All earthly organisms live in a perilous environment. We humans, to take a familiar example, are under a barrage of repeated attacks by a host of bacteria, viruses, fungi, and parasites all the time (I'm going to ignore bigger enemies like tigers, snakes, and spiders. In the modern world they take a lesser toll than smaller adversaries and aren't relevant to the topic at hand). These omnipresent tiny agents aren't necessarily bad guys. They, like us, are just trying to make a living, attempting to pass their genetic material to their offspring. Unfortunately their livelihood comes at our expense. Without some sort of protection, we wouldn't last long under their constant onslaught. According to all the sources that I've encountered, our defense is commonly divided into two categories: innate and adaptive immunity.
While this distinction is valuable, it turns out that these two parts of the immune systems aren't entirely separate. In fact, they interact with each other to a considerable degree. Nevertheless, I'll begin this post by listing how they're different from one another, ignoring their overlap for the time being, and then start to describe the properties of the innate system. This last bit will take several posts, and it's going to be a while before I get to adaptive immunity and how it has been engineered to combat disease. Be patient. Innate immunity is interesting in it own right. And, because of the interaction between the two systems, it's going to be important in later discussions. Anyway, here's a list of how the two systems differ.
1. The innate immune system is older than the adaptive one. That is, a much larger group of organisms, including plants and invertebrates, show an innate response when attacked by pathogens. All the references indicate that the adaptive response is limited to vertebrates, animals with a backbone like fish, reptiles, birds, and mammals. The evidence therefore suggests that it wasn't until the evolutionary appearance of the vertebrates, which occurred about a half billion years ago, that the innate system, as we know it, first arose.
2. The innate immune system kicks in faster than the adaptive one. Once challenged by a pathogen that has been introduced by a lesion in the skin that might happen after a fall and a skinned knee, the innate system mounts a defense right away, within minutes or hours. The adaptive system takes much longer to get going, a week or more.
3. The innate system is hard wired. The adaptive response is bespoke. The innate system knows about certain categories of invaders that it has "learned" about over evolutionary time. When it detects one of these marauders, it mounts a suitable attack. However, if a micro-organism that the innate system isn't prepared for, one that isn't in its repertoire, attacks, it is largely helpless. Protecting the body from previously unencountered pathogens is the job of the adaptive system.
4. Innate immunity has no memory. If the same organism attacks subsequently, months or years later, the innate system isn't any better at mounting a response than it was originally. One of the great features of the adaptive immune system, the basis for vaccinations, is that it remembers the nature of a previous attack, and mounts an improved secondary response.
Our first line of defense against the invading hordes are physical barriers to their entry. Our skin forms one such barricade that microbes have to penetrate. Even more important are the surfaces of epithelial cells that line our digestive and respiratory tracts. They're full of slimy mucous that entrap invaders and make it difficult for attackers to grab a toehold. These barriers also house a myriad of helpful bacteria that compete with the harmful ones. Moreover, the skin and internal epithelia are covered by a variety of toxic molecules that are designed to kill bacteria and viruses. However, despite the strength of this array of protective devices, these barriers can be broken or overwhelmed. That's where the more interesting part of the innate immune system comes into play.