Milk is a usually white liquid food (but can be shades of yellow, cream, pink, or brown) produced by the mammary glands of lactating mammals. It is the primary source of nutrition for young mammals before they are able to digest solid food. Milk contains many nutrients, including calcium and protein, as well as lactose and saturated fat; the enzyme lactase is needed to break down lactose. Immune factors and immune-modulating components in milk contribute to milk immunity. The first milk, which is called colostrum, contains antibodies and immune-modulating components that strengthen the immune system against many diseases.