A role for IgM in the in vitro opsonisation of Staphylococcus aureus and Escherichia coli by bovine polymorphonuclear leucocytes.

The phagocytosis and killing of Escherichia coli (strain P4) and Staphylococcus aureus (strain M60) by bovine polymorphonuclear lymphocytes (PMN) suspended in phosphate buffered saline, requires the presence of calcium ions and opsonins. The highest dilution of normal decomplemented adult sera in which the opsonins are still active is approximately 1/1000 for S aureus and 1/200 for E coli. In fetal and precolostral sera heat labile factors are also required for opsonising E coli but these are not required for S aureus. IgG1 and IgG2 from adult sera did not opsonise the bacteria even though receptors for IgG2 anti-erythrocyte antibodies have been reported on bovine and ovine PMN. A systematic separation of adult serum proteins was carried out by salt fractionation, anion exchange and gel filtration chromatography. The results suggest strongly that the opsonin in adult bovine sera is IgM.