[. . .] activated T cells can also induce disease. Disregulation of TH1 cytokine production underlies chronic inflammation, such as occurs in rheumatoid arthritis, psoriasis, and inflammatory bowel disease. Disregulation of TH2 cytokine production underlies the development of atopic disorders, including atopic dermatitis and asthma. In the asthma setting TH2 cytokines drive eosinophil activation, IgE production, IgE mediated mast cell activation and degranulation, and accumulation of mononuclear cells and granulocytes into the lung tissue. These cells continue to secrete TH2 cytokines, chemokines, and effector molecules, thereby fostering chronic lung inflammation, which leads to tissue damage and remodeling, causing airway restriction.
Asthma and other atopic disorders are increasingly common diseases in developing countries. It has been suggested that the rise in asthma prevalence is linked to improved hygiene, and to a dramatic drop in exposure to viral and bacterial infections. This concept, termed the hygeine hypothesis, states that the recent increase in asthma is due to a disruption in the normal induction of TH1 immunity, leading to a pathogenic shift to predominant TH2 immunity, thereby causing atopic disease. Thus, improvements in living standards have reduced communicable diseases only to give rise to an increase in certain immunological diseases.
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