Hypersensitivity or Allergy? - Why their is too much itching in my body?

Histamine H1 receptors are responsible for itch reactions in the skin.

Something more with H1 Histamines: Vasodilatation, increased capillary permeability etc, and to some extent for contraction of smooth muscle in the bronchi and gastrointestinal tract.

Other Histamines:  Peripheral effects of histamine are chiefly mediated by 2 sets of receptors termed H1 and H2. Other effects that are mediated by H2 receptors include cardiac accelerating effects and, in particular, the stimulating action of histamine on the secretion of gastric acid. 

Recently H4 and H3 receptor have also been identified in a number of systems including the CNS and peripheral nerves.

Allergy or itching, in medical term hypersensitivity, a word is most commonly used to identify health problems that are related to allergy or itching. So "Histamin, allergy or itching, finally Hypersenisitivity", these terms are very closely related with each other. 

What are the causes of allergy/itching? 

What is Hypersensitivity? 

Hypersensitivity commonly defined as an exaggerated or inappropriate immune response causing tissue damage. 

Hypersensitivity reactions are generally classified into 4 types (types I to IV) though this may be considered an oversimplification and more than one type can often be postulated for a patient’s hypersensitivity.

The term ‘allergy’ by definition originally covered all types of hypersensitivity reactions as well as the induction of immunity in an individual. Nowadays, both terms "Hypersensitivity and Allergy" are more commonly applied to “type I” hypersensitivity reactions.

Type 1 Hypersensitivity

Type I, immediate hypersensitivity reactions occur after exposure to an antigen (the allergen) in a sensitized subject. 

It may occur as in one in whom the initial exposure to the antigen has caused the production of specific antibodies, mainly IgE (immunoglobulin E), which are bound to the surface of mast cells and basophils. 

At subsequent exposure, antigen binds to antibody resulting in degranulation of mast cells and basophils with release of mediators. 

These include preformed mediators such as histamine and chemotactic factors, and newly synthesized mediators such as leukotrienes, platelet-activating factor, and prostaglandins.

Although "type I reactions" are usually described as being acute and short-lived, clinically there may often also be a late-phase and more prolonged reaction affecting the skin and bronchi. 

Examples of type I hypersensitivity reactions maybe allergic conjunctivitis, allergic rhinitis, urticaria and angioedema, and anaphylactic shock etc.

Type II Hypersensitivity

Type II, cell-surface hypersensitivity reactions are caused by the interaction of circulating antibodies, mainly IgG (immunoglobulin G) and IgM (immunoglobulin M), with antigens that are on the surface of specific cells or tissues. 

This interaction results in the activation of complement and of phagocytic and killer cells leading to cell damage or lysis. Thus type II reactions are responsible for blood transfusion reactions, some drug-induced blood disorders, and many auto-immune disorders. 

Type III Hypersensitivity

Type III, immune complex hypersensitivity reactions are caused by the interaction of fixed or circulating antigens with circulating antibodies, mainly IgG and IgM (either soluble or particulate), resulting in the formation of immune complexes. 

The immune complexes trigger a variety of inflammatory processes including complement activation, mediator release from mast cells and basophils, and platelet aggregation. 

Examples of type III reactions include serum sickness, some auto-immune and neoplastic disorders, type 2 lepra reactions, particularly in the lung, to some particulate antigens such as micro-organisms.

Type IV Hypersensitivity

Type IV, cell-mediated or delayed hypersensitivity reactions, are caused by the interaction of an antigen with sensitized T-lymphocytes (lymphokines are released by T-lymphocytes and inflammation ensues). 

Type IV reactions usually occur at least 24 hours after contact with the antigen. A type IV reaction is responsible for tuberculin reactions used for sensitivity testing, contact dermatitis, and some reactions to chronic infectious disease.

Treatment for Hypersensitivity 

Most of the common Allergic reactions are caused due to Type I hypersensitivity, closely related to H1 and H2 histamine groups (mostly H1 group). Thus, drugs used to treat Hypersenitvity or Histamin H1 reactions are known as Antihistamines. 

Histamine H1-antagonists (termed ‘antihistamines’) diminish or abolish the major actions of histamine in the body by competitive, reversible blockade of histamine H1-receptor sites on tissues; they do not inactivate histamine or prevent its synthesis, nor, in most cases, its release (although some are claimed to have mast-cell stabilizing properties).

Again, many of the sedating antihistamines also possess antimuscarinic, adrenaline-antagonizing, serotonin-antagonizing, and local anesthetic effects. Some have calcium-channel blocking activity.  

Foods that causes Allergy

More than 120 foods have been described as causing food allergies. However, allergy experts only consider a limited number of those to be of public health concern. 

Eight things cause about 90% of food allergy reactions:

• Milk (mostly in children)
• Eggs.
• Peanuts.
• Tree nuts, like walnuts, almonds, pine nuts, brazil nuts, and pecans.
• Soy.
• Wheat.
• Fish (mostly in adults)
• Shellfish (mostly in adults)

Drugs That causes allergy 

Any medication can trigger an allergic reaction. That said, some are more likely to cause these types of problems than others:

• Antibiotics -- amoxicillin (Moxatag), ampicillin, penicillin (Bicillin L-A), tetracycline (Sumycin), and others
• Nonsteroidal anti-inflammatory drugs like ibuprofen and naproxen
• Aspirin
• Sulfa drugs
• Chemotherapy drugs
• Monoclonal antibody therapy -- cetuximab (Erbitux), rituximab (Rituxian and others
• HIV drugs -- abacavir (Ziagen), nevirapine (Viramune), and others
• Insulin
• Antiseizure drugs -- carbamazepine (Tegretol), lamotrigine (Lamictal), phenytoin, and others
• Muscle relaxers given by IV -- atracurium, succinylcholine, or vecuronium

Use of Antihistamines in various Hypersensitivity problems

• Antihistamines are used primarily for the alleviation of conditions such as urticarial rashes and nasal allergy that are characterized by "type I hypersensitivity".
• The antihistamines can improve or relieve the symptoms of seasonal allergic rhinitis (‘hay fever’) in many patients.
• Antihistamines may also be of value in vasomotor rhinitis.
• Sedating antihistamines are also widely marketed for the symptomatic treatment of coughs and colds. 
• Sedating antihistamines are still marked for their antiemetic activity and are used to control nausea and vomiting caused by a variety of vestibular disorders.
• Sedating antihistamines have also been widely used for premedication in anesthetic practice and still have a major role in the prevention of postoperative nausea and vomiting.
• Some of the antihistamines with very pronounced sedative effects, such as diphenhydramine and promethazine, have been marketed for the treatment of occasional insomnia.
• Some antihistamines are also available as preparations for topical application for the alleviation of insect bites.
• Antihistamines are of value in preventing urticaria and are used to treat urticarial rashes and mild angioedema.
• They are also used as adjuncts to adrenaline in the emergency treatment of anaphylaxis and severe angioedema.

(Important Tips: Most 1st generation antihistamines such as Chlorpheniramine, are short active, non-selective and sedation occurs. 

Other 2nd generation antihistamines like Loratidine, Fexopheanidine do not have any or very less sedative effect, also give long-acting effect. 3rd generation antihistamines e.g. Levocetirizine, are still under investigation and they actually contain active metabolite or enantiomers of 2nd generation antihistamines).