'Husbands' sisters bring up blisters,' but this one wouldn't hurt a fly.
A gall mite (Phytoptus pyri) sometimes severely injures the leaves, on which it forms blisters - the best remedy is to cut off and burn the diseased leaves.
The manifold plate is then heavily punched from one side, so that the opposite face protrudes in broken blisters, which are then hammered down until each becomes a centre of wave propagation.
The numerous small blisters or vesicles thus derived coalesce, forming a large sac full of "blister-fluid."
Blisters also cause local dilatation of vessels, but are usually applied to the skin for inflammation in deep-seated parts, such as the lungs, though they also relieve pain in the joints in acute rheumatism.