Why does histamine cause bronchoconstriction?
Basics first: Crash Course

We will be introducing:
Autonomic divisions, Histamine and Vasoactivity (or Broncho-)
Starring: Innervation of the Lower Respiratory Tract

Lower respiratory tract is mostly innervated by the parasympathetic division, it has only scarce sympathetic-receptors there.

So, why it matters?



Autonomic divisions serve as fight-or-flight and rest-and-digest mechanism for the body as to respond for sudden changes.

You are having a good picnic, your body is in a state of rest. Your body is absorbing nutrient and enjoying your day. Blood isn't needed so much on your hands, or skin, or muscle.
This is the Parasympathetic Division

Suddenly a snake appeared in front of you, you scream, you panicked, and you flight away (or fight?). Your eyes dilate (where is that snake!), your muscles work, your respiratory tract dilates, as to prepare to avoid being killed by the snake. Blood rushs in to your muscles, but shunts on your digestion part, as what every you eat doesn't matter.
This is the Sympathetic Division

What matters?    


Parasympathetic division only knows how to lock down things.
Sympathetic division can lock things, open things, and so, and so, and so.


How does sympathetic division work with so many things?
It has different receptors which reacts differently on input
Imagine people when reacting to clown: NOOOO and YAYYY

Histamine serves as modulation to outputs of autonomic division, directly or indirectly

Histamine is a broad-response chemical used to respond against an infection. As different cells have different ratio of autonomic division, it can cause dilation or constriction on different places.

One thing to note that, in a nutshell, histamine serves to buffer the work of autonomic innervation. It is entirely up to the regular function of the "autonomics" there to do their business.

Vasoactivity (or Broncho-) is achieved by state of balance between innervation by the autonomic division and local signals

Local signals play a part. A balance of signal determines the effect of innervation.
For bronchi, when histamine puddles, local signal sends stronger input than global innervation. Hence, the parasympathetic takes over.




  histamine-caused constriction serve in lungs during allergies


Histamine serves as to protect the lower respiratory tract from inhaled irritants. The lower respiratory tract is innervated by parasympathetic stimulation. Parasympathetic stimulation only knows one command: constrict and strike.
When histamine buffs up the signal, it constricts.




  vasodilator in other parts of body  


Indeed it is. The vasodilation effect is achieved by GPCR-mediated sympathetic innervation.
Histamine buffs a diverse response of sympathetic division, hence dilates the blood vessels.




  Wouldn't it be more practical to vasodilate lungs so that white blood cells can arrive and toxins can be flushed more quickly


In line with WYSIGYG, vasodilation is different from bronchodilation (or constriction). We have noted above that the operators are different, and they serve different functions.

As a note, white blood cells can't just come anywhere they want. Ever seen your nasal mucus being very viscous? That will happen your lung when white blood cells arrive.
White blood cells also cause fibrillation, which succumbs people when does.   

It has been a reason why pneumonia and lung-infection related diseases, tend to succumb people by drowning them in "their own mucus".

Another part of sending much things down your lower respiratory part is...
How could you clean the mess up?
The upper respiratory part is the defense capable of trapping irritants by mucus and send the mucus to your stomach.
Bronchus? Not so much. More mucus will eventually drown your lungs.



I invested 50 minutes for this.