MARCO and infection: MARCO (macrophage receptor with collagenous structure) is a class A scavenger receptor encoded by the MARCO gene, which is constitutively expressed by subsets of macrophages and has been shown to be up-regulated in the presence of infection or in inflammatory conditions in vivo (van der Laan et al., 1999; Seta et al., 2001; Milne et al., 2005).