Breastfeeding accounts for up to 40% of all infant human immunodeficiency virus (HIV) type 1 infections in resource-limited settings, where prolonged breastfeeding is the only available and safe infant feeding option [1, 2]. However, most breastfed infants remain uninfected even after prolonged exposure to breast milk [3-5]. The factors in breast milk that protect the majority of breastfed infants of HIV-infected mothers from infection remain largely undetermined. Breast milk contains a multitude of immune parameters, including immunoglobulins, antimicrobial substances, pro-and anti-inflammatory cytokines, and leukocytes . Moreover, breast milk not only provides passive protection, but also can directly modulate the immunological development of the infant [7, 8]. Cell-mediated immunity in breast milk has not been as extensively studied as humoral immunity, described in Chap. 10. There is, however, increasing interest in the role that lymphocytes, macrophages, and other immune cell types play, both in innate and in adaptive breast milk immunity . © 2012 Springer Science+Business Media New York.