Invasion via sialic acid (SA)-dependent or -independent pathways. The PfRh1/2a/2b proteins are represented by coloured blocks as shown. Erythrocyte binding proteins (EBAs) and their receptors are shown in orange. The limited space model proposes that the apical tip of the merozoite is a confined space such that few PfRh proteins can be inserted. As a result, the PfRh proteins actually making contact with the erythrocyte reflect the proportion of each PfRh protein produced by that parasite. For example, the W2mef parasite is SA-dependent as its PfRh1 gene is amplified threefold, leading o a ratio of PfRh1:2a:2b of 3:1:1. Triglia T, Duraisingh MT, Good RT, Cowman AF. Reticulocyte-binding protein homologue 1 is required for sialic acid-dependent invasion into human erythrocytes by Plasmodium falciparum. Mol Microbiol. 2005 55(1):162-74. PMID: 15612925.