Allergy and the Immune System.

ROLCAM · Posted: Wed Oct 10, 2007 1:21 am Post subject: Allergy and the Immune System.

Allergy and the Immune System.

Many people suffer from allergies, and these can happen in different ways for different people. This page aims to give you some understanding of the process that underlies all of these different symptoms. At the bottom of the page is a guide to many of the immunology words that we use to talk about allergy - these can be very confusing!

An allergy is a kind of disease that happens because the body misdirects the attack of its powerful immune system against a relatively harmless substance, and accidentally damages itself in the process.

What is an Allergy? How does it relate to the Immune System?

Allergy, Immunity and Heredity

Common Allergens

Phases of the Allergic Response

Some helpful terms

What is an Allergy? How does it relate to the Immune System?

The immune system has the very important role of protecting us against infection. Most of the time, the immune system functions well, with an immune response directed against a potentially harmful organism (such as a bacterium; or virus). Because the immune system can easily damage cells it is important to limit the response to just those things which are harmful to the body. But in some people, the immune system attacks a harmless substance along with those things it should - those that are harmful. The immune system has very powerful effects the tissues of the human body, just as it does on the harmful substances that it is intended to attack. When this system attacks something harmless, the attack can lead to damage to human tissues, resulting in allergic reactions;. When someone has a reaction to a harmless substance, we call that substance an allergen.

Allergy, Immunity and Heredity

Allergy and the immune system; are, as a result, very closely linked. The tendency to develop allergic reactions has a strong hereditary factor. This is because the genes that affect immunity affect allergy, and genes that affect immunity are important to pass on to children - so that they too will be able to fight infections like their parents can. The tendency to have allergic reactions is termed atopy.

Atopy is defined as the presence of an allergic reaction to an allergen as demonstrated by a skin-prick test; or RAST . Just because a person is atopic doesn't mean they have an symptoms: approximately 20-30% of the population is atopic, and two-thirds of atopic individuals have allergic disease.

Mast cells have a large role in both allergy and the immune system. Mast cells are partly responsible for linking the Innate Immune System (to do with inflammation) and the Acquired Immune System (which is very specific and tightly controlled) - which is normal.

The acquired immune system includes antibodies that are specific to a particular substance. Antibodies may also be called immunoglobulins. In allergy, a particular class of antibody called IgE are often responsible for these misdirected attacks. Mast cells are very sensitive to the moment when IgE recognises its target (a target for an antibody, harmless or not, is called an antigen).

When this happens, mast cells release the contents of special granules held inside them; this event may be called degranulation. The granules contain hormones called histamine and leukotrienes. It is histamine and leukotrienes that produce the inflaming response of allergy.

The release of histamine and luekotrienes defines atopy - when this process leads to symptoms, it is called allergic disease. It is still unknown why some people have this abnormal reaction to harmless substances in the environment, or why certain substances are more likely to cause allergies than others.

Common Allergens

Some common allergens include:

Pollens such as grass or pine pollens.

House dust mites.

Animal dander, produced by animals such as cats, dogs and horses.

Moulds.
Phases of the Allergic Response

Allergic reactions occur in two distinct phases:

The initial phase response usually occurs within 15 minutes of contact with the allergen. It usually recovers within 2-4 hours. This response involves mast cell degranulation as the primary cause of symptoms.

The late phase response occurs several hours later, usually starting 4 hours after contact with the allergen and can last 12 to 24 hours in some individuals. This response occurs after recruitment of white blood cells called eosinophils and T lymphocytes to the site of the allergy (eg. the lungs in asthma) where they release a number of mediators that lead to continued allergic symptoms.

Some helpful terms

These are some of the terms you may hear when doctors discuss allergies:

Allergen: An allergen is a name for any substance that causes an allergic reaction.

Allergic rhinitis: This is group of symptoms of nose and upper airways caused by allergy, commonly including a runny nose, cough and an itchy sensation, also known as hay fever. To the right is an image of polyps being removed from the nose, in a surgical operation. Polyps can form in some people who suffere from allergic rhinitis.

Allergy: An allergy is an exaggerated immune reaction to substances that are normally harmless. Allergy can appear in many forms and cause many symptoms, such as sneezing, respiratory symptoms such as asthma or hay fever, itching, skin rashes, or generalised reaction such as anaphylaxis. Below is an image of a skin rash reaction to an food allergen.

Anaphylaxis: Anaphylaxis is a dangerous generalised allergic reaction to an allergen.

Antibody: Antibodies are proteins (also known as immunoglobulins) that are produced by B-cells after these cells have been stimulated by an antigen. Antibodies act specifically against that antigen in immune responses, which may be normal (such as fighting infection) or abnormal (such as allergy).

Antigen: An antigen is a substance foreign to the body that causes an immune response.

Asthma: Asthma is an inflammatory lung condition characterised by short periods of 'bronchoconstriction', or tightening of the airways, making it more difficult to breathe. This condition is often caused by allergy, or may be worsened by allergy.

Asymptomatic: A word used to describe someone who has no symptoms.

Atopy: People who are 'atopic' have a reaction to allergens on skin-prick tests or RAST, but they may or may not have symptoms of allergic disease.

B-cell: A B-cell (sometimes called a B-lymphocyte) is a type of white blood cell that produces antibodies against foreign antigens.

Desensitisation: A treatment of allergy where a person is exposed to small amounts of an allergen until they no longer produce an allergic reaction to the substance.

Eczema: Eczema is an inflammatory skin condition characterised by redness, itching and oozing lesions that can become scaly, crusted or hardened, that is often caused by allergy.

Eosinophil: An eosinophil is a type of white blood cell that is often elevated in people with allergic disease. Its name comes from a kind of dye called 'eosin' that can be used to show it up from the background when using a microscope.

Histamine: Histamine is a local-acting hormone, only active for a short time, that is released by mast cells. Histamine causes many of the symptoms of allergy.

Hypersensitivity: This term is used to describe exaggerated, inappropriate immune responses to usually harmless substances. This term may be used instead of 'allergy'.

IgE: a class of antibody (or immunoglobulin), that function especially in allergy. The other antibody classes are IgA, IgD, IgG and IgM.

Leukotrienes: Leukotrienes are a group of fast-acting, short-range hormones that are released by mast cells. Leukotrienes cause many of the symptoms of allergy.

Mast Cell: A mast cell is a type of white blood cell involved in allergic reactions by releasing mediators such as histamine and leukotrienes in allergic reactions, and by being sensitive to the presence of IgE.

Mediators: This term is almost interchangeable with hormone, but often means chemicals produced by cells of the body that have particular roles in communicating or controlling neighbouring cells, some of which are involved in allergic disease. Examples include leukotrienes and histamine.

Perennial: A term meaning 'occurring all year round'. Originally used to refer to plants that grow all year round, this is now commonly used in allergy, since many allergens are from plants - such as pollen.

RAST: RadioAllergoSorbent Test (or RAST), often used to determine what antigens a patient is allergic to.

T-cell: A T-cell (sometimes called a T-lymphocyte) a type of white blood cell involved in immune responses and often involved in allergic reactions.

Seasonal: This word simply means 'occurring at a particular time of the year'.
_________________
Roland Camilleri

Moderator

Sydney , Australia.