The association of purified, iodinated bovine immunoglobulins with the various fractions of whole milk and with cell-free skim milk and with two different mammary cell populations was studied. Associations in whole milk were concentration independent over a 6-fold range and revealed that SIgA and IgM were 5-fold more prevalent in milk fat than IgG1 and IgG2; the concentration of IgM and SIgA was 3-fold and 2-fold higher in fat than whey, respectively. A significant proportion of the IgM, ca 20% and to a lesser extent IgG2, ca 10%, were found in association with the casein pellet. Greater than 85% of the IgG2, >90% of the IgG1, ≅80% of the SIgA and 70% of the IgM were found in milk whey. Isoelectric precipitation of casein significantly reduced the amount of IgM which associated with fat. When labelled Igs were incubated with milk leukocytes alone, only SIgA and IgM became significantly associated with them. However, when 106 cells were added, the amount of SIgA and IgM in the casein-cell pellet was not additive, although the increase for IgM was significant. These Igs also associated with the casein pellet of cell-free skim milk. When whole milk was used, the milk fat competed with cells and casein for association with SIgA and IgM. Homogenization of the fat layer from normal milk with or without added cells, caused significant release of the Igs which sedimented in the pellet.
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