Histamine

Get an overview of histamine including its biological role effects on the body and involvement in allergies and immune responses.

Overview of Histamine:

• Chemical Nature: Histamine is an amine compound (hist + amine) released by tissues during allergic and inflammatory reactions, as well as in response to injury.
• Function: Acts as a local response against parasite infections and protects cells from foreign substances.

Structure:

• Histamine consists of an amine group (NH₂) attached to an imidazole ring.

Biosynthesis:

• Precursor: Histidine
• Enzyme: L-histidine decarboxylase catalyzes the decarboxylation of histidine to produce histamine.

Storage of Histamine:

• Primary Sources: Basophils and mast cells (types of white blood cells) store most of the body’s histamine in granules.
• Other Locations:
  • Brain’s hypothalamus (acts as a neurotransmitter)
  • Enterochromaffin-like (ECL) cells of the stomach
  • High concentrations in skin, mucosal cells of the lungs, intestine, urinary tract, and cerebrospinal fluid.

Mechanism of Release:

Histamine is released from mast cells through a process involving:

1. Sensitization: Exposure to an allergen leads to the production of Immunoglobulin E (IgE) antibodies.
2. Binding: IgE antibodies attach to receptors on the surface of mast cells.
3. Activation: Upon re-exposure, the allergen cross-links the bound IgE antibodies.
4. Degranulation: This triggers mast cells to release histamine and other inflammatory mediators such as prostaglandins, kinins, and platelet-activating factor.

Common Antigens

• Antigens that can trigger histamine release include proteins and polysaccharides from sources like dust, pollen grains, food substances, venoms, toxins, detergents, and various chemicals.

Physiological Effects:

• Hypersensitivity Reactions: Itching, sneezing, watery eyes, runny nose.
• Smooth Muscle Contraction: Bronchoconstriction (asthma) and ileum contraction.
• Vasodilation: Causes hypotension.
• Gastric Secretion: Increases HCl secretion, leading to acidity.
• Inflammation: Causes pain and allergic reactions.

Catabolism or Degradation of Histamine

Histamine is inactivated by enzymes that alter its structure, preventing it from binding to its receptors:

• Diamine Oxidase (DAO): Metabolizes extracellular histamine.
• Histamine-N-Methyltransferase (HNMT): Metabolizes intracellular histamine.

Thank you for reading from Firsthope's notes, don't forget to check YouTube videos!