Bacteria Make "Surface-to-air" missiles to shoot down our immune system
Bacteria need moisture, the right temperature, degree of acidity and nutrients to grow. For these reasons, our mucous membrane i.e. mouths and nasal passages make great places for some bacteria to live and grow. Many types of bacteria colonize these relatively attractive environments without causing us any harm. Our bodies have developed specialized antibodies, defensive weapons generated by our immune systems. These antibodies are called immunoglobulin A, or IgA for short. They are shaped a bit like the letter Y and can catch invading bacteria, sticking them to each other so that they end up in large clumps. These clumps of bacteria then get caught in the mucous that our bodies secrete. The moist skin that lines the mouth and nasal passages is covered in small hair-like structures called cilia, they act like a brush, sweeping the clumps of bacteria up and out so that you can expel them. You will probably have noticed that when you have a throat infection you produce greenish coloured mucous. That is a combination of the bacteria, antibodies and various cells and chemicals that your immune system has used to defeat the invading bacteria. The overall process is known as immune exclusion.
An antibody, showing the Y shape and red, sticky patches that can grab the surface of invading bacteria
The IgA protease, showing its teeth in red!
An Arms RaceSome bacteria have developed weapons that can cut our antibodies into small pieces so that they become unable to stick the invading bacteria into clumps. These anti-antibodies are called IgA proteases and are produced inside bacteria like the ones that cause meningitis and gonorrhoea. The bacteria make proteins inside their cells and then these proteins get secreted through the bacterium's surface. The protease can then attack the antibodies and allowing the bacteria to gain a foothold. These proteases also seem able to attack other human proteins and help the bacteria burrow through the skin and eventually into the bloodstream.