Objectives-To test whether secretory immunoglobulin A (sIgA) to human immunodeficiency virus (HIV) antigens in breast milk of HIV-positive women is associated with protection against HIV transmission among breast-fed infants.Study design-Nested, case-control design in which HIV-specific sIgA was measured in breast milk collected from 90 HIV-positive women enrolled in a study in Lusaka, Zambia. Milk samples were selected to include 26 HIV-positive mothers with infected infants (transmitters) and 64 mothers with uninfected infants (nontransmitters).Results-HIV-specific sIgA was detected more often in breast milk of transmitting mothers (76.9%) than in breast milk of nontransmitting mothers (46.9%, P = .009). There were no significant associations between HIV-specific sIgA in breast milk and other maternal factors, including HIV RNA quantities in breast milk, CD4 count, and plasma RNA quantities.Conclusions-HIV-specific sIgA in breast milk does not appear to be a protective factor against HIV transmission among breast-fed infants.Human immunodeficiency virus (HIV) can be transmitted from an HIV-infected mother to her child through breast-feeding. This poses a serious dilemma for health policy-makers. Although complete avoidance of breast-feeding would eliminate the risk of breast-feeding-associated transmission, breast milk substitutes are unaffordable, unavailable, unacceptable, and unsafe for many HIV-infected women in low resource settings. The quantity of viral RNA and cellassociated viral DNA in breast milk from HIV-positive women strongly predicts mother-tochild transmission among breast-fed infants. [1][2][3] What is puzzling, however, is why the majority of breast-fed infants born to HIV-positive mothers remain uninfected despite prolonged exposure. Although breast milk is clearly a route of transmission, human milk is also a rich source of a multitude of innate and specific immune factors that may play a role in modulating the risk of infection. 4 Better understanding of anti-infective properties of breast milk may assist Secretory immunoglobulin A (sIgA) is the predominant immunoglobulin in breast milk and is associated with passive immunity to other (non-HIV) pathogens among breast-fed infants. 5 HIV-specific IgA antibodies are detected in breast milk of a high proportion of HIV-positive, lactating women. [6][7][8] In an in vitro model, sIgA purified from colostrum was able to block one of the pathways involved in HIV penetration across mucosa, that is, transcytosis through epithelial cells, 9 suggesting that sIgA may be related to decreased infectivity of breast milk. Although persistence of HIV-specific IgA and IgM in breast milk was associated with reduced transmission in one study in Rwanda, 10 no protective association was observed in two other studies. 8,11 HIV-specific sIgA has been detected frequently in cervicovaginal samples from exposed but persistently uninfected cohorts of high-risk women, 12-14 suggesting a role for these mucosal responses in resistance to HIV. Purified sIgA from ex...