How does the immune system identify and fight off different types of pathogens, and why do some pathogens evade our immune responses?
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The immune system uses two general strategies:
1. Identify general traits associated with pathogens. For example, bacteria are structurally different from your cells so if your body notices cells with tails (flagella) and cell walls swimming around, it will destroy them. Likewise if a cell notices DNA outside the nucleus or other weird things that happen in virus infected cells it will signal all the cells around it to be on high alert for viruses and if those cells detect signs of a virus they’ll destroy themselves (which prevents the virus from replicating).
2. Identify very specific things about one pathogen. This is where antibodies and vaccines come in. Certain immune cells will produce antibodies at random and if, by chance, they stick to something that isn’t one of your cells (like a pathogen) you start mass producing and fine tuning that antibody. On second exposure the immune system can remember past infections and start making those antibodies again (but much faster snd better this time), this is why vaccines work.
Theres a lot of other things going on too this is simplified.
There are lots of strategies to evade the immune system. The problem with the specific approach (antibodies) is a pathogen (typically a virus) can mutate so the antibodies no longer stick as well. We saw this with COVID, but even more infamously it happens with HIV which is part why we still don’t have an HIV vaccine. HIV is actually a master at evading the immune system, it is a special type of virus that makes our cells convert it into DNA and then it integrates into our genome. People with HIV can have no HIV viruses in their body but the DNA/instructions to make them remains in some of their cells DNA, if they ever stop taking their medications that HIV DNA will make more viruses again. Larger pathogens like worms/parasites will make little sacks to live in, separating themselves from your immune system. Other parasites like malaria will actually hide inside your red blood cells (which are somewhat uniquely ignored by the immune system), malaria is famous for cyclical fevers caused when the parasites leave the cells to find new cells to infect briefly becoming visible to the immune system again. Theres many other strategies, literally entire textbooks worth of ways to try to evade the immune system.
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