Do primary and secondary defenses always prevent pathogen from entering?

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Tj789
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I know they dont but why? How are pathogens adapted in overcoming both primary and secondary defenses to cause us harm?
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Another
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If they prevented antigen entry 100% of the time, we'd never become ill! There's no real way to learn all the methods of immunity evasion (literally, if you can think of it, a bacteria can probably perform it) but here's a few:

1) Some bacteria have capsules which can prevent it from being phagocytosed, or prevents against death in the stomach

2) When a bacteria has been engulfed in a phagocytic lysosome, some of them use the opportunity to invade the phagocyte from the inside straight to the cytoplasm

3) Some pathogens will infect the immune cells themselves
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kanra
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^ This.

Also, the immune system isn't impenetrable. Diagrams in textbooks always show one or two pathogens being dealt with by the immune system, but in reality millions and millions of pathogens are present in the environment and some will slip through simply by chance.
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nexttime
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Everything the immune system can do, bacteria can counter. The only reason bacteria don't just kill us is because that would not be in their interest. An organism that kills a human quickly does not spread. Instead they partially resist and we're used as a bacteria transport system, each carrying about 10x more bacterial cells in our gut than we have human cells in our entire bodies, periodically dropping off some of these bacteria like some kind of bus service. If aliens landed one day and wanted to talk to the organisms in charge, they would not be talking to us!
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HenryHiddler
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(Original post by Another)
If they prevented antigen entry 100% of the time, we'd never become ill! There's no real way to learn all the methods of immunity evasion (literally, if you can think of it, a bacteria can probably perform it) but here's a few:

1) Some bacteria have capsules which can prevent it from being phagocytosed, or prevents against death in the stomach

2) When a bacteria has been engulfed in a phagocytic lysosome, some of them use the opportunity to invade the phagocyte from the inside straight to the cytoplasm

3) Some pathogens will infect the immune cells themselves
Just to build on this with an example, I want to add that mycobacterium tuberculosis, the causative agent for TB, works this way - it prevents the fusion of the lysosome with the phagosome.
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