Why do people have allergies? What biological purpose do they serve? Of what earthly use could it be for me to itch when certain plants are in season?
Or is this just an inherent design flaw in the human race that hadn’t evolved out of us yet?
(Please pardon any typos; I’ll be rubbing my eyes pretty hard until the Claritin kicks in.)
Allergies are your immune system (and mine, too) overreacting to certain otherwise harmless irritants. One of the current theories is that they are in a way a product of civilization…our immune systems evolved in an dirty enviroment with infections and pathogens threatening us all the time. Civilization has tended to eliminate things like plagues and other real threats so the immune system sees danger where there is none, in things like house dust and ragweed pollen.
Allergies are proof that humans, like everything else on earth, are not designed in any sense at all. They merely are good enough systems to not die out.
Only a tiny few people will die from an allergy. The very recent cultural world has made it possible for allergic people to be exposed on a regular basis to allergens they wouldn’t have had to worry about before modern trade and industry. The rest is just an irritation. Irritations have no special meaning in evolution.
Not trying to start a debate, but:
Where do people get the idea that every single detail of life requires some sort of evolutionary explanation? Some things just randomly got into our DNA; they neither further our survival, nor kill us. Allergies, like many other things, just are.
You have no idea how refreshing it is to find somebody who understands this. I am constantly trying to explain to people that evolution is not some sort of conscious, guiding force. We are not here because we are perfect, we are here because we are good enough to have persisted. My favorite way of saying it is that ‘our environment hasn’t killed us yet’.
Having said that, I remember reading once that it has been theorized that food allergies at one time conferred an evolutionary pro-survivalistic advantage because they caused populations to move away from indigenous foodstuffs, thereby decreasing population load in a particular area.
Allergies are mediated by a unique immune response that originally evolved to fight parasitic infections. The antibody subtype responsible for your sneezing and itching is called IgE. Other animals besides humans are susceptible to allergies as well. One conjecture is that in today’s very clean, parasite-free environments, the immune system malfunctions, treating harmless things as parasites instead.
A related theory is that today’s landscaping practices have contributed to higher (and imbalanced) levels of plant reproductive material in the air. A contrived environment, if you will. At least that’s what I’ve read; I claim no authority.
Surely you’ve been around here long enough to have read many posts by qualified evolutionary biologists and knowledgeable lay posters to know that current doctrine is that evolution is not, repeat not, purposeful as your title says.
An inexpert WAG: Let’s go with the parasite theory. Parasites have an advantage if they can suppress the immune system and keep it from killing them off. If these immunosuppressive parasites are common, hosts with an overly aggressive immune system will have an advantage, since once the parasite mellows it out, it will be at just the right level of aggressiveness. Take away the parasites, and you have lots of formerly fit hosts with hyper-aggressive immune systems that flip out over pollen and cat hair. What do you think?
Not really; I’m much more interested most of the time in politics than I am in biology.
However, in some cases allergies can kill, even leaving aside anaphylactic shock. I’m asthmatic, and my asthma is sometimes triggered by allergic reactions. My lungs aren’t so terribly crummy, but someone with more severe asthma than mine could have a fatal attack brought on by an allergic reaction.
If they have a purpose at all, it’s to tell you “Stay away from this shit”. There are some claims, and I can’t speak to their legitimacy, that allergies are on the rise because humans have added so many strange chemicals to the environment that we would otherwise never encounter.
One theory to explain the apparent rise in allegies among people in Western society attributes the development of allergies (as well as other so-called ‘autoimmune diseases’) to our good sanitation.
In many third world areas, people are exposed almost non-stop to infections. That keeps their immune systems “busy”. Their immune systems remain robust. So, should they develop some immune system cells that have as their targets self-directed antigens or “targets” (as we all do from time to time), those cells are kept in check or even deleted.
In the absence of such ongoing immune stimulation (i.e. in areas of good sanitation and low exposure to infections), the immune system tends to be left unchecked. Self-directed immune cells are more likely to become established. The result is the development of autoimmune diseases and allergies.
Another way to look at this, I suppose, is that if the immune system is kept busy doing what it’s supposed to do, it won’t get into mischief.
Here is a study showing that asthma (which is often an allegic disease) is less common in kids who attend day-care and/or who have siblings. In other words, kids who are exposed to lots of other people’s germs get less asthma. This sorta supports the “immune-mischief” hypothesis.
A major flaw in this appealing theory is that is does not explain the rise in asthma (an allergic type condition) in underprivileged urban children whose environment is often more third world in nature than it is Western.
It is possible that the decline in breast feeding has also augmented the rise in allergy rates. According to this line of thought, early exposure of young children to various dietary substances leads to allergies later in life (perhaps young kids’ immature digestive tracts allow the absorption of culprit molecules into the circulation. Alternatively, maternal milk contains antibodies which may prevent the child from developing his/her own allergy-promoting antibodies.).