A<unk>  is a small pocket of fluid within the upper layers of the skin.<unk> s can be filled with blood (known as blood<unk> s) or with pus (if they become infected). However, most<unk> s are filled with a clear fluid called serum. Serum is the part of the blood that remains after red blood cells and clotting agents have been removed.
A<unk>  usually forms because the outer layer of the skin has become damaged. Fluid collects under the damaged layer of skin, cushioning the tissue underneath, protecting it from further damage and allowing it to heal.
A blood-blister usually forms when a small blood vessel close to the surface of the skin ruptures (breaks) and blood leaks into a tear between the layers of skin. This can happen if the skin is crushed, pinched or squeezed very tightly.<unk> s can also form as the result of certain medical conditions.
Blister
Sulcus 
Eyelash
Chimera 
Formothion
Prosector's wart
Phlegmon
Deaf-mute