Types Of Allergic Reactions

Hypersensitivity Reactions / Allergic reactions:

These are allergic reactions and there is involvement of immune system.  Trial exposure is necessary for hypersensitivity reactions.

Types of allergic reactions:

There are 4 types of allergic reactions

  Type I hypersensitivity reactions

2.       Type II hypersensitivity reactions

3.       Type III hypersensitivity reactions

4.       Type IV hypersensitivity reactions

Type I hypersensitivity reaction:

Majority drugs are metabolized and have molecular weight < 1000 Dalton but the weight of antigen must be > 8000 Dalton.

In order to increase the weight of drug metabolite from 1000 to 8000 Dalton drug metabolite attach itself with body proteins Heptin called Heptination . Thus the weight of drug metabolite increased to 6000-8000 Dalton and metabolite become complete antigen.

Antigen sensitize B cell to produce IGE antibody . Complete antigen binds to IGE antibody which is present on the surface of mast cell or basophil cells to activate the immune system.

By degranulation release histamine , 5-HT , bradykinin, serotonin all are inflammatory mediators cause anaphylaxis (severe allergic reaction)* types of allergic reactions 

Common clinical features of type I hypersensitivity reaction:

Mild to moderate:

Angioedema,  urticaria,  rashes, redness / erythma, bradycardia.

Severe:

Bronchoconstriction , hypotension,  asthma .

Management of type I hypersensitivity reaction:

Mild to moderate:

•Antihistamines such as cetirizine

•Corticosteroids

•Oxygen

Severe:

Epinephrine (adrenaline)

Dose of epinephrine for type I hypersensitivity reaction:

>16 year = 0.5ml

6-12 year = 0.3ml

<6 year = 0.15ml

Until reaction disappear.

How to administer epinephrine?

Head down , legs up at 60° angle and administered in thigh muscle.

Causes of type I hypersensitivity reaction:

•B lactam antibiotics

•pencillin,  cephalosporins

Type I hypersensitivity reaction Chances is low for higher generation.

Type II hypersensitivity reaction:

Type II hypersensitivity reactions is mediated by IgG, IgM, IgA but most common is IgG antibody.

They cause destruction of cells. Drug metabolite in blood combine with the other proteins on the blood cells surface and become complete antigen.

It produce IgG, IgM, IgA antibodies,  these antibodies react with drug present on the surface of blood cell and cause the activation of complement system.

All haemolytic ( blood related ) allergic reaction are type B allergic drug reaction.* types of allergic reactions 

Examples of type II hypersensitivity reaction:

Cytotoxic reactions

•thrombocytopenia: destruction of platelets

•neutropenia: destruction of neutrophils

•haemolytic anaemia: destruction of RBCs

Causes of type II hypersensitivity reaction:

•quinine

•qunidine

Type III hypersensitivity reactions:

Type III hypersensitivity reactions are also called immune complex because immune complex is formed between antigen and antibodies.

Drug metabolite combine with body protein and form complete antigen which stimulate B cells which cause the production of IgG antibody which form complex with antigen inside circulation.

Some complex are small and some are large , immune complex circulated by blood and due to small size complex leave blood circulation and deposit in some organs where blood flow is turbulent or slow. I.e. kidney

During secondary exposure there will be activation of complement system which result in inflammatory response and chemotaxis,  attraction of phagocytes and neutrophils and other cells causing damage to the organ where this process is happening.* types of allergic reactions 

Examples of type III hypersensitivity reaction:

•interstitial nephritis caused by steroids and older penicillins.

•serum sickness caused by anti thyroids , penicillin,  streptomycin , sulfonamide

Clinical representation of type III hypersensitivity reaction:

Enlarge lymph nodes,  fever, urticaria, rash .

Type IV hypersensitivity reaction:

Type IV hypersensitivity reaction are also called delayed hypersensitivity reactions.

Type IV hypersensitivity reaction is mediated by T cells. There is no detectable antibody. Occur after 48-72 hours after secondary exposure.

Drug metabolite attach with protein sensitized and bind with T cells and waiting for sensitization .

If exposure T cells release cytokines either TH1 or TH2 . If TH1 is sensitized,  release IL2 and interferon alpha which are inflammatory mediators. If TH2 sensitized,  release IL4 and IL5.

Reaction appears after 48 hours that’s why called delayed hypersensitivity reaction.* types of allergic reactions 

Examples of type IV hypersensitivity reaction:

Contact dermatitis caused by topical antifungal, antibiotics, antihistamines.

 

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