Differences between "allergies" and "colds"

December 21, 2007 4:42 PM

1) Why are some sicknesses "allergies" while others are things such as "colds"? Also, 2) how does the body know which is which?

Question 1) When I get allergies my eyes get itchy, my nose starts to run, I start to sneeze, etc. The same type of stuff happens when I get a cold. As far as I know, both sets symptoms are caused by the body trying to protect itself from foreign agents.

With allergies, it is supposedly a "false alarm" and the body is reacting to pollen or other benign agents. For this we take anti-histamines.

But a cold is a real attack, so we take cold medicne which contains different types of compounds such as decongestants and painkillers.

Why the difference?

Question 2) I also noticed that that the human body reacts stronger to a cold, with fevers and whatnot. Those are also self defense mechanisms. How does the body know to pull out the big guns in a cold but keep those guns in reserve if it just a false alarm?

posted by sideshow to health & fitness (11 comments total)

The difference is that a cold is a virus while the allergy is a reaction to a particular substance. Fevers are trying to kill the virus by rising your body temp. The allergy is not attacking the body the same way a virus would so it has no reason to cause a fever. The biology of the two are very different even though the symptoms are similar.
posted by pwally at 4:52 PM on December 21, 2007

But a cold is a real attack, so we take cold medicne which contains different types of compounds such as decongestants and painkillers.

For the record, while anti-histamines help with the allergic reaction, cold medicines do not help with the cold, what they do is try to reduce the misery of the symptoms you must endure while waiting for your body to get over the cold.

For example, if you didn't have anti-histamines, then you might likewise be reduced to using decongestants for your allergies - doing nothing to help against the allergic reaction, but attempting to reduce the misery of its effects. (And just as when using those medicines with a cold, it would be a somewhat unsatisfactory experience :)

In short, you take different medicines for allergies because there actually are medicines for allergies, while there are not any medicines for a cold, so all you can do is pain relievers and wait it out. (With the exception of new prescription anti-virals like Tamiflu)
posted by -harlequin- at 5:13 PM on December 21, 2007

Allergies can cause fever, thus the term "Hay fever".
posted by Manjusri at 5:15 PM on December 21, 2007

I also noticed that that the human body reacts stronger to a cold, with fevers and whatnot. Those are also self defense mechanisms. How does the body know to pull out the big guns in a cold but keep those guns in reserve if it just a false alarm?

The body doesn't know how to tell them apart. The immune system reacts to them both the same way. But the blood level curve over time of allergens and viruses is different. Typically allergens peak at the beginning and fade off with time, whereas the blood load of viruses climbs until the immune system gets the upper hand. That's the main reason why the self defense mechanism you describe act differently. As long as T4 cells keep running into things they think are enemies, they keep producing interferon, which directly or indirectly causes most of those symptoms. With an allergy, since the allergen level ramps off rapidly, that doesn't happen for as long or as intensely.

Another difference is something with the strange name of endoplasmic reticulum. It's feature of epithelial cells (and maybe muscle cells; I'm not sure about that). What it does is to grab random proteins from inside the cell, divide them into pieces using cutting enzymes, and stick the pieces out through the cell wall.

As long as those proteins are normal ones, produced from your own genes, that has no interesting effect. But if the cell has been invaded by a virus, which has hijacked the cell's own mechanisms and turned it into a virus factory, then some of the proteins will be those the virus wants to produce. Those are "foreign" -- and their presence on the cell wall will cause the immune system to attack and kill that epithelial cell. That's the primary mechanism by which your body wins its war against viruses.

If allergens cause much of that, it isn't really a lot, whereas in a typical virus attack thousands, probably even millions, of epithelial cells have to be sacrificed for the good of all.

Fevers are trying to kill the virus by rising your body temp. Actually, that's not the function of a fever. Chemical activity runs faster at higher temperature. A fever makes your immune response faster, but it also makes the viruses run faster. A fever doesn't fundamentally change the progress of the disease, but it does make it happen quicker. Evolutionarily that was an advantage because it meant that you were incapacitated for less time and thus there was a shorter window of vulnerability in which you could be killed some other way, for instance by a predator.
posted by Steven C. Den Beste at 5:49 PM on December 21, 2007

Other than anti-histamine, there's very little difference between cold medications and severe allergy medications.

In short, allergies are caused by your immune system recognizing a potential invader. This causes a certain type of immune response, which causes inflammation, extra mucus production, and irritation. One of the chemicals released in this immune cascade is histamine. Since your immune response in this case is inappropriate, you take an anti-histamine to block the response. Basically, you're telling your body "Woah there - this pollen isn't going to hurt you, so just chill".

Since these anti-histamines usually do a pretty good job of blocking the response, additional medications to treat symptoms are often unnecessary. For people with severe allergies, though, you may still see a cocktail of decongestants, painkillers, etc, thrown into the mix.

With a respiratory viral infection, (commonly referred to as a cold, or the flu) viruses have invaded your nasal passages and are hijacking cells and churning out copies of themselves. This results in a somewhat different immune response, where your body ramps up to fight off the virus. To you, this feels much the same, since it also results in inflammation, irritation, and extra mucus production.

With a virus, you do not want to suppress this immune response. You want your body to fight off the virus using all the tools at it's disposal. Thus, you're just using a cocktail of painkillers, decongestants, etc, to make life tolerable until your system kills off the virus.

How does the body know to pull out the big guns in a cold but keep those guns in reserve if it just a false alarm?

A viral infection will result in lots of signals that the immune system can recognize. Certain viral proteins trigger immune response, and more generally, the viruses cause cells to explode like balloons after they are full of viral copies. This causes all sorts of signals that warn the immune system that something is wrong. Allergens like pollen won't' trigger these same immune response pathways.
posted by chrisamiller at 5:54 PM on December 21, 2007

Actually, that's not the function of a fever. Chemical activity runs faster at higher temperature. A fever makes your immune response faster, but it also makes the viruses run faster.

Actually, the poster was closer to correct. Raising body temperature can cause proteins to denature, making things more difficult for invading bacteria or viruses. Your body has an extensive network of heat-shock proteins that help you deal with the extra heat, while many invaders don't.

It's controversial whether the increased metabolism that helps cause the fever actually has any useful effects (other than, of course, causing the fever)

Another difference is something with the strange name of endoplasmic reticulum.

You're half right on the Endoplasmic Reticulum. ER is found in all eukaryotic cells, and has a variety of important functions. The display of foreign antigens on the surface of cells is a complex task that involves multiple systems and organelles, including the ER, cytoskeleton, transport proteins, and specialized membrane receptors (among others). To say that the ER alone is responsible is like saying that the carburetor is responsible for making the car go.
posted by chrisamiller at 6:08 PM on December 21, 2007

The body doesn't always know to not pull out the big guns. Ask anyone who's got a severe food allergy. :) Or lupus.

Also, just as a note, the route of infection and subsequent spread of a virus varies from agent to agent (not to mention we're leaving out bacteria, fungi, amoebas, etc.) so what happens to a cell is a bit more complex than getting sacrificed for the greater good by a leukocyte. As a matter of fact, some (most, I would say) viruses assemble themselves within the cell and use the cell's own membranes as envelopes. These membranes are studded with glycoprotein, which are known to function in cell-cell recognition, and so when these viruses to infect new cells, all they have to do is say, "Hello, look at my glycoprotein. I am obviously one of you." And the new cell says, "Well, I see no reason to be suspicious. Come on in!"

Very smart, these little buggers.
posted by reebear at 6:37 PM on December 21, 2007

As Reebear says, sometimes the body does pull out the big guns in response to allergens. Extreme cases of it are known as anaphylaxis.

That's why when you get a flu shot you have to wait around for fifteen minutes. Once in a great while someone reacts by going into anaphylactic shock, which can include extreme bronchospasm. (Which is Latin for "you can't breathe".)

The nurse has vials of epinephrine, and will give you a shot of it to prevent you from suffocating.

ChrisAMiller: I stand corrected. (College zoology was 30 years ago.)
posted by Steven C. Den Beste at 7:04 PM on December 21, 2007

Question 2:
Typical allergies (like pollen and environmental, non-food allergies) occur because your body produces a certain type of antibody (IgE) against a foreign particle (the allergen). When multiple IgE molecules bind to an allergen, they are able to bind to and activate certain non-specific immune cells that then release chemicals (like histamine) that produce the allergy symptoms. So what happens is a relatively small subset of the immune system is triggered and the part that is activated works on a relatively local scale.

In the case of a viral infection, you elicit a much broader portion of the immune system. Infected cells present parts of the virus (an antigen) on their exterior causing them to be killed and eaten by antigen presenting cells that then display those viral parts to T-cells that release all kinds of chemicals that stimulate almost all parts of the immune system. Some of theses activated cells do kill infected cells, others churn out a crap load of antibody to neutralize (bind and interfere with) the virus made by infected cells, while other cells secrete chemical messages that modulate the activity of immune and non-immune cells. Also, the chemicals are the ones like interleukin-1, which causes the fever, interleukin-2, which cause a huge expansion in the number of immune cells in your body (and thus makes you tired); and interferon-gamma, which causes weakness & muscle pain; these chemicals work and spread body-wide.

In summary, the body knows to pull out the big guns because of what part of the immune system is triggered. Also, remember that the virus is replicating so its treat multiplies over time whereas the allergen is sort of limited time/dose exposure.

Does that answer your question?

Now, corrections:
1) A lot of viruses do not use the cells' membranes as part of themselves. There are quite a few non-enveloped viruses (polio, SARS, etc...) out there. Also, I don't know (could be wrong on this) of any surface glycoproteins that function to suppress innate immune responses -- they generally just function to allow the virus to grabs the cell & either trigger the cell to take up the virus or force its way in. All viruses assemble inside the cell; for some viruses, leaving the cell triggers a last step that is necessary for the virus to be mature and infectious, but it still assembled inside the cell.
2) Like chrisamiller said, nearly every single eukaryotic cell has an endoplasmic reticulum. The only reason it really functions in antigen presentation is because getting inside the ER is the first step to getting put on the outside of the cell for pretty much all molecules. In fact, for the immune cells that present antigen, the ER has nothing to do with it. All of the processing steps that prepare the viral protein to be presented occur in the cytoplasm/nucleus before it is loaded onto MHC I in the ER or MHC II in late endocytic vesicles.
posted by The Bishop of Turkey at 11:54 PM on December 21, 2007

That's why when you get a flu shot you have to wait around for fifteen minutes. Once in a great while someone reacts by going into anaphylactic shock, which can include extreme bronchospasm.

Just to clear this up, I'm pretty sure this is because of the egg protein, and potential egg allergies, in the flu vaccine, not anything to do with viral protein.

As said above, the type of allergic response you're thinking of works through IgE, which stimulates release of histamine. There are other types of allergic reactions, like poison ivy and certain kinds of arthritis, that work in other ways. Off the top of my head, I believe lupus works through IgG. Viruses and bacteria stimulate many different pathways, like TLR3 recognizes the dsRNA in some viruses. Also, all nucleated cells have MHCI on them, which shows bits of cellular protein, and basically tells immune cells that it is "self". When a cell is infected by a virus, the MHCI starts showing viral protein, which immune cells like cytotoxic T cells see as "foreign", so the whole cell gets killed. Since it activates multiple pathways, some of which lead to fever, others to infiltration of immune cells, and other to the killing of cells in your throat etc that you had previously enjoyed, there is a different reaction.
posted by fermezporte at 5:20 AM on December 22, 2007

Flu vaccines, depending on the year, will ask you to disqualify yourself if you are allergic to iodine or mercury or both. Someone I knew forgot he was allergic to both and lived to tell the tale, so there may not be much in there. It might have been mostly mercury the last few years.
posted by unrepentanthippie at 12:52 PM on December 23, 2007