Your body's immune system will make antibodies to kill any bacteria. After successfully fighting off the bacteria, these antibodies are still there to fight off any future infection by the same bacteria.
The bacteria or any foreign particle to enter the human body have to cross many barriers and they are classified as line of defense. Skin is the first line of defense and it work as physical barrier for all the microbes and particle. Skin has many secrete products which kill the bacteria. Then there comes some internal barriers like mouth which secrete cell dissolving enzymes, eyes which secrete lachrymal secretions which act on bacteria to destroy it in stomach low pH and many bile salts are responsible for the prevention of bacteria to attack.
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