Antihistamines: Definition, Classification, H1, H2, H3, H4 Antihistamines

Medicines are generally used to cure many different types of diseases.

Antihistamines are defined as the drugs that are used to cure many allergic diseases such as allergic rhinitis. These kinds of drugs are easily available and they are low in cost so people take this kind of drug to cure many different types of infections that cause allergies. This kind of drug helps to relieve sneezing and some of the pollen dust mites and animal allergies also. Antihistamines do not cause any side effects to humans so they can be consumed easily. This type of medication is a short-time therapy and they give an instant relief to any of the allergic conditions such as sinusitis.

Some conditions cannot be treated with the help of antihistamines which includes the respiratory tract infection, particularly the lower respiratory tract. These are used to cure allergic reactions that include common cold infections.

Define Antihistamines

Antihistamines are the type of drug that blocks the action of the histamine secretion. Histamine is defined to be as the organic compound that can control the activity of the immune responses and It also plays a role as the neurotransmitter in the transmission of the nerve impulse. Histamines play a great role in the inflammatory responses that are produced by many types of allergic reactions. Histamine is secreted by the basophil cells of the white blood cell and is also present in the connective tissue that is present inside the human body.

Antihistamines Classification

There are mainly four antihistamines classification that include H1 antihistamine, H2 antihistamine ,H3 and H4 antihistamine.

H1 Antihistamine

This type of antihistamine is defined as a compound that suppresses and blocks the function of the H1 receptor.

H1 antihistamine binds to the H1 receptor and blocks the function of the H1 receptor as it includes many different functions such as the constitutive function. H1 antihistamine Works antagonistically to the histamine as histamine binds to the H1 receptor and activates the function of this receptor whereby the H1 antihistamine blocks the function of the H1 receptor.

Antihistamine classification are into two different types of generation that are first generation and second-generation

First generation antihistamines

These are those antihistamines that are available at a very cheap rate. These first generation antihistamines play a great role in preventing the allergy symptoms but only to some amount.

Second-generation antihistamines These antihistamines are defined as the drugs that are more selective than the first-generation antihistamines.

These antihistamines work very selectively for the peripheral H1 receptors but not as much effectively as central nervous system H1 receptors.

The reason behind this selectivity is that the peripheral receptors occur in the form of Zwitter ionic concentration at pH of about 7.4. The peripheral receptors are very polar in nature so they cannot cross the blood-brain barrier and these are used to play the role outside the central nervous system so antihistamine such as the cetirizine is very reactive against the peripheral receptors.

H2 Antihistamines

These antihistamines are known to play a very important role at the H2 histamine receptor site. H2 antihistamine plays a different role in the mucosa parietal cell and these cells are the part of endogenic signaling pathway that secrete the secretion for the gastric acids. These are used as a first line therapies that are used to cure many gastrointestinal abnormalities that includes gastroesophageal reflux disorder and also the peptic ulcers that occur in the human body. Cimetidine is an example of the H2 antihistamine that play a great role as an antagonist against the testosterone and androgens.

H3 Antihistamines

These are defined as the medicines that block the activity of the histamine in the H3 receptors that are usually present in the brain. The H3 receptors are defined to be as the inhibitory Autoreceptors that are located on the histaminergic nerve terminals. These receptors play a very gross role in the release of the histamine and modify the release of the histamine.

H4 Antihistamines

This antihistamine helps to block the activation of the H4 receptors.

Bad Effects Of Antihistamines

• The use of antihistamine can lead to constipation in a human.

• It can also lead to trouble urinating.

• It can cause blurred Vision in a human being.

• Some very difficult symptoms such as the dry mouth and dizziness can also be seen due to the intake of the antihistamine.

• These drugs can lead to the degrees in the tear production.