We have recently developed densely glycosylated fluorescent quantum dots (glycan-QDs) as new mechanistic probes for MLGIs. These glycan-QDs were not only able to quantify MLGI affinity via a ratiometric QD sensitized dye Förster resonance energy transfer (FRET) readout, but also dissect the exact binding modes and affinity enhancing mechanisms of MLGIs via hydrodynamic size analysis and STEM imaging of lectin binding induced glycan-QD assemblies. Here, two closely-related, immunologically important tetrameric glycan-binding viral receptors, DC-SIGN and DC-SIGNR (collectively denoted as DC-SIGN/R hereafter), were used as model lectins due to their differences in viral transmitting properties despite sharing 77% amino acid identity, the same tetrameric architecture and identical monovalent mannose-binding motifs. Using QDs bearing -1,2-manno-biose (DiMan)-glycans (QD-DiMan) as probes, we were able to extract key structural and mechanistic information for DC-SIGN/R related MLGIs. We revealed that, although both lectins bound multivalently with QD-DiMan, it was achieved through different binding modes, resulting in very different affinities. DC-SIGN was found to simultaneously bind to the same QD-DiMan with all 4 carbohydrate-recognition-domains (CRDs), giving rise to strong binding (apparent equilibrium binding dissociation constant, 𝐾 d , = 2.1 nM). However, DC-SIGNR was found to crosslink between QD-DiMan particles, with a much weaker affinity (𝐾 d = ~633 nM, ~300-fold weaker than that of DC-SIGN). Additionally, DC-SIGN binding, monitored via FRET, was detected at very low protein to QD molar ratios (PQRs) and increased linearly with PQR till the QD surface was fully saturated with protein. However, for DC-SIGNR, due to low affinity and crosslinking binding nature, saturation occurred at much higher PQR and significant binding was only observed as PQR was increased above a certain threshold.