Binding of immunoglobulins and immune complexes to erythrocytes of vertebrates

Previous investigations which have referred to red cell Fc receptor in rabbit, guinea pig. sheep and man, have been extended to other vertebrates, such as reptiles, amphibia, birds and mammalians.

In these investigations, red cells from different animal species have been analyzed. As ligand DNP-BSA anti-DNP non-precipitating antibody, aggregated IgG by the bis-diazotized benzidine method. IgM 7S and non-modified IgG, IgA and IgM were used. The ligand binding to red cells was detected by Coombs test with specific anti-immunoglobulin serum.

The analyzed red cells showed exposed Fc receptor. In the case of human red cells, to render such receptor evident it is necessary to submit the cells to trypsinization.

All the analyzed erythrocytes bind IgM 7S, Ab-Ag complex and aggregated IgG, and some of these cells bind non-modified immunoglobulins, although none of them bind IgM 19S. These bindings were inhibited by Fcγ, Fcμ and Ag-Ab complex, but they were not inhibited by Fabγ. When specific anti-immunoglobulin serum was added to red cells with bound IgM 7S or Ag-Ab, the clusters which formed by nucleated erythrocytes were bigger than those obtained with non-nucleated red-cells.

In the case of chicken (nucleated) and sheep (non-nucleated) erythrocytes. the interaction between¹³¹I-IgM_s-red cells at equilibrium showed a similar K₀ for both red cells (chicken K₀ = 1.8 × 10⁸ L/M; sheep K₀ = 1.1 × 10⁸ L/M). The number of Fc receptors by red cell were 1.23 × 10⁶ and 1.21 × 10⁵ for chicken and sheep erythrocytes, respectively. Taking into account these results, the different behaviour of nucleated and non-nucleated red cells with bound IgM 7S, when they react with anti-IgM antibody, would be the consequence of different avidity for both equilibrated systems and not a particular characteristic of their Fc receptors.