If you have allergies, chances are that you inherited this trait. If you have read How Your Immune System Works, you know about lymphocytes, also known as white blood cells. Lymphocytes are a fundamental component of the immune system, and when they make a mistake it can create an allergic response.

Terms to Know

Allergen - an allergy-causing substance
Antibody - a B-cell-produced protein molecule made in response to, and reactive with, a specific antigen
Antigen - a substance capable of causing the production of antibodies and then reacting specifically with these antibodies
B-cell (B-lymphocyte) - a type of white blood cell capable of producing antibodies
Basophil - a white blood cell containing histamine and other allergy mediators
Histamine - a chemical present mainly in mast cells that, when released, causes many of the symptoms of allergy
IgE - the type of antibody most instrumental in allergic reactions
Lymph nodes - small organs within the lymph system located at the intersections of lymph channels
Lymphocyte - a type of white blood cell instrumental in antibody production
Mast cell - a type of cell containing histamine and other allergy mediators instrumental in the allergic response

There are two types of lymphocytes:

B-lymphocytes (B-cells)
T-lymphocytes (T-cells)

Both types help guard your body against foreign substances such as invading bacteria, viruses and toxins. They move freely through and among the tissues of the body, travel through the walls of blood vessels, and move between the various lymph nodes and lymph channels. B-cells and T-cells go everywhere.

Lymphocytes act like traveling customs agents. Everywhere they go, they are busy checking the passports of every cell they encounter. Whenever they discover a cell that seems threatening, they immediately begin countermeasures against it. The biochemical process behind these countermeasures is amazing!

Allergic Threats
When a lymphocyte encounters a particle or cell with surface marker molecules that identify it as a foreign invader, it performs a microscopic version of taking fingerprints and mug shots of the invader. Because these foreign invaders cause the production of antibodies, they are called antibody generators, or antigens. After a B-cell identifies an antigen, it will make its way back to a lymph node, change into a plasma cell and produce antibodies specifically engineered to fight that particular threat.

There are five basic types of antibodies, called immunoglobulins, or Igs. Each is classified by type with a letter suffix:

IgA
IgD
IgE
IgG
IgM

The Ig responsible for allergic reactions is IgE.

IgE antibodies are present in everyone -- but remember those immune response genes mentioned previously? In a properly functioning immune system, the genetic code contains enough information to enable the lymphocytes to distinguish between threatening and non-threatening proteins. In an allergic person's immune system, the lymphocytes can't tell that the protein ingested as part of a meal containing shellfish isn't invading the body. The B-cells of an allergic person -- "misinformed" at the genetic level -- cause the production of large quantities of IgE antibodies that attach themselves to mast cells and basophils throughout the body. This is known as the sensitizing exposure.