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''irritant contact dermatitis''
common - non-allergic reaction due to weak acids or alkalis (e.g. detergents) 
Often seen on the hands. Erythema is typical, crusting and vesicles are rare

''allergic contact dermatitis'' 
type IV hypersensitivity reaction 
Uncommon - often seen on the head following hair dyes. 
acute weeping eczema which predominately affects the margins of the hairline rather than the hairy scalp itself. 
Topical treatment with a potent steroid is indicated

Cement is a frequent cause of contact dermatitis. The alkaline nature of cement may cause an irritant contact dermatitis whilst the dichromates in cement also can cause an allergic contact dermatitis

response to external agents
itchy rash - redness scaly crusting, swelling , thickening,
associated finger nail dystrophy

allergic - type 4 reaction to allergen, maybe on palms, look for vesicles, may occur at distant sites where Tcells are.

Irritant - non-immune reaction to irritant - more common, often on backs of hands (check occupation), limited to sites of exposure

beware allergy to aqueous cream (common)

need to use enough steroid 
remember palms are thick so seldom problem with skin thinning
1 fingertip unit should cover a whole hand

Flare
may be bacterial infection - needs Fluclox or E'mycin
oral pred may help - 2/52 but may need systemics via 2ndary care (eg Ciclosporin or ?Alitretinoin)

RAST - for type 1 reactions
Skin prick test - for pollen, bee sting or food
Patch test - for type 4 reactions