Scars form after destruction of some part of the dermis. Like an erosion, only deeper, penetrating at least part of the dermis. The causes are the same as for erosions. A small, fluid-filled spot less than 0.2 of an inch in diameter. Swelling in the skin that produces an elevated, soft, spongy area that appears relatively suddenly and then disappears.
Wheals are common allergic reactions to drugs, insect bites, or something that touched the skin.
Excerpted with permission from pages 950 to 952 of The Merck Manual of Medical Information.