My first cat (many years ago) suddenly started pulling his fur out by the
back legs.  It turned out that he had fleas, and they were deeply nested in
the groin area -- I couldn't even find any signs of them elsewhere.  The vet
said there was an allergic reaction.  When we got rid of the fleas, the
problem with pulling his fur (actually, biting at it) also disappeared.

MaryL

Mine licked her fur off her belly, bit off one front paw, both back flanks
and a bit on her side.  Vet could find no reason for it; definitely wasn't
fleas or ringworm.  It's almost a year later and all of the fur has grown
back except a bit on her belly.

Absotively. Its called "Allergic Dermatitis".

http://www.maxshouse.com/Dermatology-Illus/allergic_dermatitis.jpg

Flea saliva contains about dozen allergens.  Some cats develop horrendous
reactions from flea bites- that's called "Flea Allergy Dermatitis" or "Flea
Bite Hypersensitivity".

Cats can also develop very serious allergic reactions from one or more
ingredients or protein sources in cat food:

http://www.maxshouse.com/Dermatology-Illus/Food_allergy-dermatologic-002.jpg

Cats can also lick off their fur from stress.

Phil