It is due to the release of chemicals such as histamine by immunity cells within the skin layers. This is known as ‘immediate hypersensitivity’.

Slower acting forms of response by the immune system occur because the population of cells that react against the invading substance or organism (allergen) needs to be built up from a small number of ‘memory’ cells that recognize the allergen.

Two or three days these cells give rise to others that in turn switch on other defense mechanisms such as the release of chemicals that attract the scavenger cells of the immune system. Because of the lag between initial contact and subsequent reaction, this is called ‘delayed hypersensitivity’. The skin reactions underlying allergic contact eczema are of this type.

It is not known why some substances are particularly prone to causing contact allergy, but common culprits are:
nickel - a white metal often used in jewellery, belt buckles, jean studs, metal watch straps, bra fasteners. Many other metals can also cause reactions.
perfumes - Balsam of Peru is a component of fragrances that can cause contact allergy. rubber and related chemicals - shoes and clothing.
potassium dichromate - cement, leather and matches.
hairdressing chemicals (paraphenylenediamine).
medications - lanolin, neomycin (antibiotic), fluidic acid (antibiotic), steroid creams and cetearyl alcohol (used in emollient creams).
epoxy resins - in glues used in hobbies, woodworking and glass fiber construction.
colophony - resin from spruce trees, used in sticking plaster adhesive.
plants - both by direct contact and by contact with airborne particles.
In the food industry a wide variety of foods are known to potentially cause skin reactions. These include:
sugar
flour/dough
citrus fruits and their peel
other fruits and vegetables
spices, herbs, seasonings (eg horseradish, mustard, garlic)
fish and seafood
meat and poultry.
These are the most common of all possible allergens that can cause contact eczema.

Some of the materials known to potentially cause skin reactions are themselves components of common treatments for eczema (eg lanolin in moisturizers and hydrocortisone in steroid creams). Therefore if the condition is worsening or proving resistant to eczema treatment, the possibility that the eczema treatment itself is contributing to the skin problem should be stop immediately.

Rashes first appear only at the site of skin contact with the allergen. Later it often spreads out and may involve skin well away from the contact site. This is because the immune cells become activated and migrate out through the bloodstream to other parts of the body.

When allergic contact eczema is suspected, it can therefore be important to remember where it first started when taking the medical history behind the development of a rash finding the cause.

Sometimes the cause and effect relationship is obvious, with a red scaly reaction around an earring or under a watch strap. At other times, especially if a long time has passed since the initial exposure and sensitisation , it can be impossible to identified contact allergic from other types of eczema.

It may also be difficulties distinguishing allergic contact eczema (which is an immune response) from irritant eczema (which is a direct skin reaction against an irritant substance). In practice, this difference is not so important because the same lines of treatment are required for both.

Hay fever is the most common of all the allergic diseases - about 15 per cent of the population in industrialized cold countries suffer from this condition. Symptoms usually appear in childhood first and then lessen by the age of 30 or 40.

Perennial allergic rhinitis is a similar allergy that occurs all year round and is caused by things such as house dust, mites and pets. However the predominant allergen changes from time to time.

: Frequent sneezing is characteristic of hay fever.

Different microscopic substances get into the nose and cause the body to produce antibodies and release histamine.

Histamine irritates the upper respiratory passages, making them swell and producing the typical hay fever symptoms.

A tendency to suffer allergies is often hereditary. The most common causes of hay fever are:
tree pollen such as elder, elm, hazel and especially birch (spring hay fever).
grass pollen (any flowers, plant or trees).
mugwort and hybrids such as chrysanthemum (autumn hay fever).
house dust mites and mould fungus - particularly associated with perennial allergic rhinitis.