Causes of Asthma

For unknown reasons, asthma and allergies have dramatically increased in prevalence over the last several decades. Although about 1/2 of people with asthma also have allergies, not all people with allergies have asthma. The association between asthma and allergy, although clearly present, is poorly understood.
Asthma is a complex disease caused by the interaction of host and environmental factors at some critical period during the development of the immune system. Each child with asthma has a different profile of contributing factors; research into therapies specific for each factor may improve treatment and prevent the progression of asthma. Additionally, factors that contribute to the development, persistence, and severity of asthma in any individual child may be different.
The most important host factors are genetic factors. About 1/3 of children with asthma have another immediate family member with asthma. If the child's mother has asthma, the child is more likely to have asthma than if the child's father has asthma. If both parents have asthma, their child has a 50% chance of having asthma. Several genes have been identified that may be important in the transmission of asthma, including ADAM33, which regulates a metalloprotease enzyme that is involved in airway smooth muscle. Genetic factors may also be involved in determining the persistence, severity, and development of asthma; research is also in progress regarding phenotype-genotype correlations.
A subgroup of lymphocytes, T helper (Th) cells type 2 (Th2) are the major allergy-asthma controlling cells. Th2 cells are responsible for making immune factors called interleukins, a type of cytokine involved in the inflammatory response. Some interleukins are responsible for the first-phase asthma attack by producing IgE, which then binds to mast cells, triggering release of leukotrienes, which are chemicals responsible for airway spasms and increased mucus production. Other interleukins are responsible for the late-phase asthma attack, leading to eosinophil accumulation and release of further immune factors. Current medications for asthma target this system, but recently T killer cells have also been implicated in the asthma response and may be responsible for why some individuals might not respond optimally to medication.
In contrast to Th2 cells, which are the asthma and allergy T cells, Th1 cells are involved in the response to infection. As the Th1 cell response increases, the Th2 cell response is thought to decrease. The Th1 response is known to increase with less frequent use of antibiotics, increased exposure to other children, and exposure to certain infections, such as the common cold with a subsequent decrease in asthma frequency and a decreased Th2 response. This has given rise to the "hygiene hypothesis" in which early exposure to allergens and infection might actually prevent the allergy and asthma response.
On the other hand, some viral infections, such as the respiratory syncytial virus (RSV) and even early infection with rhinovirus, responsible for the common cold, may be environmental triggers for asthma. Other environmental factors that are important in the development and persistence of asthma include airborne allergen exposure, especially house-dust mite and Alternaria. Exposure to smoke, air pollution, and diet are also thought to be involved, but studies proving cause are not currently available. Immunizations are not implicated in the increase in asthma prevalence. [National: 2007] [Akinbami: 2006]

Authors & Reviewers

Initial publication: September 2008; last update/revision: November 2019
Current Authors and Reviewers:
Author: Lynne M. Kerr, MD, PhD
Reviewer: Khanh Lai, MD

Page Bibliography

Akinbami L.
The state of childhood asthma, United States, 1980-2005.
Adv Data. 2006(381):1-24. PubMed abstract
U.S. Department of Health and Human Services Centers for Disease Control and Prevention Advance Data's statistics on asthma in the United States from Vital and Health Statistics.

National Asthma Education and Prevention Program Expert Panel.
Expert Panel Report 3 (EPR-3): Guidelines for the Diagnosis and Management of Asthma - Summary Report 2007.
National Institutes of Health: National Heart, Lung, and Blood Institute; (2007) Accessed on 11/17/2020.
A 60-page summary of the 440-page comprehensive guideline; published by the National Heart, Lung, and Blood Institute.