Immunological protection of the vulnerable marsupial pouch young: two periods of immune transfer during lactation in Trichosurus vulpecula (brushtail possum).

Marsupial young are born with an underdeveloped immune system and are dependent upon passively acquired immune protection provided by the mother's milk. Colostrum and milk samples were collected from the brushtail possum throughout lactation and the concentration of secretory IgA (sIgA), IgG and transferrin was determined by Western blotting. Two periods of immune transfer were identified. The first, a colostral phase, occurs immediately after birth and involves sIgA, IgG and transferrin. During the early lactation stage, pouch young receive milk of a unique composition as they undergo developmental changes in the pouch that occur in utero for eutherian mammals. At the end of this external gestation, the composition of the milk changes (switch phase) to resemble that of eutherian mammals in the late lactation phase. The second transfer of immunity consists of IgG and transferrin, and occurs during the switch phase prior to maturation of the immune response.