Despite being surrounded by harmful microorganisms, toxins, and the threat of our own cells turning into tumor cells, humans manage to survive; largely thanks to our immune system. The immune system is made up of organs, tissues, cells, and molecules that all work together to generate an immune response that protects us from microorganisms, removes toxins, and destroys tumor cells - hopefully, though, not all at once! The immune response can identify a threat, mount an attack, eliminate a pathogen, and develop mechanisms to remember the offender in case you encounter it again - all within 10 days. In some cases, like if the pathogen is particularly stubborn or if the immune system starts attacking something it shouldn’t like your own tissue, it can last much longer, for months to years, and that leads to chronic inflammation.
Your immune system is like the military - with two main branches, the innate immune response and the adaptive immune response. The innate immune response includes cells that are non-specific, meaning that although they distinguish an invader from a human cell, they don’t distinguish one invader from another invader. The innate response is also feverishly fast - working within minutes to hours. Get it? “Feverishly” - that’s ‘cause it’s responsible for causing fevers. The trade-off for that speed is 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 might 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 that line the airways to keep invaders out.
In contrast, the adaptive immune response is highly specific for each invader. The cells of the adaptive immune response have receptors that differentiate one pathogen from another by their unique parts - called antigens. Adaptive immunity is also diverse, meaning it can recognize almost an infinite number of specific antigens and mount a specific response against each of them. The trade off is that the adaptive response relies on cells being primed or activated, so they can fully differentiate into the right kind of fighter to kill that pathogen, and that can take a few weeks. But the great advantage of the adaptive immune response is immunologic memory. The cells that are activated in the adaptive immune response undergo clonal expansion which means that they massively proliferate. And each time the adaptive cells see that same pathogen, they massively proliferate again, resulting in a stronger and faster response each time that pathogen comes around. Once the pathogen is destroyed, most of the clonally expanded cells die off, that’s called clonal deletion. But some of the clonally expanded cells live on as memory cells and they’re ready to expand once more if the pathogen ever resurfaces.