And fibronectin, present in large quantities in colostrum, can make certain phagocytes more aggressive so that they will ingest microbes even when the microbes have not been tagged by an antibody. As is true of defensive molecules, immune cells are abundant in human milk. They consist of white blood cells, or leukocytes, that fight infection themselves and activate other defense mechanisms. The most impressive amount is found in colostrum. Most of the cells are neutrophils, a type of phagocyte that normally circulates in the bloodstream.
Some evidence suggests that neutrophils continue to act as phagocytes in the infant's gut.
Yet they are less aggressive than blood neutrophils and virtually disappear from breast milk six weeks after birth. So perhaps they serve some other function, such as protecting the breast from infection. The next most common milk leukocyte is the macrophage, which is phagocytic like neutrophils and performs a number of other protective functions. Macrophages make up some 40 percent of all the leukocytes in colostrum. They are far more active than milk neutrophils, and recent experiments suggest that they are more motile than are their counterparts in blood.