Description, Causes and Risk Factors:
The immune system is an integral part of human protection against disease, but the normally protective immune mechanisms can sometimes cause detrimental reactions in the host. Such reactions are known as hypersensitivity reactions, and the study of these is termed immunopathology. The traditional classification for hypersensitivity reactions is that of Gell and Coombs and is currently the most commonly known classification system.
Type I hypersensitivity is also known as immediate or anaphylactic hypersensitivity. The reaction may involve skin (urticaria and eczema), eyes (conjunctivitis), nasopharynx (rhinorrhea, rhinitis), bronchopulmonary tissues (asthma) and gastrointestinal tract (gastroenteritis). The reaction may cause a range of symptoms from minor inconvenience to death. The reaction usually takes 15 - 30 minutes from the time of exposure to the antigen, although sometimes it may have a delayed onset (10 - 12 hours).
In type 1 hypersensitivity, an antigen is presented to CD4+ Th2 cells specific to the antigen that stimulate B cell production of IgE antibodies also specific to the antigen. The difference between a normal infectious immune response and a type 1 hypersensitivity response is that in type 1 hypersensitivity the antibody is IgE instead of IgA, IgG, or IgM. During sensitisation, the IgE antibodies bind to Fc? receptors on the surface of tissue mast cells and blood basophils. Mast cells and basophils coated by IgE antibodies are "sensitised." Later exposure to the same allergen cross-links the bound IgE on sensitised cells, resulting in degranulation and the secretion of pharmacologically active mediators such as histamine, leukotriene (LTC4 and LTD4), and prostaglandin that act on the surrounding tissues. The principal effects of these products are vasodilation and smooth-muscle contraction.
A subsequent exposure to the same allergen cross links the cell-bound IgE and triggers the release of various pharmacologically active substances. Cross-linking of IgE Fc-receptor is important in mast cell triggering. Mast cell degranulation is preceded by increased Ca++ influx, which is a crucial process; ionophores which increase cytoplasmic Ca++ also promote degranulation, where as, agents which deplete cytoplasmic Ca++ suppress degranulation.
Mast cells may be triggered by other stimuli such as exercise, emotional stress, chemicals (e.g., photographic developing medium, calcium ionophores, codeine, etc.), anaphylotoxins (e.g., C4a, C3a, C5a, etc.). These reactions, mediated by agents without IgE-allergen interaction, are not hypersensitivity reactions, although they produce the same symptoms.
The reaction is amplified by PAF (platelet activation factor) which causes platelet aggregation and release of histamine, heparin and vasoactive amines. Eosinophil chemotactic factor of anaphylaxis (ECF-A) and neutrophil chemotactic factors attract eosinophils and neutrophils, respectively, which release various hydrolytic enzymes that cause necrosis. Eosinophils may also control the local reaction by releasing arylsulphatase, histaminase, phospholipase-D and prostaglandin-E, although this role of eosinophils is now in question.
Cyclic nucleotides appear to play a significant role in the modulation of immediate hypersensitivity reaction, although their exact function is ill understood. Substances which alter cAMP and cGMP levels significantly alter the allergic symptoms. Thus, substances that increase intracellular cAMP seem to relieve allergic symptoms, particularly broncho-pulmonary ones, and are used therapeutically. Conversely, agents which decrease cAMP or stimulate cGMP aggravate these allergic conditions.
Type I hypersensitivity is short-lived in terms of its reaction, however due to different diseases that affect the body, the prognosis can be different as well. Common diseases such as allergic rhinitis (hay fever), allergic conjunctivitis and bronchial asthma rarely cause deaths except in severe asthma. However angioedema and anaphylaxis can potentially cause death if not treated immediately. Type I allergic hypersensitivity diseases do not have a cure. The aim of the treatment is to control the symptoms.
Symptoms usually begin within minutes of allergen exposure (eg, drug administration, insect sting, food ingestion, allergen immunotherapy) but can recur hours after the initial exposure (late-phase reaction).
Diagnostic tests for immediate hypersensitivity include skin (prick and intradermal) tests, measurement of total IgE and specific IgE antibodies against the suspected allergens. Total IgE and specific IgE antibodies are measured by a modification of enzyme immunoassay (ELISA). Increased IgE levels are indicative of an atopic condition, although IgE may be elevated in some non-atopic diseases (e.g., myelomas, helminthic infection, etc.).
There appears to be a genetic predisposition for atopic diseases and there is evidence for HLA (A2) association.
- Symptoms consist of congestion; sneezing; itchy, runny nose and eyes; and itching of the palate and inner ear. Patients may also report postnasal drip, which can cause sore throat, coughing, or throat clearing.
- Long-term allergen exposure can cause chronic changes of increased difficulty breathing and chest tightness, and the patient may give a history of repeated rescue inhaler use or reduced peak flows.
- Patients may also report significant pruritus that causes scratching, which produces the lesions. Superinfection with staphylococcal organisms can occur, particularly in severely excoriated or cracked lesions.
- Patients may also report nausea, vomiting, abdominal cramping, and diarrhea after ingestion of the offending food.
Symptomatic treatment is achieved with antihistamines which block histamine receptors. Cromolyn sodium inhibits mast cell degranulation, probably, by inhibiting Ca++ influx. Late onset allergic symptoms, particularly bronchoconstriction which is mediated by leukotrienes, are treated with leukotriene receptor blockers (Singulair, Accolate) or inhibitors of the cyclooxygenase pathway (Zileuton). Symptomatic, although short term, relief from bronchoconstriction is provided by bronchodilators (inhalants) such as isoproterenol derivatives (Terbutaline, Albuterol). Theophylline elevates cAMP by inhibiting cAMP-phosphodiesterase and inhibits intracellular Ca++ release is also used to relieve bronchopulmonary symptoms.
The use of IgG antibodies against the Fc portions of IgE that binds to mast cells has been approved for treatment of certain allergies, as it can block mast cell sensitization.
Hyposensitization (immunotherapy or desensitization) is another treatment modality which is successful in a number of allergies, particularly to insect venoms and, to some extent, pollens. The mechanism is not clear, but there is a correlation between appearance of IgG (blocking) antibodies and relief from symptoms. Suppressor T cells that specifically inhibit IgE antibodies may play a role.
NOTE: The above information is for processing purpose. The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition.
DISCLAIMER: This information should not substitute for seeking responsible, professional medical care.