Thank you! Much appreciated! n/t

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: : I am hoping to explain to you guys why there are differences in the : type of "allergic" reactions to PPD and other sensitizing ingredients. : What follows is summarized and quoted directly from "Immunology: A : Short Course" by Eli Benjamini and Sidney Leskowitz; this is my text : from various immunology courses and is as succinct as it gets. The : study of immunology always did make my head spin.... : : There are four types of hypersensitivity reactions characterized: : : Type I: Anaphylactic reactions, mediated by the antibodies that are : also responsible for protecting you from parasite invasion (called : IgE). Reactions are rapid, occurring within minutes after exposure. : : Type II: Cytolytic/Cytotoxic reactions, the antigen binds to the : surface of a cell and activates a separate arm of the immune system : called the Complement Cascade. : : Type III: Immune complex reactions happen when the foreign : substance and antibodies form complexes in the tissue and the : antibodies are called to the reaction site. The complement cascade is : also involved. This reaction takes hours to set in. : : Type IV: Cell-Mediated Immunity or Delayed Type Hypersensitivity. : This is mediated by the T cells rather than the antibodies. The : defining characteristic is local tissue damage. This type of reaction : has a delayed onset and may occur within 1-2 days after challenge. : : It looks to me like Types I and IV are the characteristic reactions : to PPD exposure for those who are allergic. Types II and III are : usually caused by repeated injection of substances, but I can't rule : that out in the case of black dyes because they often find their way : under the skin. The people who don't know any better are the ones who : look for the Type I reaction, and when it doesn't happen, they assume : everything has gone OK. An experienced allergist or immunologist can : spot the difference between Type I and IV immune reactions but your GP : can only store so much information is his/her head, so they are unsure : about what they are seeing. : : A Type I reaction will show on the surface what is called the "wheal : and flare" reaction, an area of redness (erythema) and swelling : (edema). Again it's very fast, it will take about 15 minutes to show : up. Any of you who have had reactions to makeup or lotion will : recognize it. The important thing about Type I which scares me a bit : is sensitization. : : "...once adequate exposure to the allergen has been acheived by : repeated mucosal contact, ingestion or...injection, and IgE antibody : has been produced, then an individual is considered to be sensitized. : IgE antibody is made in small amounts....IgE molecules persist at the : site for weeks..." : : That means another exposure could make you feel even worse. : Corticosteroids are the indicated treatment for Type I : hypersensitivity, as well as antihistamines and in severe cases, : epinephrine. : : Type IV reactions are a bit tricker to explain...there is a longer : period of sensitization (1-2 weeks, which may explain a lot of the : stories we hear). It is the T cell's second exposure to the substance : that causes the reaction. The gross appearance is redness and : induration, or a raised thickening of the skin, reaching its peak at : 24-48 hours after the initial reaction. A classic example of Type IV : is poison ivy dermatitis, as well as nickel-chromium sensitivity from : jewelry. "An additional pathologic component of contact sensitivity : reactions in humans is the separation of epidermal cells, spongiosis, : and blister formation." In other words you will be a flaky, pus-y : mess! : : I only wish I could find a driver for my scanner so I could scan the : "lovely" colour plates showing each type of reaction taking place on : some poor person's skin. Then there is the whole matter of the stuff : finding its way into your lymph nodes and your organs, but that is : another story. : : I hope this clears things up, in your heads anyway....or at least I : hope I haven't been blowing the dust off my books for nothing :^)

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