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immune system consist of an external barrier (the skins, the stomac acid) and an internal barrier which are the white blood cells.The internal barrier consist of two systems,
the innate system and the adaptive system.
The adaptive system is basically a machine that at random produces different combinations of antibodies(flags) that can bind to surface of pathogens. When a flag is bound to a pathogen and a macrophage finds the pathogen, the antigen (flag) will enable the phage to eat it. Once the phage has eaten the pathogen it will message the immune system that a match has been found for the flag and that the immune system should mass produce these flags so that they can stick to more pathogens and that way enable the killing of all similar pathogens.
Now that is just for pathogens outside the cells, how does the immune system differentiate infected cells from normal cells? What stops the immune system from killing our healthy cells if elimination is based on antibodies sticking to their surfaces that subsequently entices macrophages to kill them. This is imo the interesting part of immunology. Every cell produces proteins and every protein it produces will leave a trace on the cells cellmembrane(surface). These traces are subtracts of the protein and are called MHC. These traces the cell leaves (MHC) is what antibodies can attach to and macrophages only eat shit that has antibodies on them. All of this together with intercellar communication plays a role in eliminating infected cells. This was a simplified explanation, if i try to go more in detail or simplify more i will just confuse you, there are many complex functions of the immune system and it's a really interesting read.
