How Do Antihistamines Function in the Human Body?
Antihistamines are a class of drugs that are used to cure the allergies triggered by the physiological action of the histamines. Histamines are the organic compounds that are involved in the local immune responses and act as a neurotransmitter. Histamines take part in the inflammatory response and act as a mediator of pruritus. Basophils produce histamines, and they are found in the connected nearby tissues. Histamines increase the permeability of the white blood cells, and this makes the pathogens to get more engaged with the infected tissues. Antihistamine examples are ranitidine, loratadine, meclizine, cetirizine, etc.
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The Action of Antihistamines on the Human Body, How do they Suppress Antihistamines?
Our immune system might start to mistake a harmless substance as a dangerous material or pathogen, this shows the start of the development of an allergy. These harmless foreign substances that are considered dangerous by the immune system, are named as the allergen. One of the most common allergens are pollen and milk (Lactose intolerance).
When the body comes in contact with any of the allergen, the immune system responds harshly in order to suppress the upcoming foreign substance named allergen. To deal with the allergens the immune system releases various chemicals and antibodies. One such chemical is named histamine, the histamine is the chemical which transports the information regarding the allergen to the brain and thus causing the major symptoms of the allergies. These symptoms can be nasal swelling, runny eyes, and, in some cases, mouth itching.
Antihistamines, as the name suggests (Anti-histamine), are the chemical that stalls and suppresses the action of the histamines. Thus protecting them from the various symptoms of the allergies. Antihistamine fights for a space in the neural receptors or the binding site of neural receptors with the histamine and thus stopping them from sending the information regarding the allergy to the brain.
However, Antihistamine has a limited range of applications and can not be used to treat many types of symptoms like for the symptoms of nasal congestion and swelling, the doctors do recommend the use of decongestant rather than Antihistamine. In many conditions, Antihistamine and decongestant are prescribed together along with some other types of medication depending on the patients.
Antihistamine Drugs Classification
Antihistamines drugs can be classified into two types according to the H receptor-targeted: -
H1: They are generally used to treat allergic reactions as well as mast cell-mediated disorders. This category can be further divided into two classes: first-generation H1 antihistamines, which have a central effect and are used as sedatives; and second-generation H1 antihistamine which have a lesser central effect and are used as antiallergic drugs.
H2: They are used for gastric reflux disease as they help in reducing the production of stomach acid by reversibly blocking the H2 histamines receptors in the parietal cells of the gastric mucosa.
Example of Drugs and Antihistamine Uses According to Antihistamine Classification
H1 First-Generation: Meclizine, Clemastine, Hydroxyzine, Brompheniramine, Dimetindene, Doxylamine, etc. that are used as sedative agents, antiallergic agents, to cure motion sickness, and an antiemetic agent. These drugs have strong sedative action and anticholinergic side effects.
H1 Second-Generation: Loratadine, Cetirizine, levocetirizine, azelastine, fexofenadine, etc. that are used as antiallergic agents, or adjuvant treatment for anaphylactic shock. These drugs are non-sedative or mildly sedative.
H2: Ranitidine, Cimetidine, Famotidine, etc. that are used to reduce the production of stomach acid and are generally used as a second-line treatment or in combination with proton pump inhibitors (PPIs).
Antihistamine Side-Effects
The severity and the frequency of the adverse effects of histamines vary according to the type of antihistamine. The H1 first-generation antihistamines are considered to be more harmful, and they usually cause drowsiness. H1 antihistamines cause anticholinergic effects like dry mouth and eyes, dizziness, urinary retention, mydriasis tinnitus, and tachycardia along with headaches. Benadryl and Chlor-Trimeton are examples of such drugs. The side effects of H2 antihistamines include confusion, dizziness, headaches. For example, Allegra, Clarinex, and Zyrtec. Cimetidine can lead to erectile dysfunction and gynecomastia because of its antiandrogenic effect. Cimetidine and ranitidine decrease the renal excretion of creatinine.
Solved Questions
Question 1. What is the difference between Claritin and Clarinex?
Answer: Claritin is loratadine that is metabolized in the liver to produce Clarinex that is desloratadine.
Question 2. How do antihistamines protect against allergies?
Answer: Antihistamines block the action of the histamines that gets triggered when our body comes in contact with components like pollen, dust mites, etc. These components make the histamines come in action to cause allergies like swelling of the nose, running nose and eyes, etc. Antihistamines block the course of action of histamines and protect us from allergies.
Question 3. Can antihistamines be used to cure skin allergies?
Answer: Yes, antihistamines can be used to take care of insect bites and skin allergies. Zyrtec and Claritin are both best antihistamines to help you with your skin allergies.
Question 4. What is an allergy?
Answer: An allergy is an immunological hypersensitivity that is mediated by immunoglobulin E antibody (IgE). An allergy is not related to any disease or infections.
Question 5. What causes an allergy?
Answer: Allergies generally occur when your immune system reacts to a foreign substance like pollens, bee venom etc. Allergies are basically the reactions of your body coming into contact with foreign substances.
FAQs on Antihistamines: Classification, Benefits & Side Effects
1. What is an antihistamine and what does it do?
An antihistamine is a type of drug designed to counteract the effects of histamine, a chemical released by the body's immune system during an allergic reaction. Its primary function is to relieve symptoms associated with allergies, such as sneezing, itching, watery eyes, and a runny nose.
2. How do antihistamines work to control allergy symptoms?
Antihistamines work by blocking histamine receptors on cells. When an allergen enters the body, the immune system releases histamine, which then binds to these receptors, causing allergic symptoms. Antihistamines compete with histamine and prevent it from binding to the H1 receptors, thereby suppressing the allergic response and providing relief from its symptoms.
3. What are some common examples of antihistamine drugs?
There are many types of antihistamines. Some common examples mentioned in the NCERT syllabus for the 2025-26 session include:
- Brompheniramine (Dimetapp)
- Terfenadine (Seldane)
Other widely known examples include Cetirizine (Zyrtec), Loratadine (Claritin), and Diphenhydramine (Benadryl).
4. What is the main classification of antihistamines?
Antihistamines are primarily classified into two main groups:
- First-generation antihistamines: These were the first to be developed. They are effective but can cross the blood-brain barrier, often causing side effects like drowsiness. An example is Diphenhydramine.
- Second-generation antihistamines: These are newer drugs that do not easily cross the blood-brain barrier, so they are less likely to cause drowsiness. Examples include Cetirizine and Loratadine.
5. What are the common side effects associated with taking antihistamines?
The most common side effect, especially with first-generation antihistamines, is drowsiness or sedation. Other potential side effects may include:
- Dry mouth, eyes, and nose
- Dizziness
- Headache
- Upset stomach
Second-generation antihistamines generally have fewer side effects.
6. Why do some antihistamines cause drowsiness while others do not?
The difference lies in their ability to cross the blood-brain barrier. First-generation antihistamines are lipid-soluble and can easily pass this barrier, affecting histamine receptors in the central nervous system which control alertness, thus causing drowsiness. In contrast, second-generation antihistamines like Cetirizine are designed to be less lipid-soluble and do not readily enter the brain, so they block allergy symptoms in the body without causing significant sedation.
7. Can a person develop a tolerance to antihistamines, making them less effective over time?
Yes, it is possible for the body to develop a tolerance to a specific antihistamine after prolonged, continuous use. This happens because the body may adapt to the drug's presence, reducing its effectiveness. If this occurs, a doctor might recommend switching to a different type or class of antihistamine to restore the desired therapeutic effect.
8. Besides allergies, what are some other medical uses for antihistamine drugs?
While primarily used for allergies like hay fever, some antihistamines have other therapeutic applications. For example, due to their sedative effects, first-generation antihistamines like Diphenhydramine are often used as short-term sleep aids. Certain antihistamines are also effective in preventing motion sickness, treating nausea, or managing symptoms of the common cold.
9. How do antacids and antihistamines differ in their chemical action and purpose?
Antacids and antihistamines are different classes of drugs with distinct functions. Antacids, such as sodium hydrogencarbonate, work by chemically neutralizing excess stomach acid to relieve heartburn and indigestion. In contrast, antihistamines work by blocking the action of histamine at receptor sites to prevent allergic reactions. They do not neutralize any substance but instead interfere with a biological signalling pathway.
10. In which situations should a person avoid taking antihistamines without consulting a doctor?
It is crucial to consult a doctor before taking antihistamines, especially for individuals with certain pre-existing conditions. These include people with:
- Glaucoma
- An enlarged prostate
- Epilepsy
- High blood pressure
- Kidney or liver disease
Furthermore, pregnant or breastfeeding women should always seek medical advice, as not all antihistamines are considered safe for them.
