Key developments in the understanding of the immune functions of milk and colostrum are reviewed, focusing on their proteinaceous components. The topics covered include the immunoglobulins, immune cells, immunomodulatory substances, and antimicrobial proteins. The contributions of new technologies and the introduction of fresh approaches from other fields are highlighted, as are the contributions that mammary biology research has made to the development of other fields. Finally, a summary of some current outstanding questions and likely future directions of the field are given.
The concept that milk, mammary secretions and the mammary gland have major roles in immune defense is an old one. The bactericidal property of milk was recorded in the scientific literature in the late nineteenth century [1, 2]. Moreover, observations at this time on the ability of milk to provide immunity to the newborn  played a key role in the development of modern immunology. The aim of this review is to provide a modern audience with a timeline for the key discoveries in milk immunology, illustrating how the current understanding of the immune function of milk evolved, and to offer some pointers for the future direction of the field. This review is divided into three sections: the immunoglobulins and immune cells, immunomodulatory components, and antimicrobial components, covering elements of both innate and adaptive immunity, immune defenses in the mammary gland and the participation of the mammary gland in the mucosal defense system. In this we focus largely on the proteinaceous components of milk, some of which are depicted in Fig. 1. Reviews describing physical barriers, the role of probiotics, and the carbohydrate and lipid components of milk that have host defense functions have recently been presented elsewhere.