Type I hypersensitivity (or immediate hypersensitivity) is an allergic reaction provoked by reexposure to a specific type of antigen

Antigen

An antigen is a foreign molecule that, when introduced into the body, triggers the production of an antibody by the immune system. The immune system will then kill or neutralize the antigen that is recognized as a foreign and potentially harmful invader. These invaders can be molecules such as...

 referred to as an allergen

Allergen

An allergen is any substance that can cause an allergy. In technical terms, an allergen is a non-parasitic antigen capable of stimulating a type-I hypersensitivity reaction in atopic individuals....

, or to a nonimmunologic stimulus like cold weather or exercise. Type I is not to be confused with Type II

Type II hypersensitivity

In type II hypersensitivity the antibodies produced by the immune response bind to antigens on the patient's own cell surfaces...

, Type III

Type III hypersensitivity

Type III hypersensitivity occurs when antigens and antibodies are present in roughly equal amounts, causing extensive cross-linking.-Presentation:...

, or Type IV

Type IV hypersensitivity

Type IV hypersensitivity is often called delayed type hypersensitivity as the reaction takes two to three days to develop. Unlike the other types, it is not antibody mediated but rather is a type of cell-mediated response....

 hypersensitivities.

Exposure may be by ingestion

Ingestion

Ingestion is the consumption of a substance by an organism. In animals, it normally is accomplished by taking in the substance through the mouth into the gastrointestinal tract, such as through eating or drinking...

, inhalation

Inhalation

Inhalation is the movement of air from the external environment, through the air ways, and into the alveoli....

, injection

Injection (medicine)

An injection is an infusion method of putting fluid into the body, usually with a hollow needle and a syringe which is pierced through the skin to a sufficient depth for the material to be forced into the body...

, or direct contact.

Pathophysiology

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 sensitization, 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 "sensitized." Later exposure to the same allergen cross-links the bound IgE on sensitized cells, resulting in degranulation

Degranulation

Degranulation is a cellular process that releases antimicrobial cytotoxic molecules from secretory vesicles called granules found inside some cells...

 and the secretion of pharmacologically active mediators such as histamine

Histamine

Histamine is an organic nitrogen compound involved in local immune responses as well as regulating physiological function in the gut and acting as a neurotransmitter. Histamine triggers the inflammatory response. As part of an immune response to foreign pathogens, histamine is produced by...

, leukotriene

Leukotriene

Leukotrienes are fatty signaling molecules. They were first found in leukocytes . One of their roles is to trigger contractions in the smooth muscles lining the trachea; their overproduction is a major cause of inflammation in asthma and allergic rhinitis...

 (LTC4 and LTD4), and prostaglandin

Prostaglandin

A prostaglandin is any member of a group of lipid compounds that are derived enzymatically from fatty acids and have important functions in the animal body. Every prostaglandin contains 20 carbon atoms, including a 5-carbon ring....

 that act on the surrounding tissues. The principal effects of these products are vasodilation

Vasodilation

Vasodilation refers to the widening of blood vessels resulting from relaxation of smooth muscle cells within the vessel walls, particularly in the large arteries, smaller arterioles and large veins. The process is essentially the opposite of vasoconstriction, or the narrowing of blood vessels. When...

 and smooth-muscle

Smooth muscle

Smooth muscle is an involuntary non-striated muscle. It is divided into two sub-groups; the single-unit and multiunit smooth muscle. Within single-unit smooth muscle tissues, the autonomic nervous system innervates a single cell within a sheet or bundle and the action potential is propagated by...

 contraction.

Type 1 hypersensitivity can be further classified into an immediate and late-phase reaction. The immediate hypersensitivity reaction occurs minutes after exposure and includes release of vasoactive amines and lipid mediators, whereas the late-phase reaction occurs 2-4 hours after exposure and includes the release of cytokines.

Overview of mediators released by mast cells in type 1 hypersensitivity, and their actions:

Vasodilation Vasodilation Vasodilation refers to the widening of blood vessels resulting from relaxation of smooth muscle cells within the vessel walls, particularly in the large arteries, smaller arterioles and large veins. The process is essentially the opposite of vasoconstriction, or the narrowing of blood vessels. When...  and increased permeability	Histamine Histamine Histamine is an organic nitrogen compound involved in local immune responses as well as regulating physiological function in the gut and acting as a neurotransmitter. Histamine triggers the inflammatory response. As part of an immune response to foreign pathogens, histamine is produced by... PAF Leukotriene C4 Leukotriene C4 Leukotriene C4 is a leukotriene.... , D4 Leukotriene D4 Leukotriene D4 is a leukotriene....  and E4 Leukotriene E4 Leukotriene E4 is a leukotriene.... Prostaglandin D2 Prostaglandin D2 Prostaglandin D2 is a prostaglandin that binds to the receptor PTGDR, as well as CRTH2. It is a major prostaglandin produced by mast cells – recruits Th2 cells, eosinophils, basophils. In mammalian organs, large amounts of PGD2 are found in the brain, in mast cells and found nowhere else... Neutral proteases
Smooth muscle spasm	Histamine PAF Leukotriene C4, D4 and E4 Prostaglandin
Leukocyte extravasation Leukocyte extravasation Leukocyte extravasation is the movement of leukocytes out of the circulatory system, towards the site of tissue damage or infection. This process forms part of the innate immune response, involving the recruitment of non-specific leukocytes...	Cytokines (e.g. chemokines and TNF Tumor necrosis factor Tumor necrosis factor is a cytokine involved in systemic inflammation and is a member of a group of cytokines that stimulate the acute phase reaction... Leukotriene B4 Leukotriene B4 Leukotriene B4 is a leukotriene involved in inflammation. It is produced from leukocytes in response to inflammatory mediators and is able to induce the adhesion and activation of leukocytes on the endothelium, allowing them to bind to and cross it into the tissue... Chemotactic factors for neutrophils and eosinophils
Unless otherwise specified, the reference for this table is:

The reaction may be either local or systemic. Symptoms vary from mild irritation to sudden death from anaphylactic shock

Anaphylaxis

Anaphylaxis is defined as "a serious allergic reaction that is rapid in onset and may cause death". It typically results in a number of symptoms including throat swelling, an itchy rash, and low blood pressure...

.

Treatment and prognosis

Treatment usually involves epinephrine

Epinephrine

Epinephrine is a hormone and a neurotransmitter. It increases heart rate, constricts blood vessels, dilates air passages and participates in the fight-or-flight response of the sympathetic nervous system. In chemical terms, adrenaline is one of a group of monoamines called the catecholamines...

, antihistamines, and corticosteroid

Corticosteroid

Corticosteroids are a class of steroid hormones that are produced in the adrenal cortex. Corticosteroids are involved in a wide range of physiologic systems such as stress response, immune response and regulation of inflammation, carbohydrate metabolism, protein catabolism, blood electrolyte...

s. If the entire body gets involved, then anaphylaxis can take place, which is an acute, systemic reaction that can prove fatal.

Examples

Some examples:

• Allergic asthma
  Asthma
  Asthma is the common chronic inflammatory disease of the airways characterized by variable and recurring symptoms, reversible airflow obstruction, and bronchospasm. Symptoms include wheezing, coughing, chest tightness, and shortness of breath...
  
  
• Allergic conjunctivitis
  Conjunctivitis
  Conjunctivitis refers to inflammation of the conjunctiva...
  
  
• Allergic rhinitis
  Allergic rhinitis
  Allergic rhinitis, also known as pollenosis or hay fever, is an allergic inflammation of the nasal airways.It occurs when an allergen, such as pollen, dust or animal dander is inhaled by an individual with a sensitized immune system...
  
   ("hay fever")
• Anaphylaxis
  Anaphylaxis
  Anaphylaxis is defined as "a serious allergic reaction that is rapid in onset and may cause death". It typically results in a number of symptoms including throat swelling, an itchy rash, and low blood pressure...
  
  
• Angioedema
  Angioedema
  Angioedema or Quincke's edema is the rapid swelling of the dermis, subcutaneous tissue, mucosa and submucosal tissues. It is very similar to urticaria, but urticaria, commonly known as hives, occurs in the upper dermis...
  
  
• Urticaria
  Urticaria
  Urticaria is a kind of skin rash notable for pale red, raised, itchy bumps. Hives is frequently caused by allergic reactions; however, there are many non-allergic causes...
  
   (hives)
• Eosinophilia
  Eosinophilia
  Eosinophilia is a condition in which the eosinophil count in the peripheral blood exceeds 0.45×109/L . A marked increase in non-blood tissue eosinophil count noticed upon histopathologic examination is diagnostic for tissue eosinophilia. Several causes are known, with the most common being...
  
  
• Penicillin
  Penicillin
  Penicillin is a group of antibiotics derived from Penicillium fungi. They include penicillin G, procaine penicillin, benzathine penicillin, and penicillin V....
  
   allergy
• Cephalosporin
  Cephalosporin
  The cephalosporins are a class of β-lactam antibiotics originally derived from Acremonium, which was previously known as "Cephalosporium".Together with cephamycins they constitute a subgroup of β-lactam antibiotics called cephems.-Medical use:...
  
   allergy
• Food allergy
  Food allergy
  A food allergy is an adverse immune response to a food protein. They are distinct from other adverse responses to food, such as food intolerance, pharmacological reactions, and toxin-mediated reactions....
  
  

External links

• Type I hypersensitivity (Flash Animation)