Histamine- the why and why

#KYJ - Histamine and Mast cells. (H1)

In a course on Respiratory nursing last week I was asked about the role of Histamine. Specifically H1.  Not the H2 variety in gut.

So here is my analogy:  imagination needed.

Say there was a bad guy (Buggsy Peanuts) outside your house.  To deal with him you needed to get out of your house.

You are too big to fit out the window so but you you don't have a key to get out the door.

You are stuck inside with Basil, your frightened mate.  Buggsy the bad guy is outside.  Picture it?

Ok- your friend Basil, can't deal with the bad guy but does have a door key.   Basil opens the front door for you, and you you go and deal with Buggsy.

Specialised White blood cells called Basophils (Basil) possess a key that opens pores inside blood vessel walls.   That key is a protein called Histamine.

Ordinarily, large allergen or pathogen fighting phagocytes can't get out of the blood vessels to fight allergens or pathogens, so basophils inside blood vessels and Mast Cells in your tissues (mast cells are just basophils outside the blood stream), produce this Histamine key to make blood vessels porous to allow white blood cells out to attack invaders.

Think of histamine as a protein like insulin, but instead  of opening doors for Glucose to move , histamine opens doors for immune fighting cells to migrate to the tissues where the epic battle between Buggsy Peanut and the White knights of the immune system attack.

Now the downside to this histamine, is that its very action of Vasodilation and increased permeability, causes plasma and water attracting proteins (Albumin) to leak into tissues - Oedema is the result.

Engorgement of vessels close to the invasion causes the blood flow to increase giving rise to redness.

Now reflect on the last ant sting or mosquito bite.
It was red, hot , swollen and painful/itchy wasn't it.

Local histamine responses in skin causes these same symptoms.  They are called urticaria or hives, and are typical in allergies.

If histamine response is in the nasal mucosal or sinus tissues, then the swelling and congestion is called Allergic Rhinitis (Hay Fever), or in sinuses-sinusitis.

If the histamine response occurs in upper airway structures like Lips, tongue and throat, it is referred to as Angioneurotic oedema, or simply angiooedema.  This is a common symptom in a life threatening allergic reaction, and often compromises the patient's airway.   You will know it as the deadly symptom in Anaphylaxis.
When the response is in the lungs, and bronchial tissues become swollen then histamine is one player in the diseases of Asthma and Broncheolitis.

The above examples are all local effects
When the histamine release is systemic, the massive overwhelming release of histamine in all vessels, causes such vasodilation and capillary leakage, that the patient loses their circulating blood volume (hypovolaemia) and their distribution of blood between arteries and veins becomes critically imbalanced (maldistribution).   This is a complex and life threatening cause of shock.  In severe allergic response it is called anaphylactic shock, and in severe pathogenic responses, it is called Septic Shock.

Recap:
Anaphylaxis causes anaphylactic shock.
Infection causes septic shock.

Histamine is a potent vasodilator . And makes blood vessels engorge and leak.

Can you now see the role (in mild allergies) of drugs that block or inhibit histamine?  These are a family of Anti-cholinergic drugs called antihistamines.   Examples include Loratidine, Zadine, Clarityne, polaramine, Telfast and some of the older phenothiazine drugs like the Rock Star -Promethazine (Phenergan).

All antihistamines cause sedation but only if the drug can cross the blood-brain-barrier.  Phenergan and Polaramine are examples.

Any how- as usual, I digress.

The point is Basil (Basophils) holds the key (Histamine) to the door (vessel wall) so you (Phagocyte) can get out and kill Buggsy Peanut (pathogen or allergen).

If it was you who asked the histamine question, Thankyou for stimulating such a great topic for our #knowingyourjargon (KYJ) series.

Check out the blog for more. There is heaps of #FOANed