Your immune system is like the military - with two main branches, the innate immune response and the adaptive immune response.
Key features of the innate immune response are that the cells are non-specific, meaning that they don’t distinguish one invader from another invader, the response is really fast - occurring within minutes to hours, and that there’s no memory associated with innate responses.
In other words, the innate response will respond to the same pathogen in the exact same way no matter how many times it sees the pathogen.
The innate immune response includes things that you may not even think of as being part of the immune system.
Things like chemical barriers, like lysozymes in the tears and a low pH in the stomach, as well as physical barriers like the epithelium in the skin and gut, and the cilia which line the airways to keep invaders out.
Now if a pathogen happens to get in, then the immune system kicks in and it usually begins with the macrophage - which is the garbage truck of the body.
Macrophages eat up dead and dying cells, so that the tissue doesn’t become cluttered with them, and that makes room for new cells. They also eat invading pathogens.
Since macrophages live in the tissue they begin recognizing pathogens within minutes of an infection.
And the way that a macrophage figures out if something is a healthy host cell or a pathogen is by the molecules that a cell or pathogen has on it’s surface.
This is because cells of the innate immune response don’t distinguish one invader from another invader.
You see - pathogens have molecules that humans don’t have and they’re called pathogen associated molecular patterns or PAMPs.
PAMPs include bacterial wall components like peptidoglycan, lipopolysaccharide or LPS, and lipoteichoic acid, fungal wall components like mannan, and flagella proteins which can be found on some parasites and bacteria.
For intracellular pathogens, like viruses, PAMPs might include the viral RNA or DNA.
Now, PAMPs are recognized by Pattern Recognition Receptors or PRRs which are receptors on various immune cells including macrophages, neutrophils, eosinophils, basophils, and mast cells.