Asthma and the Hygiene Hypothesis



Since 1970 The prevalence of asthma has increased over 30% world wide particularly in developed and recently industrialized countries . In Sweden the number of children with atopic disorders (Allergic rhinitis , Asthma , Eczema) has increased tow folds in the past 12 years.

In USA annually 15 million out patients visitis and more than 2 million day hospitalization occur because Asthma . In addition , over $6 billion is spended directly and indirectly per year , 80% of which is related to direct treatment expeces . Asthma is currently the most frequent cause of chronic ailment and also is the most plevalent chronic disease among children. Drastically esclated prevalence of atopic disorders including asthma in western Europe, America and Australia denotes the importance of environmental factors in recent years . In fact , the incidence of atopic disorders of environmental factors in recent years.In fact, the incidence of atopic disorders depends on the relation between environmental  factors and genetic bases.

New theories support the relation between improvement of health in the society especially in fancy and the increased prevalence of asthma.

Reduction of childhood infections due to better health condition and early administration of anti-biotics for mild diseases results in decreased interaction of the body with microbial antigens. In fact microbial antigens active Th1 cells and finally decline Th2 response .  Studies in Europe and Africa have shown that life in rural and agricultural areas result in protection against atopic disorders because of bacterial pollution of the environment . It appears that incidence of bacterial and viral infection in infancy and early childhood prevent the allergic diseases including asthma lately in life.

Although, there are many hypothesis in this matter , it is generally assumed that infections play a substantial role in the evolution of immunity system of the child . In the intrauterine life, contact of the foetus with common environmental antigens passing the placenta result in predominance of the 2 response . It is Known that in the early life non-atopic newborns the immune system evolve to Th1 response (Immune Deviation ), while in atopic children there is an increase in Th2 response. In fact, microbial agents are the main cause of this evolution. After digestion of microbes, macrophages release  interleukin 12 which cause the release of Inter Feron y  from Th1 and Natural killer cells, which causes the deviation of child’s immune response from Th2 to Th1.

Recent hypotheses suggest the importance of balance between Th1 and Th2 in the recent increased prevalence of atopic disease and Asthma . In fact the immunological situation in atopic and asthmatic adults showes Th2 predominance while in non-atopic individuals the Th1 response is dominant. The characteristic of Th2  dominance is the frequency of cytokines such as interlukins 4,5,13 which are found in asthmatic patients . Where as in Th1 immune response interferon y and interleukin 2 are prevalent. Thus , the occurrence of infections in childhood lead the dominant situation from Th2  to Th1 and the absence of such infection in infancy causes expansion and maturation of Th2 cells and then persistence of this atopy phenotype in the child.

Recent studies in Eastern Europe and newly unified Germany have confirmed the in creased prevalence of asthma in the first children and among well and healthy families . In fact,  good health will lead to less infection and more asthma.

However, in this theory , the occurrence of allergic disorders is dependent upon condition of target organ, in the other word , the presence of a cofactors is of importance in the incidence of those disorders .  For instance , the intervention of RSV, infection exposure to allergens , cigarette  smoke , air pollution probably change the immune regulation of mucosal surface and predispose to Th2 response and then development of asthma.

Some of the authors are of opinion that for the reduction of asthma and allergy in high risk children (e.g: those with familial history of atopy) It is preferable to expose children to irritants which induce Th1 response such as infections with MTB , measles , hepatitis A, etc. 


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