Turk et al. (2021) found that proteins involved in host defense such as α2- macroglobulin (A2M), α1-microglobulin/bikunin protein (AMBP), proteins involved in transport such as apolipoproteins AII and F (APOA2, APOF), and retinol-binding protein 4 (RBP4), are increased in milk but reduced in the serum of mastitis cows26. Here, RBP4 is linked to mastitis.