Pathogenic Trickery—How Germs Thwart The Immune System

The body has an elaborate defense system against bacteria and viruses. The only ones that are successful are those that have managed to defeat it. They use every trick in the book and then some.pathogen-trickery

We have two immune systems, both of which are very effective against bacterial and viral invaders. The older one, the innate immune system,  is found in all animals from worms on up. The newer one, the adaptive immune system, is a vertebrate only feature. The innate immune system cells, called phagocytes, are fast acting and on the spot. It’s a little clumsy at recognizing bacteria, but will try to engulf or poison it if it does. Another innate immune system cell, the ‘natural killer’, will kill any of our own cells that have been invaded.  The innate system gets most of the invaders. But if it doesn’t, we get sick. The adaptive immune system is a lot slower, but far more specific and harder to fool. If it sees a bacteria or virus, it will react by generating millions of antibodies which will both disable the virus or bacteria and label it for destruction. The adaptive immune system takes about 7-10 days to kick in.  That’s why colds and many other diseases take that long to end. But, if it’s seen the disease before, it will attack it immediately. Vaccinations get this system to work and to create immunity.

The Nature of the Enemy

It is hard to contemplate how ubiquitous bacteria are. They literally coat you inside and out. They coat everything else. We are home to 100 trillion. The ‘inside’ of us they are allowed to coat are the regions the outside has access to: the intestinal tract and the lungs. Further inside though, where blood flows, we are sterile—bacteria free. Since food and waste have to get in and out, this is quite an undertaking.

Bacteria see us as a big meal or at least meal providers by our own eating. The vast majority of them are harmless. However, if they get to the sterile insides, they can be dangerous, and are immediately killed.

Bacteria are small. You could probably put 1000 bacteria inside a typical human cell.

In order to survive, they have to have developed some method of defense or evasion. Now, bacteria and viruses are constantly mutating, so by trial and error, they will find an exploitable weakness if one exists. But if they do get inside, here are some typical tricks they pull:

  • Look like something the human cell wants. Human cells are studded with receptors and transporters that are looking for some chemical floating by in the blood stream. If the bacteria can deceive the cell’s receptor into believing it is the desired chemical, the cell may let the bacteria in. Now the cell is infected, and the bacteria is safely out of harms way from the numerous immune system white cells also circulating in the blood stream.
  • Inject toxins into the cell.
  • Some bacteria attack the defense itself. The white cells will engulf bacteria and poison them. However, some bacteria cloak themselves, causing the engulfing white cell to become confused and to not recognize the bacteria.
  • Some bacteria thrive inside the white cell. An engulfing white blood cell will try to poison the bacteria after engulfment. Some bacteria can block this and can then start to multiply inside the white cell. Soon the white cell pops, releasing hundreds of bacteria to continue the process. This case is part of the process that leads to arterial disease that leads to strokes and heart attacks. The infected white cells get behind the arterial wall and then pop and infect the wall.
  • Some bacteria fool the adaptive immune system. These bacteria can in effect change their spots. The adaptive immune system floods the bloodstream with antibodies that are looking for certain patterns. Some bacteria can switch patterns about in real time.

Which Diseases Use Which Tools?

Bubonic Plague. This bacteria gets inside a cell, where it can then replicate. To get in, it matches up to a cell receptor. Plague is also able to neutralize engulfing white cells.

Tuberculosis. These invade the white cell engulfers. The white cell tries to poison it, but the TB bacteria has a protective oily coat which defeats them. The TB bacteria then start replicating and eventually kills the white blood cell.

Cholera. Cholera manages to evade the adaptive immune system antibodies. Cholera kills by quickly dehydrating the victim. If water is administered by IV, the victim’s immune system will knock out the cholera in a few days.

Rabies. Rabies is a virus and apparently blocks the production of antibodies by the adaptive immune system.

Ebola. This virus, lately in the news, invades the engulfing white cells and also breaks a communication path between the innate and adaptive immune system, thus shutting down the latter.

At this point we have done well against bacteria and viruses. In prior times they would kill half of us before our 16th birthday. But they are still evolving, and the battle never ends.

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