Immunoglobulin G subclasses in human colostrum, milk and saliva

Previous studies have suggested local production of IgG4 in human colostrum and mature milk. We extended these observations to examine all IgG subclasses in mammary secretions and in saliva, a mucosal secretion. In human colostrum and milk, the geometric mean percentages of IgG contributed by IgG2 were 44% and 43%, respectively, and by IgG4, 6% in both. These percentages are significantly increased compared to the contributions in matched plasma, 29% for IgG2 and 2% for IgG4. The contribution of IgG1 (47%) and IgG3 (<4%) were decreased compared to plasma which contained 64% IgG1 and 6% IgG3. Similarly, in salivary secretions the percentages of IgG contributed by IgG2 and IgG4 were increased compared to serum while the percentage of IgG1 was decreased. IgG3 was not measurable in any saliva specimen by the technique used. These data demonstrate that IgG subclass distribution in two separate mucosal secretions is uniquely different from that in matching plasma or serum.