Bioactive molecules in human milk are important components of the innate immune system. Differences in cytokines, growth factors and lactoferrin between preterm and term milk are most dramatic in colostrum and early milk and mostly resolve by 4 weeks after delivery (Figure 1B). Leptin is produced by mammary glands, secreted into human milk, and may be important in post-natal growth. Human milk leptin does not appear to differ between preterm and term milk. Bile salt-stimulated lipase activity is similar in term and preterm milk while lipoprotein lipase activity is higher in term milk.