Dimeric IgA, also called secretory IgA, is formed by mucosal epithelium. Of the choices, colostrum is the only example of a secretory fluid.
With breast-feeding, dimeric IgA contained in the breast milk is transferred to the newborn. Upon reaching the gut of the newborn, it is able to provide mucosal protection from pathogens. This is an example of passive transfer of immunity. Unlike IgA, IgG is found circulating in maternal serum and can cross the placenta into fetal circulation. This provides the newborn with protection during the first few months following birth. Eventually, maternally-derived IgG present in the fetus is degraded, and fetal levels of IgG fall. However, at around six months, the immune system of the fetus begins synthesis of its own antibodies.
Loureiro et al. illustrate the importance of passive transfer of immunity via breast milk by showing that maternal antibodies may protect the infant gastrointestinal tract from enteropathogenic E. coli infection by inhibiting the adherence and colonization of this bacteria in the intestine.
Pitcher-Wilmott et al. discuss the subclasses of IgG that cross the human placenta and compare maternal levels of antibodies during pregnancy. Maternal donation of immunoglobulins is essential in protecting the infant which has immature immune function.
Illustration A displays a sample of colostrum (left) compared to normal breast milk (right).
Answers 2 and 3: Blood does not contain a high concentration of dimeric IgA (as the circulating form is typically in the monomeric form).
Answer 4: Fetal bone marrow does not contain high levels of dimeric IgA.
Answer 5: Fetal thymus does not contain high levels of dimeric IgA.
Loureiro I, Frankel G, Adu-Bobie J, Dougan G, Trabulsi LR, Carneiro-Sampaio MM. Human colostrum contains IgA antibodies reactive to enteropathogenic Escherichia coli virulence-associated proteins: intimin, BfpA, EspA, and EspB. J Pediatr Gastroenterol Nutr. 1998 Aug;27(2)
PMID:9702647 (Link to Abstract)
Pitcher-Wilmott RW, Hindocha P, Wood CB. The placental transfer of IgG subclasses in human pregnancy. Clin Exp Immunol. 1980 Aug;41(2)
PMID:7438556 (Link to Abstract)