What diluent reduces allergen adsorption to vial surfaces?

Human serum albumin is the correct choice because it acts as a stabilizing agent when mixed with allergens in a vial. The presence of human serum albumin reduces the adsorption of the allergen molecules to the surface of the vial, which can be an issue with other diluents. By minimizing this adsorption, the allergen remains more available and active for subsequent use in immunotherapy, ensuring better efficacy in treatment.

Saline solution, while commonly used in many medical contexts, doesn’t have the same stabilizing properties that human serum albumin does, which limits its ability to prevent allergen adsorption effectively. Buffer solutions serve primarily to maintain pH levels but do not significantly influence the adsorption characteristics of allergens. Glycerol, on the other hand, can provide some stabilization and viscosity but isn't specifically noted for reducing adsorption as effectively as human serum albumin. Thus, the unique properties of human serum albumin make it the best choice in this context for reducing allergen adsorption.