What Causes Asthma?
Reviewed by: HU Medical Review Board | Last reviewed: October 2021 | Last updated: November 2021
Asthma is a common chronic airway condition that leads to respiratory symptoms. It involves an immune response called airway inflammation. This leads to airway narrowing and swelling, both of which make it harder to breathe.
Certain genetic and environmental factors increase the risk of developing asthma. These factors make some people more likely to have hyperactive inflammatory responses. This makes the airways more sensitive to asthma triggers.
How is inflammation related to asthma?
Airway inflammation has a central role in causing symptoms of asthma. Inflammation normally helps your body fight off infections. But for some people, it can be overactive or respond to harmless substances.1
Asthma and other allergic conditions involve “type 2 inflammation.” This is an immune response that activates immune cells called “type 2 helper T cells.” These cells release proteins (called interleukins) that activate other immune cells. They also promote the formation of antibodies called immunoglobulin E (IgE). IgE recognizes certain allergens and activates immune cells.2,3
Some immune cells that get activated include:3
- Mast cells
These cells all release substances that lead to symptoms of asthma. For people with asthma, this process is often hyperactive. This means the immune system becomes overly sensitive and responds to harmless triggers like allergens.3
How does airway inflammation cause asthma symptoms?
Inflammation causes many changes to your airways. These changes lead to the symptoms of asthma, such as wheezing and shortness of breath.
A major result of inflammation in asthma is airway narrowing. This is also called bronchoconstriction. It makes it harder for air to travel out of your lungs.3
Bronchoconstriction happens when IgE activates mast cells. Mast cells are immune cells that release chemicals such as histamine and leukotrienes. These chemicals cause airway muscles to expand, which narrows the airway.3
Airways of people with asthma are hypersensitive. This means that the airways have an exaggerated response to certain triggers. Inflammation increases airway sensitivity to allergens. When IgE antibodies are released, they bind to mast cells and basophils. This tells them to quickly release chemicals that lead to symptoms of asthma.2,4
As inflammation gets worse, other factors can obstruct airflow. This includes:3,4
- Swelling of the airway (edema)
- Excess mucus
- Thickening of airway muscles (hypertrophy and hyperplasia)
These factors combine to narrow and block the airways. This makes it harder for air to get in and out of the lungs.
For many people with asthma, airway narrowing and obstruction are reversible. This means that anti-inflammatory drugs can open up the airway. However, chronic inflammation sometimes leads to permanent changes in the airway. This is called airway remodeling.2,3
Airway remodeling involves activation of structural airway cells. Possible permanent changes include thickening of airway muscle, mucus secretion, and lung tissue scarring. This can make someone with asthma less responsive to treatment.2
What causes airway inflammation for people with asthma?
The inflammation process starts when airway cells called dendritic cells identify an allergen. Dendritic cells break it down into small pieces and display the pieces on their surface. Immune cells called T cells then recognize the allergen pieces. For people with asthma, this transforms T cells into active type 2 helper T cells.4
We do not yet know why people with asthma have this inflammation response. Experts think it involves an imbalance between type 1 and type 2 helper T cells. An immune response that involves mostly type 2 helper T cells is linked to conditions like asthma.5
Genetic factors and environmental exposures increase the risk of developing asthma. They may do so by influencing the balance of types of immune cells. This may make some people more likely to have a type 2 inflammatory response. Some risk factors include:1,2,6
- Family history of asthma
- History of other allergic conditions
- Viral respiratory infections during early childhood
- Exposure to certain allergens during early childhood
- Exposure to cigarette smoke in the womb or early childhood
- Air pollution or workplace exposures