Introduction: It has been pointed out in previous articles in this series that infants are not completely dependent on receiving immune support from mother’s milk in the form of antibodies, the star players of the adaptive immune system (Article 1). Milk also contains multiple components of the innate immune system (Article 1 – Table 1). Several important members of milk’s innate immune system have already been described including lactoferrin in Article 2 and glycomacropeptide, oligosaccharides, glycosaminoglycans, alpha-lactalbumin and lysozyme in Article 3. These molecules can be broadly classified as being proteins or carbohydrates. In this 4th article in the series, milk fat takes to the stage, or at least a subset of milk fats referred to as phospholipids. These molecules form the structure that encapsulates the energy dense triglyceride fraction of fat packaged in the milk fat globule. This envelope is referred to a milk fat globule membrane (MFGM).