Peptide-lipid constructs of the structure L-S-F are disclosed, where F is a peptide, S is a spacer covalently linking F to L via an oligomer of ethylene glycol, and L is a diacyl- or dialkyl-glycerolipid (including glycerophospholipids). The spacer ideally has 6 to 14 ethylene glycol repeats, corresponding to PEG with a molecular weight of approximately 250 to 600. Also disclosed is a method of detecting reactive antibodies in serum by contacting serum with cells modified to incorporate a peptide-lipid construct, where the peptide is an epitope of the antibody, and determining the degree of aggluthination of the cells.