Let me try to bring together the salient points that I've learned so far about innate immunity.
Point 1 - There are two parts to the immune system: innate and adaptive immunity. What's the difference? For one thing, innate immunity is older in evolutionary terms than the adaptive system. Organisms up and down the tree of life utilize various versions of it. It's also faster to respond to attack, taking minutes and hours rather than days. But the main difference between the two systems is that the adaptive defense is preprogrammed. It has an evolutionary designed repertoire of defensive agents and can make no more (at least not in less than millions of years). In general, if confronted with a threat that it hasn't anticipated, it isn't able to respond. The adaptive system, which seems restricted to vertebrates, has adopted a different strategy that I'll expand upon in later posts. But briefly, its game plan is to construct millions of different weapons at random without worrying about whether they'll be effective or not. One other difference between the two systems. Innate immunity apparently hasn't any memory, a second attack by the same agent doesn't result in a heightened response. That's not true of the adaptive system.
Point 2 - The systems are distinguishable but inseparable. That's something that I haven't emphasized enough, but is extremely important. Although most of my references divide immunity in two, they also warn that there exists no real barrier between them. One often works in conjunction with the other. I'll try to point out where this occurs in later posts.
Point 3 - The various components of the innate system make use of receptors to distinguish between the harmful and harmless. Receptors are proteins that lie in the membranes of cells, with portions protruding inside and out, one end binding to intruders. Once an enemy has been detected, receptors pass signald across the membrane, thereby releasing a cascade of events that prime the cell for an appropriate response.
Point 4 - Macrophages are sentinel cells. They are long term residents in many tissues and send out signals upon a microbial invasion. Upon activation, they ingest microbes and cells that are killed or injured. And they secrete cytokines to alert other immune components to come to their aid.
Point 5 - Neutrophils are the major phagocytic cells of the body. They more swiftly move in the bloodstream, awaiting a signal to slow down. When macrophages send out an alarm (through the release of releasing cytokines), they slow down, stop, and leave the blood stream and become killing machines.
Point 6 - The complement system consists of blood borne proteins that contribute to the innate and adaptive immune response, It also is responsible for the direct killing of bacteria and some viruses. Complement makes use of proteolytic cascades to initiate and amplify its effects.
Point 7 - Natural killer cells target viruses, injured cells, and tumors. Unlike the other cellular components of innate immunity, its main function is to detect invaders that have gotten inside cells. It does its job by making use of the major histocompatibility complex, a system for displaying fragments of the proteins located in the interior of the cell on its outer membrane.
Point 8 - Mast and dendritic cells are sentinels. Mast cells rapidly release toxic chemicals upon activation. Dendritic cells are the major link between innate and adaptive immunity.
That's a lot of food to ingest in a relatively brief sitting. Take a deep breath. The next topic is even more filling.