A Question For The Lone Ranger

[Shelli]

Dear Lone Ranger;

I'm presently healing from a cat fight that I got caught in the middle of a few nights ago which prompted me to question why wounds itch when they're healing.

Do you know, and if so, could you please tell me how the body works in regards to that?

Signed;
Itchy in Springfield

[The Lone Ranger]

Hi Shelli!

You might be surprised to know that there is no universally agreed-upon explanation for why healing wounds itch. The most widely-accepted explanation has to do with secretion of histamine by damaged tissues.


When you're wounded, mast cells in the damaged tissue release histamine. Histamine has a number of effects. It makes nearby blood vessels dilate and it makes capillaries become more porous. The result is that more blood flows to the site of the injury, causing reddening, heating and swelling. Histamine also stimulates nerve endings, causing pain.

That much is clear.


It's commonly thought that as the wound heals, low levels of histamine secretion by mast cells and basophils (basophils are a type of white blood cell) cause low-level excitation of pain receptors -- and that's why the wound itches. That would explain why antihistamines are often effective in relieving itching. Since they block production of histamine, they prevent the stimulation of nerve endings by histamine that seems to cause itching. (If histamine is directly applied to the skin, it causes an itching sensation, which lends support to the hypothesis that it's continued low-level histamine secretion that causes a healing wound to itch.)


Hope that helps!

Cheers,

Michael

[LadyShea]

Interesting! Our bodies are so fucking cool.

[Shelli]

aHA! Histamines.. that makes sense.

Thanks, TLR.

* Shelli gets up to take an antihistamine

[Stormlight]

Bollocks. It itches because angels lick the wound.

[Watser?]

Originally Posted by Stormlight

Bollocks. It itches because angels lick the wound.

Now I have an image of angels licking bollocks

[Shelli]

[Stormlight]

[viscousmemories]

I was wondering why the doctor prescribed Benadryl when I had that mysterious rash on my arms recently.

[livius drusus]

That's fascinating, TLR. This should become the masked man's version of the Auntie Unmentionables thread.

[Shelli]

Originally Posted by livius drusus

That's fascinating, TLR. This should become the masked man's version of the Auntie Unmentionables thread.

[D. Scarlatti]

Originally Posted by livius drusus

This should become the masked man's version of the Auntie Unmentionables thread.

"Ask Auntie Histamine."

[Crumb]

And he should totally use the Unmentionables to answer the questions!

Dear The Lone Ranger,

It hurts when I do this.

Sincerely,

Admiral Fulton J Boondesvill XIV, Esq, Mrs

[JoeP]

Dear Lone Ranger:

Talking about wounds and histamine making blood vessels dilate, what's the reason for inflammation in general? It seems like the benefits of getting more blood to the wound, joint, or diseased area are outweighed both by the psychological impact of the pain and the long-term (oxidant?) damage caused by chronic inflammation.

Yours,
Inflamed* of Johannesburg

* Metaphorically only

[The Lone Ranger]

Originally Posted by JoeP

Dear Lone Ranger:

Talking about wounds and histamine making blood vessels dilate, what's the reason for inflammation in general? It seems like the benefits of getting more blood to the wound, joint, or diseased area are outweighed both by the psychological impact of the pain and the long-term (oxidant?) damage caused by chronic inflammation.

Yours,
Inflamed* of Johannesburg

* Metaphorically only

Unpleasant as inflammation is, we'd have rather shorter life expectancies if it weren't for inflammation.

Inflammation is all about increasing blood flow to the site of an injury. This has the immediate effect of causing it to bleed. That's absolutely important. A wound, especially a puncture wound, provides an ideal means of entry into the body by bacteria, viruses and parasites. The initial bleeding of a wound helps to clean it out and drastically reduces the likelihood of bacteria, etc. getting into the general circulation, where they could do tremendous damage.

Because blood flows into the site of injury faster than it flows out, and because the capillaries in the injury site become porous and so leak fluid, there is swelling. It actually works rather well to isolate the site of injury, making it very difficult for bacteria, etc. to get out of the injury site and into the general circulation.

There are several other important components to the inflammatory response. The increased flow of blood to the area of injury brings in complement proteins and antibodies that help fight infection. The increased blood flow also brings in neutrophils and other white blood cells that help to fight infection. The increased porosity of the capillaries makes it easier for macrophages and other specialized white blood cells to move into the injury site, where they'll destroy bacteria and damaged or dead body cells.

The release of histamines and other chemicals by affected tissues not only stimulates inflammation, but also helps to attract white blood cells to the injury site.

Another reason inflammation is a good thing is that the increased flow of blood to the injury site causes the temperature of the area to rise. (The blood coming from the interior of the body is warmer than is blood in the outer portions of the body, so increasing blood flow to an injury site will cause it to heat up.) Increasing the temperature by just a few degrees increases the metabolic rate of (surviving) cells, speeding the repair of injured tissues and increasing the efficiency with which the various white blood cells can function to dispatch invaders. Also, many bacteria are quite temperature-sensitive, and raising the temperature of damaged tissues by just a few degrees can seriously slow down bacterial reproduction, giving the body's defenses a better chance of fighting off the infection.


The fact that inflamed tissue is so painful is probably adaptive, too. The very fact that inflammation is so painful encourages us not to put any pressure on an injury site or to otherwise use it. That reduces the chance of further injury, speeds healing time, and reduces the chance that pressure on the injury site might force pathogens into the general circulation.



So, while the inflammatory response is most-definitely unpleasant, it's definitely a good thing.

Cheers,

Michael

[livius drusus]

[JoeP]

And btw, the Wikipedia page on inflammation has some truly lovely photos. You should not visit it.

[Wandering Wonderer]

Originally Posted by JoeP

And btw, the Wikipedia page on inflammation has some truly lovely photos. You should not visit it.

Why did I click on the link?

[JoeP]

Originally Posted by Wandering Wonderer

Why did I click on the link?

I'm guessing Satan made you do it?

[Shelli]

Wow! That is so cool!

Thanks, TLR.

[livius drusus]

Now that is a quality pun.

[The Lone Ranger]

The subject of nut allergies came up in chat yesterday, and it occurred to me that a discussion of anaphylaxis might be in order.


We discussed above some of the components of an immune response. Any chemical that triggers an immune response (and in particular, production of disease-fighting proteins called antibodies) is an antigen.

You're born with innate immunity; that is, from the time of your birth, your immune system "knows" how to defend you against some diseases. For instance, the blood proteins that determine your blood type (ABO) help protect you against some diseases. People who are Blood Type A or O have greater resistance to the bubonic plague than do people who are Blood Type B, to take an example. This is probably why it's relatively rare to find someone of northern or western European descent who has Type B blood. The bubonic plague killed millions of Europeans during the 13th and 14th centuries, but especially people with Blood Type B.

Indeed, it has been suggested that one of the reasons the plague outbreaks had virtually disappeared by the 15th century was because most of the people in Europe with Type B blood had been killed off, making it more difficult for the disease to spread.

It has been shown that people with Blood Type O are more resistant to SARS (severe acute respiratory syndrome) than are people of other blood types.

If you have Type A blood, it tends to clot more readily than blood of Types B or O. On the other hand, the very fact that it tends to be more prone to clotting means that people with Type A blood are at higher risk of blood clots forming inside the blood vessels and blocking blood flow, causing a myocardial infarction (heart attack).



Anyway, the point of all this is that you're born with some ability to defend yourself against disease. (There are over 300 known blood antigens; at least some of them help protect against disease.)

Most immunity, however, is acquired. Your immune system has to "learn" to protect you against most antigens. When your system encounters such an antigen for the first time, it takes a relatively long time to mount a full immune response, because the specialized white blood cells (lymphocytes) that produce protective antibodies (B lymphocytes) and that initiate and coordinate the immune response (T lymphocytes) must first go through a process in which they "learn" to recognize the antigen and produce antibodies that will be effective against it.

After that first encounter, the presence of antibodies circulating in the blood, plus the presence of "memory cells" (B- and T-lymphocytes that can defend against that specific antigen) ensures that the next time the antigen is encountered, the immune response will be quicker and (hopefully) more efficient.




Now, of course, the immune system doesn't always work perfectly. Sometimes, the immune system responds to substances that are perfectly harmless. When an immune response is mounted against a substance that is not harmful to the body, a substance that does not trigger an immune response in a person with a normally-functioning immune system, that is an allergy.

Normally, an allergy is little more than an annoyance. You remember that part of a normal immune reaction is release of histamine, which causes dilation of blood vessels. As blood vessels in the affected area become dilated and more porous, more blood flows to the area, and the increased porosity of capillaries in the area means that fluid leaks out of the blood and into the affected tissues. (This accumulation of fluid in body tissues is edema, and is one of the principle causes of swelling of the affected tissues.)

Histamine causes the muscles surrounding the bronchial tubes to contract, and so these tubes narrow. Naturally, this makes breathing more difficult, but if you're breathing something in that irritates the respiratory passages, the narrowing of the bronchial tubes means that more of the air moving through the respiratory system is in direct contact with the mucous membranes lining the airways. This increases the efficiency with which smoke, bacteria, and other irritating or harmful substances are removed from the air before it reaches the delicate and easily-damaged alveoli in the lungs, where gas exchange occurs.


So, this explains what happens if you're allergic to something. If you're allergic to some substance and it comes into contact with your skin, then release of histamine and other chemicals at the site of contact will cause an inflammatory response. The increased blood flow to the contact site will cause redness and swelling (edema) of the skin, not to mention itching.

If you breathe in a substance to which you're allergic (pollen, say), then the histamine reaction will cause narrowing of the respiratory passages and therefore difficult breathing. It's also likely to trigger increased mucous production, which will have the effect of trapping more of the offending substance before it can reach the lungs -- but will also cause a runny or stuffy nose. It's also likely to trigger sneezing and coughing, both of which are mechanisms that help clear the respiratory passages.


It's worth keeping in mind that this sort of thing will probably not occur on your first exposure to the substance in question. But after the first exposure, your immune system is sensitized to the substance, and if it is hypersensitive to it, you'll suffer from an allergy. (Lots of people make the mistake of assuming that they're insensitive to the toxins secreted by Poison Ivy because there's little or no reaction the first time they touch the plant. But just because there's no reaction the first time doesn't mean there won't be a reaction on subsequent exposure.)



It seems to be the case that certain allergies are becoming more common. Recent research suggests that one reason this is true -- paradoxical though it might seem at first -- is because we expend too much effort to keep our environments clean. It appears that if a young person's immune system is not sufficiently challenged, it doesn't "learn" to properly distinguish between harmful and harmless substances, and so is more likely to show an allergic reaction.

So, it seems that trying too hard to make sure your kids are never exposed to dirt and germs can be counterproductive, because it increases the likelihood that the immune system will be unable to distinguish between harmful and harmless substances, and so increases the likelihood of allergies. Letting your kids get dirty and grimy -- up to a point, anyway -- is probably good for them.




This leads us to the subject of anaphylaxis. In the event of a severely malfunctioning immune system, exposure to a substance that triggers an immune response (even if the substance is perfectly harmless) can cause a systemwide release of histamine. Remember that histamine causes dilation of blood vessels at the site of an injury or infection and constriction of respiratory passages. Now imagine what would happen if histamine were simulataneously released throughout the body. It will kill you PDQ.


A hyperallergic reaction leading to a systemwide response is anaphylaxis. A severe allergic reaction to nuts (for example) can cause anaphylaxis, and anaphylaxis can be fatal if the victim doesn't receive prompt treatment. (Incidentally, there are different kinds of nut allergies. People who are allergic to peanuts aren't necessarily allergic to "tree nuts" such as Brazil nuts, and vice-versa.)

Constriction of the respiratory passages will cause someone experiencing anaphylaxis to have difficulty in breathing. In severe cases, the respiratory passages can become completely blocked, causing the person to suffocate.

Dilation of blood vessels in the skin will cause redness and swelling in the skin (urticaria or hives), especially the skin of the face, lips, neck and throat (angioedema). Angioedema can be life-threatening, because rapid swelling of the mucous membranes of the throat can block the air passages and cause suffocation.

But perhaps the most dangerous thing about anaphylaxis is that dilation of blood vessels throughout the body causes a sudden, drastic drop in blood pressure (anaphylactic shock). This very low blood pressure (hypotension) is life-threatening because it means the brain may not receive enough blood -- and therefore not enough oxygen -- to survive.

This, incidentally, is why it's important to keep a person's head lower than his or her heart if (s)he is in shock. It's vital to ensure that as much blood flows to the brain as possible.



Anaphylactic shock can be treated by rapid administration of epinephrine (adrenaline). It causes relaxation of the muscles surrounding the respiratory tubes, and so eases the person's breathing. If the anaphylaxis has caused the person to stop breathing, epinephrine can at lease keep him or her breathing long-enough for the hypotension to be treated. That's why people who are hypersensitive to such things as bee stings or nuts sometimes carry epinephrine with them. (It won't do much for the hypotension, but it'll hopefully keep the person alive long-enough for medical help to arrive.)


Cheers,

Michael

[Ensign Steve]

Originally Posted by The Lone Ranger

It's worth keeping in mind that this sort of thing will probably not occur on your first exposure to the substance in question. But after the first exposure, your immune system is sensitized to the substance, and if it is hypersensitive to it, you'll suffer from an allergy. (Lots of people make the mistake of assuming that they're insensitive to the toxins secreted by Poison Ivy because there's little or no reaction the first time they touch the plant. But just because there's no reaction the first time doesn't mean there won't be a reaction on subsequent exposure.)

Is that why I didn't have any allergies for an entire year after I moved to Georgia? Oh what a blissful year that was.

[LadyShea]

Thanks TLR, very informative

Incidentally, I took some flack for not being Mrs. Clean regarding dog hair and etc. when we brought the baby home, and then for letting him run around outside barefoot and play in the yard, and taking him swimming in the river and ocean at a few months old. I told them it was good for him to be exposed to such things...good thing I was right