Upon fixation, all intracellular trafficking of molecules is stopped. Drying and lipid solvent treatments (acetone, ethanol, etc.) create massive holes in the greater sub/cellular structure. This allows antibodies to cross membranes (extracellular, nuclear, etc.) in fixed cells. There is no detergent that causes an antigen that was previously hidden (masked) to be exposed (unmasked). Low levels of detergents such as Tween 20 in the washing solutions reduce the surface tension and allow the tissues/cells to remain wet.
