Macrophage receptor MARCO, also known as Macrophage receptor with collagenous structure and Marco, is a single-pass type II membrane protein. MARCO is a member of the class A scavenger receptor family and is part of the innate antimicrobial immune system. It is expressed in subpopulations of macrophages in the spleen and the medullary cord of lymph nodes. Although it is expressed on subsets of macrophages, it can be upregulated on other macrophages after bacterial infection. The strategic position of MARCO-expressing cells in lymphoid organs suggests an important role for this bacteria-binding molecule in removal of pathogens. MARCO has a short N-terminal cytoplasmic domain, a transmembrane domain, and a large extracellular part composed of a 75-residue long spacer domain, a 27-residue collagenous domain, and a 99-residue long scavenger receptor cysteine-rich (SRCR) domain. It is possible that cooperation between the SRCR domain and the collagenous domain is needed for high-affinity bacterial binding, or that the SRCR domain has to be in a trimeric form to effectively bind to bacteria.