As any city dweller can tell you, it is not the meek who shall inherit the Earth, but the cockroaches. These ubiquitous pests can fight off some of the strongest insecticides and withstand a dose of radiation 50 times that which would kill a human. Impressive as that sounds, though, researchers are just beginning to understand how hardy roaches really are. It appears they have an immune system that is comparable to a mammal’s in its sophistication.
In the past the dogma was that invertebrates did not have a true immune response, says Richard Karp, a University of Cincinnati immunologist. A true immune response is one that is specific--the cells produced in response to a microbial invasion will attack only the invaders- -that is long-lasting, and that has immunologic memory, or the ability to mobilize more quickly if the same invader strikes a second time. All vertebrates from human beings on down have a true immune response.
But on the basis of a few experiments--mostly with moths--and a lot of prejudice, immunologists had dismissed the whole class of insects as immunologically primitive. They said, ‘Why should they need a true immune response? They don’t live that long anyway,’ Karp explains. To Karp, though, it seemed unlikely that all insects would be so utterly vulnerable to attack when even the simplest of all vertebrates, the hagfish, can produce antibodies. This seems to say that immunity just came on the scene full-blown with vertebrates, says Karp. An evolutionary biologist would look at that and say, ‘Come on guys, there have got to be some transitions here.’
To prove the naysayers wrong, Karp turned from the moth to the cockroach. Cockroaches are long-lived, often surviving three to four years, unlike most insects, he says. Anything with such a long life span would have to deal with repeated exposures to pathogens, toxins, whatever, as humans do. So if there was any chance of seeing a real immune response in an invertebrate, it certainly would be in a long-lived one.
Karp began by injecting his roaches with honeybee venom. He gave them two weeks to mount whatever immune response they could, and then injected them with another dose--this one large enough to be lethal. Not only did the roaches survive the onslaught but their immune response seemed up to even human standards. It was specific to honeybee venom--subsequent injection of a similar dose of snake venom killed them--and it had a memory. You could rest these animals almost two months and then challenge them with the honeybee venom again, and essentially it acted like a booster, says Karp. You got these tremendous, very quick reactions to the venom.
The similarities to the human immune system did not end there. Karp found that female roaches, like female humans, have a stronger immune response than males and that very young and very old roaches, like their human counterparts, have a weaker response. What’s more, the roaches’ first response to the bee venom was to make a protein that binds to it-- presumably tagging it for neutralization, as a human antibody would. In fact, says Karp, the protein is about the size of the most primitive antibodies known. All these little parallels are very teasing, he says. But we can’t call it an antibody yet, because we don’t know its structure.
Even if the protein turns out not to be an antibody, Karp has already documented that insects can have a sophisticated immune system. His roaches fight off more than bee venom. Like all immune-competent creatures, they respond to a whole host of foreign substances, including innocuous, nontoxic ones, and unlike the moths in the old experiments, their response is specific and long-term.
Karp thinks the conventional wisdom was right for moths: being short-lived, they have less reason to be sophisticated. On the other hand, his research certainly reinforces the feeling, as if reinforcement was needed, that roaches are here to stay. The theory was that all you have to do is get these things to a reproductive age, when they lay eggs, and then they can die, says Karp. But it turns out that the roach isn’t ready to do that. So it has found a way to protect itself and keep on laying eggs. It’s kind of like humans trying to find some way to prolong life and the quality of life. And I guess the roach has done it.