II. Classification Based on the Different Mechanisms of Pathogenesis
II. Classification Based on the Different Mechanisms of Pathogenesis
Coombs and Gell (1963) proposed 5 types of hypersensitivity based on the different mechanisms of pathogenesis. They are the following::
1. Type I : Anaphylactic hypersensitivity.
2. Type II : Antibody - dependent cytotoxic hypersensitivity.
3.Type III : Immune complex - mediated 'hypersen sitivity.
4. Type IV: Cell - mediated hypersensitivity.
5. Type V: Stimulatory hypersensitivity.
Of these 5 types, type IV alone is delayed type of hypersensitivity and the other types are immediate type.
Type I: Anaphylactic Hypersensitivity (Anaphylaxis)
Anaphylaxis is defined as an allergic reaction to an foreign substance to a foreign substance to which it has previously become sensitized resulting from the release of histamine, serotonin and other vasoactive substances.
The term anaphylaxis was coined by Richet in 1902 and it means with protection. The term anaphylaxis comes from the Greek words. ana = against (without), phylaxis = protection.
Anaphylaxis is an immediate type of hypersensitivity.
Anaphylaxis is a severe life threatening allergy. It must be treated as a medical c emergency.
In anaphylaxis, antibodies are fixed on the surface of mast cell and basophis in sensitized host. The antigen combines with the cell fixed antibody, leading to release of pharmacologically active substances (vasoactive amines) which produce the clinical reaction. They affect the skin, heart and respiratory system.
Factors causing Anaphylaxis
Anaphylactic reactions are caused when the sensitized animals receive allergens for the second time.
The factors causing anaphylaxis are:
- Drugs
- Penicillin
- Venom
- Food
- Food additives
- Egg
- Shellfish.
- Pollen grains
- Temperature- Heat or chillness
Highlights
Anaphylaxis
Anaphylaxis is a life threatening allergic reaction.
It is an immediate type hypersensitivity.
It belongs to type I hypersensitivity.
It is caused by allergic substances which enter the body for the second time. .
When the allergic substance enters the body for the first time, it produces IgE antibodies.
These antibodies bind to the mast cells and basophils. They do not cause any harm at this time.
When the same antigen enters the body for the second time, they bind to the antibodies fixed on the mast cells and basophils. ation Fabide
• This binding of antigen and antibody stimulates the mast cells to produce vasoactive amines.
• The vasoactive amines bring about allergic reactions,
Causes
• Anaphylaxis is caused by the following factors. Drugs - Penicillin
• Venom from insect bites or stings of bees, wasps, etc.
• Food
• Food additives
Egg
Shellfish Symptoms
Pollen grains
Temperature - Heat or chillness.
symptoms:
The symptoms of anaphylaxis appear suddenly and quickly within minutes or within an hour. The symptoms include:
- Iching
- Hives - Elevation of skin with reddish patches.
- Blue tinged skin because of the lack of oxygen.
- Urticaria
- Oedema
- Swelling of the lips
- Swelling of the tongue
- Swelling of the throat
- Pain in swallowin
- Swelling of the conjunctiva
- Running nose
- Wheezing
- Coughing
- Vasodilation
- Low blood pressure
- Unconsciousness
- Abdominal pain
- Diarrhoea
- Vomiting
- Headache
- Cardiac arrest - Death
Diagnosis
- Hives
- Low blood pressure
- Bluish skin
- Swelling of lips and throat
- Wheezing
- Swelling of the eyes, face, etc.
- Abnormal heart rhythm
- Fluid in the lungs.
Control.
Anaphylaxis can be prevented by avoiding contact with allergens.
Treatment
The following drugs are used for the treatment of anaphylaxis.
Epinephrine
Isoprenaline
Sodium chromoglycate.
Symptoms of Anaphylaxis
Anaphylactic reactions cause the following symptoms .
1. Death:
When penicillin injection is given to a sensitive person, he shows anaphylactic reactions. There is intense constriction of bronchi and branchi oles Smooth muscles contract and the capillaries dialate. The person begins wheeze "and within minutes, he dies due to asphyxia *. Similar reactions also occur rarely, when a man is stung by bees or wasps.
2. Diarrhoea and Vomiting:
These are caused by food allergens.
3. Urticaria:
Urticaria is a vascular reaction of the skin marked by slightly cievated patches which are reddish or paler than the surrounding skin. It causes chung. It is caused by certain foods or drugs.
4. Atopy:
Atopy literally means out of place or strangeness The term atapy. was coined by Coca (1923). This term is used to refer to naturally oc curring familiar hypersensitivities such as hay fever and asthma.
The antigens responsible for atopy are called atopens. They may be inhal unts (pollens, house dust, etc.). ingestants (egg, milk, etc.) or contactants. They elicit the production of antibodies of the type IgE These antibodies are called reagin antibodies.
The tendency to develop atopy is genetically determined. Atopy therefore in families. Al l human beings are capable of forming reagin antibody small amount, but in atopics, the reagin antibodies are produced in large amount.
About 10% of persons have this tendency to overproduce reagin. It has et reparted that bottle feed babies tend to develop atopy in later life more him than breast feed babies.
The atopic allergy is due to the production of IgE artibodies which in tum is due to the deficiency of IgA antibodies. Thus IgA deficiency may predispose to atopy
The symptoms of atopy include conjunctivitis in the eyes, rhinitis in the trafors tract, dermatitis in the skin, urticaria, vomiting, diarrhoea, etc.
Mechanism of Anaphylaxis
The culprit of anaphylactic hypersensitivity is one type of immunoglobulin cailed IgE antibody. When a person receives the allergens (antigens) for the first time, the aller gens get attached to the B cells. The allergens stimulate the B cells to proliferate plasma cells. The plasma cells make IgE antibodies. The IgE antibodies are called reagins and they are made by people who are allergic. The reaginie antibodies have a strong affinity for fixation to mast cells or basophils in skin or mucous surfaces.
The IgE antibodies produced for the first time, get attached to the surfact receptors of mast cells with the help of their Fc fragment. This reaction would not harm the person and the person is now said to be immunized or sensitized for that particular antigen.
As this initial contact with antigen leads to the priming of the B cells, this is known as sensitizing or priming dose.
Subsequent contact with the allergen causes manifestations of hyper sensitivity. This is known as shock - dose.
When the animal is exposed to the same antigen for the second time the animal would be in danger. The IgE antibodies attached to the surface of mas cells, bind with the antigens. The allergens cross - link * the IgE antibodies at tached to the mast cells. Antigens B cell (Pollen) Plasma cell IgE Antibodies Mast cell .Sara obligation
This cross - linking of IgE antibodies triggers the mast cells and a series of enzymatic reactions occur inside the mast cells. As a result the mast cells lease granules,
The phenomenon of releasing of granules from mast cells is called degranu lation. These granules contain substances like histamine, serotonin, heparin, These substances are the primary cause for anaphylaxis. are of two types, namely:
The manifestations of anaphylaxis are due to the mediators. The mediators
Primary mediators
Secondary mediators,
The primary mediators include histamine, serotonin, heparin, etc. They are released from the granules of mast cells and basophils. They bring about their action quickly and immediately.
The secondary mediators are produced by leukocytes upon the stimulant of mast cells. They act slowly. They include slow reacting substance of anaphylaxis (SRS - A), prostaglandins, leukotrienes, platelet activating factor (PAF), etc.
In addition to the above mediators, complement activation releases and phylatoxins.
Symptoms
The symptoms of anaphylaxis appear suddenly and quickly within minutes or within an hour. The symptoms include:.
- . Itching
- Hives - Elevation of skin with reddish patches.
- Blue tinged skin because of the lack of oxygen.
- Urticaria
- Oedema
- Swelling of the lips
- Swelling of the tongue
- Swelling of the throat
- Pain in swallowing
- Swelling of the conjunctiva
- Running nose
- Wheezing
- Coughing
- Vasodilation
- Low blood pressure
- Headache Cardiac arrest - Death
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