Antihistamines and Allergy Treatment

Allergies, Antihistamines, and Antiallergics

• Allergies occur when the immune system reacts to a harmless substance.
• Allergens are substances that cause allergic reactions. Examples include pollen, dust mites, animal dander, certain foods, and insect stings.
• Antigens trigger an immune response. They are often proteins found on the surface of pathogens like bacteria and viruses.
• Antibodies are proteins produced by the immune system in response to antigens. They bind to antigens to neutralize or destroy them.

Allergy Terms

• Allergy: The immune system's response to a foreign substance that's usually harmless to others.
• Allergen: Any substance capable of triggering an allergic reaction.
• Antigen: A substance that triggers an immune response.
• Antibody: A protein produced by the immune system in response to an antigen.
• H1-receptor: A receptor that, when bound to histamine, activates symptoms of allergies like itching, swelling, and redness.
• H2-receptor: A receptor in the stomach that controls acid production.
• Histamine: A chemical released by the immune system during allergic reactions; causes itching, swelling, and redness.
• Rhinitis: Inflammation of the nasal mucous membranes, causing runny/stuffy nose, sneezing, and itching; can be allergic or non-allergic, like viral.
• Urticaria (Hives): Skin condition characterized by raised, itchy welts or hives. Often caused by allergic reactions.
• Eczema: Skin condition characterized by inflammation, dryness, itching, and rash that is unrelated to allergies.
• Erythema: Redness of the skin due to increased blood flow.
• Dermatitis: Skin inflammation causing redness, swelling, and itching: unrelated to allergies.
• Anaphylaxis: Severe, potentially life-threatening allergic reaction that requires immediate medical attention.

Antihistamines

• Antihistamines: Medication that blocks the effects of histamine by binding to H1 receptors.
• Antihistamines reduce symptoms of allergies such as itching, swelling, and runny nose.
• First-generation antihistamines: Often cause drowsiness, they cross the blood-brain barrier, and are more likely to be sedating.
• Second-generation antihistamines: Less likely to cause drowsiness. They don't cross the blood-brain barrier as easily, and fewer sedative effects.

Antihistamine - General Info

• Competitively blocks H1 receptors, preventing histamine from binding and exerting its effects.
• H1 receptors are primarily involved in allergic responses.
• H2 receptors are involved in gastric acid secretion.
• H3 and H4 receptors are involved in neurotransmission and immune function.

First Generation Antihistamines

• Diphenhydramine (Benadryl), Chlorpheniramine (Chlor-Trimeton), Doxylamine (Unisom), Promethazine (Phenergan), Hydroxyzine (Atarax, Vistaril), Brompheniramine (Dimetapp), Meclizine (Antivert, Bonine), Cyproheptadine (Periactin).

Second Generation Antihistamines

• Loratadine (Claritin), Cetirizine (Zyrtec), Fexofenadine (Allegra), Desloratadine (Clarinex), Levocetirizine (Xyzal).

Indications for Antihistamines

• Acute allergic reactions: Hay fever, insect bites, pet dander, minor food or drug allergies.
• Dry nasal secretions: Often found in cough and cold medications.
• Sleep aids: Due to sedative properties, but less so in later generation.
• Nausea/Vomiting/Motion Sickness: Some are used to reduce this.

Other Considerations

• Drug interactions: Older generation antihistamines can have significant sedative effects when combined with other CNS depressants like hypnotics, alcohol, or other sedatives.
• Adverse effects of antihistamines: Drowsiness, dry mouth, hypotension, rapid heartbeat, anorexia, urinary retention, etc.; often more common in first-generation drugs.
• Contraindications: Patients with cardiovascular disease, pre-existing hypertension, urinary retention, or those taking specific medications are usually advised not to use certain antihistamines.
• Cromolyn Sodium: An antiallergic drug not an antihistamine, it inhibits the release of inflammatory mediators preventing mast cell degranulation.

Additional Notes

• Different types of antihistamines have different strengths, side effects, and indications.
• Always consult with a healthcare professional before taking any medication, especially when pregnant or breastfeeding.
• The information here is for educational purposes only and should not be used as a substitute for professional medical advice, diagnosis, or treatment.