Causes

 

The cause of Alopecia Areata is complex and only partially understood. It is likely to be due to both a genetic predisposition as well as immune mechanisms. In those people who have the susceptible genes, hair growth is switched off by the immune system. It is thought that the person’s immune system recognises the hair as ‘foreign’ rather than ‘self’  which is a process we call auto-immunity. In many affected people there is a family history of other auto-immune conditions such as diabetes, thyroid disease and vitiligo. Whether the process of arresting hair growth is started off by  an environmental or local trigger is the focus of much research.

 

Genetic predisposition:

 

It is likely that Alopecia Areata is a polygenic (meaning it comes from several genes) condition.

It is possible that certain genes account for the susceptibility to develop this condition whereas other genes might account for the severity of the condition in an individual.

The evidence for this genetic susceptibility comes from numerous family studies:

 

1) There is often a family history of Alopecia Areata (10-42%).

This is particularly true in individuals who have Alopecia Areata starting early in life.

There is a familial incidence of 37% in those individuals who had their first patch by the age of 30, whereas the familial incidence is only 7.1% if the first patch starts after the age of 30.

In the case of identical twins, if one twin develops the condition, there is a 55% chance of the other twin showing symptoms.

 

2) An association has been shown with the human leukocyte antigen genetic system (HLA) Class II. There is an association between Alopecia Areata and the HLA Class II antigens -DR4, -DR5 and -DQ3. Those individuals with HLA-DR5 have a higher rate of early onset Alopecia with more extensive hair loss.

 

3) Individuals with atopy and Down's syndrome (8.8%) have an increased incidence of Alopecia Areata.

 

This suggests that the genetic basis of Alopecia Areata involves several genes (polygenic).

Clearly this makes it more difficult to identify an underlying genetic defect.

 

Immune mechanisms:

 

If one looks at a hair follicle under the microscope from an affected patch of skin with Alopecia Areata, there are many immune cells gathered around the follicle.

Immune mechanisms clearly are important in the cause of Alopecia Areata.

These can be divided into organ-specific immune mechanisms and non-specific immunity.

 

Organ-specific Immune Mechanisms:

 

There are a number of reasons to support the concept that Alopecia Areata is an autoimmune condition:

1) Significant associations are seen with other conditions that are known as organ-specific autoimmune conditions.

 

Thyroid disease is more common in individuals with Alopecia Areata (8-11.8%) compared to the general population (2%).

 

Vitiligo is a condition of pigment loss of the skin and patients with Alopecia Areata have a four times increased incidence of this condition compared to the general population.

These, and other, associations support the possibility that Alopecia Areata is also an autoimmune condition.

To confirm this, scientists need to demonstrate a proven 'autoantibody' against, for instance, the hair follicles.

 

Several antibodies have been found against different components of the hair follicle, but further work is continuing as these antibodies are also found in a proportion of individuals who have no evidence of Alopecia Areata.

 

Non-specific Immunity:

 

Alopecia Areata can be temporarily reversed by using immune suppressing treatments such as oral steroids or cyclosporin (used in transplant recipients)

 

This supports an immune basis for this condition.

 

Environmental Triggers:

 

There are variable data suggesting the possibility of environmental factors being important in the aetiology or cause of Alopecia Areata.

 

1) Certain viruses have been implicated, most recently the cytomegalovirus (CMV).

The hypothesis is that the virus instigates an immune reaction against the hair follicle in a susceptible individual.

 

However, this association has not been confirmed by different laboratories.

2) The association of emotional stress preceding the onset of Alopecia Areata is also variable.

 

In a survey carried out on behalf of Alopecia Help and Advice (Scotland) about 50% of people with Alopecia Areata could identify an episode that they considered to be emotionally or physically traumatic which preceded the onset of their hair loss.

No one environmental factor appears to be responsible for the onset of Alopecia Areata.

 

Local Triggers:

 

Certain local abnormalities have been described which might contribute to the onset or progression of Alopecia Areata/

 

1) Certain neuropeptides (calcitonin gene-related peptide, CGRP, and substance P) have been shown to be decreased in the scalp of patients with Alopecia Areata.

These neuropeptides are released from local nerve endings and can modify local inflammatory reactions (CGRP) and induce hair growth (Substance P).

 

2) Certain cytokines, which are immune-modulating proteins, have been shown to be abnormally expressed in the scalp of individuals with Alopecia Areata, and are known to affect hair follicle growth.

 

Much research continues on these various aspects of the causes of Alopecia Areata