found that the maximum number of antibody molecules that would bind to red cells of phenotype Le(a+b-) ranged from 4500 to 7300 molecules per cell for different donors.

Lewis substance in serum samples was assayed by estimating its ability to inhibit the subsequent uptake of [¹²⁵I]anti-Le^a on to red cells. Somewhat unexpectedly, no clear correlation was found between the red cell and serum concentrations of Le^a antigen of particular donors. The discrepancy between the amount of Le^a on the red cells and in the serum may be due to the presence in the serum of antigen-bearing molecules (either glycoprotein or glycolipid) which fail to bind to red cells.

IgM anti-Le^a obtained from a single donor had an equilibrium constant of 8.4 × 10⁹ l/mol. but no antibody activity was detectable in subunit preparations. It is concluded that IgM anti-Le^a binds multivalently to each red cell.