Relationship among leukocyte phagocytosis, milk immunoglobulins, and susceptibility to intramammary infection.

A study was conducted to determine the variation among cows (n = 7 and among quarters within cows in the ability of milk to support phagocytosis, immunoglobulin (Ig) concentration, and the response to intramammary challenge exposure with live Staphylococcus aureus. Before challenge exposure, the average concentration (milligrams/milliliter) of each class of Ig was: IgA = 0.05, IgG1 = 0.42, IgG2 = 0.06, and IgM = 0.05. The percentage of phagocytosis of S aureus by milk neutrophils was significantly correlated with IgA (r = 0.59; P < 0.01), IgG1 (r = 0.52; P < 0.01), and agglutination titer (r = 0.68; P < 0.01). Agglutination titer was significantly correlated with IgA (r = 0.52; P < 0.01) and IgG1 (r = 0.63; P < 0.01). Bovine serum albumin was significantly correlated with IgA (r = 0.55; P < 0.01) and IgG2 (r = 0.49; P < 0.01). An IgG1/IgG2 ratio of 7.0 in milk, as compared with an IgG1/IgG2 ratio of 0.7 in serum, indicated a selective transport for IgG1. After challenge exposure, the IgG1/IgG2 ratio in infected and noninfected quarters was 12.9 and 16.0, respectively. The fact that neither the bovine serum albumin nor the direct microscopic somatic cell count in the noninfected quarters changed indicated that the increase in the IgG1/IgG2 ratio was due to an increase in selective transport in all quarters and not to a change in permeability due to a generalized inflammatory response. The correlation between the number of challenge exposures required to produce an infection was significantly correlated with percentage of phagocytosis (r = 0.70; P < 0.01).