What is allergic reaction
Allergic reaction is the condition when a person’s immune system becomes hypersensitive to a normal harm-less objects or particles and triggers the unwanted inflammation。
Purpose and process of normal inflammation
Normal inflammation:
-used to fight foreign pathogens or bacterial.
-when a body is invaded with foreign harmful particles…the body immune response will trigger the production of antibodies (lgE). This antibodies will then bind to the mast cells or basophills cells which are also categorized under white blood cells of immune system.
-Upon second time of exposure to the same allergen or pathogens. The binding of them to the lgE secreted previously lead to the release of mediator and inflammation chemicals from the mast cells .
-This inflammatory chemicals will then in turn bind to histamine receptors on our body cells .For examples the binding of inflammatory factors to H1receptors on the blood vessel wall of nose, will cause the dilation of the blood vessels on that region, leading to swelling and redness since the dilated blood vessels allow more blood flow to that region. This also allow more white blood cells supplied to the infected region and ‘EAT’、engulf the bacterila and allergens there.
Abnormal inflammation triggered by allergic response also having the same mechanism of action but the difference is the inflammation response is over manifested and the reaction is over amplified.
Treatment
A)cetirizine.
-To treat this we usually use the drug which can block the H1 receptors and preventing the binding of inflammatory agents or chemcals (secreted by white blood cells in response to allergens ) to the receptors on our body cells exp blood vessels cells etc.
-This drug is known as antagonist since it blocks the action of agonist in the case of inflammation it is histamine. Eg: desloratine
-The other type of drug will switch off the receptors and provide favorable inverse effect (opposite of allergic effects), this kind of drug is known as inverse agonist eg cetirizine.
How an inverse agonist works?
-In our body we have many chemicals eg histamine which can only produce their effects when they bind to receptors.
- But in our body the cell receptors can exist in two states resting state or activate state. In the presence of histamine (during allergic reaction) the receptors mostly exist in activated state and the binding of histamine to activated H1 will causes inflammation response. So if we can switch off the receptors back to resting state through the drug then histamine will not be able take their action since histamine can only bind to the activated receptors not the resting one, so there will be no inflammation itchyness, rash … occur. This is the 1st mechanism of action of cetirizine.
-For the receptors which are not switched off cetirizine will bind to them and produce a reverse response example it causes blood vessel to constrict instead of relax( in inflammation of allergic reaction).This is the second way of treatment given by cetirizine.
-But then this second action is less significant due to the properties of inverse agonist.
-Although cetirizine has less sedative effect as compare to the 1st generation of drug but it still posses some effect of it. Cetirizine can cross the blood brain barrier to some extend which meant that it may affect central nervous system of our body.
B)Desloratadine
- Desloratadine is a metabolite of loratadine.
-It acts by blocking the receptors for histamine so inflammation can not be result. But it does not really reverse the effect of inflammation(not like Cetirizine).
-But the advantages of Desloratadine is it does not cause drowsiness at all because it does not readily enter the central nervous system, and does not bring sedative and adverse effect to our brain. So it is more suitable and always prescribed to children who are having less developed blood brain barrier(protecting barrier for brain)
-Desloratadine is a kind of reversible competitve antagonist which meant they compete with histamine for the same type of receptors so reducing the effect of histamine towards inflammation. Since it is reversible which means that if the histamine agonist is in large and excess amount the effect of this drug will be lost. (but this only occur in very severe rare case) |
| Desloratadine |
 |
| centrizine HCl |
Comparisons of the 2 drugs discussed above
1.Generation of drugs:
-Both are second generation of drugs(modified and better).
- Cetirizine is the metabolites of hydroxyzine while Desloratadine is the metabolites of loratadine.
2.Type:
-Both are antihistamine.
- But Cetirizine is an inverse agonist(produce opposite response to inflammation) but Desloratadine is a reversible competitive antagonist(blockere)
3.Mechanism of action:
-Both do not cure but they only relieve the condition. The symptoms can only be wiped out if the allergen is removed.
-For cetirizine it works by switching off the receptors of histamine and this action is enhances when cetirizine produce the reverse effect of inflammation (flavourable effect).
-On the other hand, desloratadine works by blocking the receptors only.
4.Minimun age:
Both are 6 months of age
5.Side effect:
-Cetirizine:Drowsiness (BUT BETTER THAN 1st generation drug); dry mouth; stomach pain (in children); tiredness; trouble sleeping (in children).
-Desloratadin: Most common side-effects are fatigue, dry mouth, headache, and gastrointestinal disturbances.
Conclusion
Cetirizine was associated with significantly greater suppression of skin reactivity to histamine compared with desloratadine. BUT desloratadine has no sedative effect on central nervous system since it can not enter the brain easily (better for children).
Invasion by Bacterial //allergen àinflammation of nose to fight bacterial or allergens àbut leads to unwanted symptoms like runny nose, red and swelling nose à bacterials and allergens are eaten by white blood cells but the inflammatin is still there àso drug is used to stop the inflammation
Just like body fever triggered to kills bacterials but caused damage to our body at the same time so paracetamol is used to stop the fever and reduce body temp.
Very informative post. I got useful things here. Thanks for sharing. recurrent abdominal pain
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