Causes of Asthma

What are the theories of the causes of asthma? Research studies on asthma have multiplied in the last few decades. This is mainly because of the rapid increase in the number of people suffering from the disease. The analysis of most studies has focused on two body systems that act on the respiratory system and influence the lungs. These are: (a) the natural defense mechanism of the body (immune system) and (b) the nervous system. Two main theories have been suggested based on these systems for defining asthma more accurately: (a) beta blockage theory that is related to the nervous system and (b) allergy theory that is related to the immune system. Better understanding of these systems and the theories associated with them helps develop more effective treatment and cure for asthma.

Beta Blockage Theory

The nervous system which consists of the brain, spinal cord and the nerves, controls several body functions. The nerves in various parts of the body have special ‘receptors’ which collect signals from the environment and send them to the brain through the spinal cord. The brain receives and interprets these signals and messages, and sends back its response in the form of necessary orders through the spinal cord to the vast network of nerves.

There are two parts of the nervous system: (1) the central nervous system which consists of the brain and the spinal cord and (2) the peripheral nervous system which includes the nerves originating from the brain and the spinal cord. The peripheral nervous system includes the autonomous nervous system which controls all involuntary actions of the blood vessels, organs and other parts of the body. There are two main branches of the autonomous nervous system: (a) the sympathetic nervous system which prepares the body for action by increasing the heart rate and stimulating the organs and (b) the parasympathetic nervous system which acts in a way opposite to that of the sympathetic nervous system and yields more control during sleep.

In the lungs, the sympathetic nervous system stimulates the respiratory passages to relax and open up, whereas the parasympathetic nervous system causes tightening and narrowing of the airways. If the lungs are normal, both these systems work efficiently in a coordinated manner and keep the airways open. Thus, air flows in and out of the lungs without any effort on your side. In case of asthma, the parasympathetic nervous system is more dominant. This kind of an imbalance between the two parts of the autonomous nervous system results in narrowing of the respiratory passages. In addition to this imbalance, some chemical substances secreted by the nerves play a very important role in the development of asthma.

All the nerves of the body secrete a chemical substance called ‘neurotransmitters’ which are used for communicating with other nerves of the body. Each type of autonomous nervous system secretes a different type of neurotransmitter. The sympathetic nervous system secretes epinephrine and adrenaline, whereas the parasympathetic system secretes acetylcholine.

The special nerve endings that respond to various types of stimuli are called ‘receptors’. There are three types of sympathetic receptors: (a) alpha, (b) beta and (c) beta2 receptors. These receptors together control all body functions, especially blood pressure, heart rate and openings of the respiratory passages.

Alpha receptors receive nerve impulses transmitted to the heart and lungs and increase heart rate, tighten respiratory passages and increase mucus secretion. This action of alpha receptors on the lungs is however not very important in asthma.
Beta receptors receive impulses that affect the heart muscles and increase the heart rate and blood pressure.

Beta2 receptors relax the muscles of the respiratory passages and decrease the secretion of mucus by the inner lining of the airways. Several studies have indicated that people with asthma have abnormal beta2 receptors and therefore the nerve impulses from the brain do not reach them. This stimulates the parasympathetic nerves to overreact and constrict the airways.

Allergy Theory

This is related to the immune system (body’s natural defense mechanism). There are three main components of the immune system: (a) the lymph nodes, (b) bone marrow and (c) the thymus gland. Lymph nodes are small oval structures that filter a fluid called ‘lymph’ and fight infections. Bone marrow is a specialized soft tissue that fills the spaces in the bones. The thymus gland is located behind the upper part of the chest bone. It is the primary central gland of the lymphatic system.

The three components of the immune system together produce the red and white blood cells. The red cells transport oxygen from the lungs to various parts of the body and carbon dioxide from all parts of the body to the lungs. The white cells defend the body against infections. There are several types of white blood cells, one of which is known as lymphocytes. There are two types of lymphocytes: (a) ‘B cells’ and (b) ‘T cells’. The T cells release a chemical substance called ‘cytokine’ which destroys disease causing agents. The B cells produce antibodies which identify the disease causing agents and help the rest of the immune system to destroy them. These agents could either be bacteria, viruses or foreign substances. Antibodies are specific to disease causing agents. This means that antibodies for one type of disease causing agent will not be effective for another type. The antibodies also help the body to remember these agents and destroy them if they enter the body again. Thus, they prevent the disease from recurring.

As soon as a disease causing agent enters the body, the white blood cells release T cells for defense against the invading agent. The ‘T cells’. ‘Meet’ the invading agents and kill them. Cytokine released by the T cells send signals to the B cells to identify the agent and to stimulate their production in the bone marrow. Increased B cells increased antibodies.
There are several types of antibodies. One of these antibodies called the Immunoglobulin E or IgE is a harmful one. Although the body does not require IgE, it manufactures it. In some people, the B cells secrete abnormally high levels of IgE. As a result, the immune system overreacts even to normal substances. Different types of IgE are produced in response to different types of substances. Thus, you will have a different IgE antibody for each substance you are sensitive to. Any of these substances can result in allergies that trigger an asthmatic attack.
Everyone has millions of small cells called ‘mast cells’ in various parts of the body. They are also present in the nose, respiratory passages and the skin. The mast cells contain large grant4les which contain three chemical substances. These chemicals are released in response to injury or infection. When an allergy causing substance is present the IgE antibodies combine with the mast cells and the three chemical substances are released in the effected tissues. This causes redness and swelling of the tissues. When the mast cells in the respiratory’ passages release the chemicals, they cause wheezing cough, swelling of the inner lining of the respiratory passages, increased mucus secretion and contractioJ1 of the airways. The end result is asthma.

One of the important chemicals released by the mast cells is histamine. This is why medicines that act against histamine are recommended for allergic conditions. These medicines are called antihistamines.

Another type of white blood cells called Eosinophils also plays a very important role in triggering off asthma. This is because they release chemicals that increase the sensitivity of the respiratory passages and leads to inflammation.

Family History

Yes, asthma can run in families, but it does not always affect the children of parents with asthma. This means that asthma can sometimes skip one entire generation and occur in grandchildren. Childhood asthma is normally inherited. If one parent has asthma, the children have fifty per cent chances of developing the disease. If both parents have asthma, this risk increases to seventy-five percent.