Liposome-mediated immunoassays for small haptens (digoxin) independent of complement

A lipid vesicle-mediated immunoassay for small haptens (digoxin) is described. Using a detergent removal procedure for vesicle formation, a water-soluble marker system like alkaline phosphatase is stably entrapped within 150-200 nm unilamellar lipid vesicles composed of egg yolk lecithin and cholesterol. Specific lysis of the lipid vesicles is achieved upon addition of a hapten-melittin conjugate. Inhibition or modulation of this lysis by the hapten-melittin conjugate can then be achieved by adding stoichiometric amounts of high affinity antibody. Finally, the antibody inhibition of hapten-melittin lysis can be modulated by the addition of competing amounts of free hapten. This assay approach is simple, fast, highly sensitive, and versatile such that it can be carried out in either a homogeneous or heterogeneous mode. Furthermore, unlike all other liposome-mediated immunoassays, complement is not required for lysis.