Allergen: A substance that can produce a hypersensitive reaction in the body.

Allergic rhinitis: A seasonal allergic, upper respiratory reaction stimulated by tree, grass or weed pollen that causes sneezing and watery, itchy eyes. Hay fever is a specific type of allergic rhinitis.

Antibiotic: A drug with the ability to kill bacteria.

Antibody: An immunoglobulin that binds specifically to a given antigen.

Antigen: A substance that reacts with antibodies or memory T cells to give rise to an immune reaction. An antigen that binds with a specific antibody or T cell can give rise to allergy.

Antihistamine: A substance capable of reducing inflammation, swelling and other effects often experienced as an allergic reaction.

Asthma: A respiratory disorder characterized by breathing trouble (wheezing).

Atopic: Hereditary tendency to experience immediate allergic reactions because of the presence of an antibody in the skin.

Bacteria: A class of microorganisms that cause infections.

B cells (lymphocytes): Cells that make and carry surface immunoglobulins.

Biopsy: Removing a section of skin so it can be examined under a microscope.

Chronic: Long-lasting, ongoing.

Clinical trial: Investigational studies of new treatments, new uses of existing treatment, or new screening methods to detect disease.

Contagious: Capable of being transmitted from person to person.

Corticosteroids: A class of substances related to a natural hormone that can diminish an inflammatory response such as itching or irritation of the skin.

Cyclosporine: A powerful prescription drug that decreases the body’s immune response.

Dermis: The middle layer of the skin, below the epidermis and above subcutaneous tissue. It is composed of connective tissue in which is embedded hair follicles, sweat glands, superficial and deeper blood vessels, and nerve fibers.

Emollient: Making soft or supple; soothing especially to the skin or mucous membrane

Emulsion: One liquid is suspended in a second liquid because the first cannot be mixed with the second, such as oil and water.

Epidermis: The outermost layer of skin. The epidermis has several active zones of skin cells, including cells that participate in immune reactions. Many eczematous skin conditions are initiated in the epidermis.

Flare-ups: Sudden outbreaks.

Hydrocortisone: A less potent corticosteroid incorporated into creams, lotions and ointments used to treat skin inflammation.

Hyperirritability: Easily irritated.

Human leucocyte antigen (HLA): Any human Class I and Class II major histocompatability complex (MHC) protein. These are important in presenting antigens to immune effector cells

Immediate hypersensitivity: A pattern of immune reactivity that begins within minutes of exposure to an antigen. Immediate reactivity is mediated primarily by mast cells. In the skin, immediate hypersensitivity often presents as hives.

Immune system: The body’s natural defense against foreign substances and organisms that could lead to illness.

Immunologic: The signs and symptoms of reactions of immune cells or antibodies to antigens.

Immunoglobulin: An antigen-binding protein secreted by specific white cells of the immune system. Immunoglobulins are antibodies.

Immunomodulation: Therapeutic manipulation of the immune system, used clinically to treat a disease in which immune system dysregulation is a factor.

Informed consent: The process by which a volunteer for a clinical trial agrees to participate after being fully informed regarding purposes of the trial, risks and benefits associated with participation in the trial, and whether volunteers will be randomized to receive treatment or placebo.

Innate: Existing in a person from birth.

Irritants: Agents that may produce inflammation if exposure is sufficient.

Lichenification: Thickening upper layers of the skin in response to trauma.

Major histocompatability complex (MHC): A specific cluster of genes that encode proteins involved in immune system reactions. See Human leucocyte antigen.

Mast cell: A tissue cell that is a primary participant in immediate hypersensitivity reactions. Mast cells make and release histamine and other mediators of inflammation.

Metabolite: The chemical compound produced when the body breakdowns a drug.

Phototherapy: Treatment with light.

Placebo: A pharmacologically inactive substance with no medicinal value, used as method for "control" in clinical trials.

Pruritus: Itch.

Subcutaneous tissue: The deepest layer of skin, under the epidermis and dermis. Subcutaneous tissue is composed of fat, tissue that separates the fat in lobules (pockets), nerves and blood vessels. The fatty tissue is biologically active and subject to inflammatory processes.

Systemic medication: Medication that circulates throughout the body and is taken orally or given by injection or infusion.

Tar treatment: Treatment consisting of chemicals related to coal tar used topically to reduce itching.

T cell: A white blood cell of the immune system that participates in a number of immune responses.

Tolerance: A state of immunologic unresponsiveness to a substance that would otherwise provoke an immune response.

Topical: On the surface of the body.

Urticaria: Hives.

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