Allergens and Antigens: Immune Response

Allergens and Antigens

• Allergen: a substance that can trigger an allergic reaction
• Antigen: a substance that can trigger an immune response, including an allergic reaction
• Antibody: a protein produced by the immune system to fight an antigen

Types of Allergic Reactions

• Rhinitis: an allergic reaction that affects the nose, resulting in sneezing, runny nose, and itchy eyes
• Erythema: a skin reaction characterized by redness and itching
• Dermatitis: a skin reaction characterized by redness, itching, and inflammation
• Urticaria: a skin reaction characterized by itchy hives
• Anaphylaxis: a severe, life-threatening allergic reaction that can cause difficulty breathing, rapid heartbeat, and a drop in blood pressure

Action of Histamine

• Vascular Effects:
  • Dilation of capillaries, leading to transient hypotension
  • Leaky capillaries, causing fluid and protein leakage, leading to irritation of nasal membranes or hives
• Extravascular Smooth Muscle:
  • Contraction of smooth muscles, leading to intestinal or bronchial constriction
• Cardiac Effects:
  • Rapid heartbeat
  • Possibly leading to arrhythmias at high levels

Indications for Antihistamines

• Acute allergic reactions:
  • Hay fever
  • Insect bites
  • Pet dander
  • Minor food/drug allergies
  • Rhinitis
  • Dermatitis
  • Itchy eyes
  • Scratchy throat
  • Urticaria
• Dry nasal secretions (anticholinergic component)
• Sleep aids (due to sedative properties)
• Reduction of nausea and vomiting (including motion sickness)

First Generation Antihistamines

• Sedation is greatly increased when taken with:
  • Hypnotics
  • Sedatives
  • Anxiolytics
  • Alcohol
  • Narcotic analgesics
  • Muscle relaxants
  • Other CNS depressants

Advantages of 2nd Generation Antihistamines

• More selective for peripheral H1-receptors
• Less sedating
• Less drying
• May be useful for patients refractory to older agents
• Longer half-life, may be dosed once daily

Side Effects of Antihistamines

• More Common SE’s:
  • Drowsiness and sedation
  • Dry mouth (xerostomia)
  • Dry mucous membranes
• Less Common SE’s (typically only seen in overdose):
  • Hypotension
  • Urinary retention
  • Rapid heartbeat (tachycardia)
  • Epigastric distress

Antihistamine Groups

• 1st Generation:
  • Ethanolamines (most sedating, highest anticholinergic effects)
  • Alkylamines (highest potential for CNS stimulation, medium for epigastric distress)
  • Phenothiazines (high anticholinergic effects, moderate sedation)
  • Other first generation (hydroxyzine, very sedating)
• 2nd Generation:
  • Cetrizine
  • Fexofenadine
  • Loratidine
  • Desloratidine

Antiallergic Agents

• Cromolyn Sodium (Sodium Cromoglycate):
  • Mechanism of Action: prevents release of histamine through degranulation of mast cell
  • Preventative strategy only
  • Indications: chronic bronchial asthma, allergic rhinitis, food allergies, and allergic conjunctivitis

Learn about the differences between allergens and antigens, and how they trigger immune responses, including allergic reactions. Understand various types of allergic reactions like rhinitis, erythema, and dermatitis.