Many helminth infections drive a specialized functional program in macrophages termed as alternative activation, associated with expression of distinct gene products including Arginase-1, Chitinase-3-like protein 3 (also known as Ym-1) and Resistin-like molecule-α (RELM-α, otherwise FIZZ-1).55 These markers of alternative activation were strikingly elevated among the expanded peritoneal macrophage populations in the SJL and BALB/c strains, in which 20–40% expressed Chi3L3 or RELMα within 7 days of infection (Figures 5a and b). Here, ARG1 is linked to helminthiasis.