If both pathogen and immune system were on Survivor, who would win?
What exactly IS the immune system?
Here’s a straightforward answer: It is the body’s solution to the problem.
What is the problem, you ask? Alien invasion. Well, not exactly. Infectious agents are the problem, and they include, but aren’t limited to, viruses, bacteria, fungi, and parasites. These pathogens use the host for survival and propagation, often causing harm to the host, which in turn, must resist in order to survive.
Okay, so what is the immune system, EXACTLY?
It is a highly integrated and complex system of cells and molecules that have specialized roles to play in defending the host organism from infection.
The immune system includes two major subsystems:
The Innate Immune System
Components of the innate immune system are the rapid response team, and serve to delay the ability of the pathogen to infect the host. They are “innate” because they are encoded in the germ line.
- The first level of defense employed by the innate immune system is physical. For example, the skin can be considered part of the innate immune system; it is a physical barrier, and actually contains molecules called – get this – “defensins”, that are toxic to pathogens.
On another level, literally, is the mucosa, or mucous membranes lining our internal organs, such as the lung. The mucosa is not as impermeable as skin, but does have finger-like projections called cilia, which secrete hydrolytic enzymes that “digest” foreign particles.
- The second level of defense is a recognition mechanism; cells of the innate immune system detect danger by sensing molecular patterns that are unique to pathogens.
The cells have receptors called TOLL-like receptors, named for the German word “wow” or “amazing”. You see, this was the word exclaimed by the German scientists who discovered the receptors. Quite fitting, don’t you think?
When these receptors recognize and bind a pattern on a pathogen, a signaling mechanism activates the kill switch in the cell.
Innate cells have also developed a crude communication system, where they are not only able to tell each other that a pathogen is present, but can also activate their neighbors.
This definition of the innate immune system is wonderfully worded!
“The most evolutionarily ancient type of immunity exists in all living multi-cellular species. When exposed to pathogens or cellular damage, cells of an organism’s innate immune system activate responses that coordinate defense against the insult, and enhance the repair of tissue injury. There is a modern day cost associated with these responses, however, because innate mechanisms can damage normal tissue and organs…human life is a balance between dual threats of insufficient innate immune responses –which would allow pathogens to prevail- and over abundant innate immune responses –which would kill or impair directly.”
Kevin J. Tracey. 2011.Science 332:673-674
Sounds like a knight in shining armor, riding to our defense, right? There is one problem – the innate immune system has no memory.
Thus, we’ve acquired the adaptive immune system.
The Adaptive Immune System
The adaptive immune system does four things:
- Recognizes antigens (components of pathogens that stimulate immune responses).
- Distinguishes between self and non-self.
- Proliferates antigen-specific cells after activation.
- Remembers specific encounters – this is why vaccines work.
Enter, the cavalry. The initial innate response protects the host while the adaptive immune system is preparing for a specific response.
At the head of the cavalry is the dendritic cell – THE most awesome cell.
(It's the red one). Right?!
Okay, now suppose we have a jalapeño-shaped pathogen, for visual and illustration purposes:
The “jalapeño” is ingested by a dendritic cell, which then presents jalapeño-shaped antigens on its surface. The antigen serves as a sign to alert cells of the adaptive immune system to its presence. These cells can kill the pathogen directly, as well as make antibodies, which act as “memory”.
Vaccines work in this way – essentially, the equivalent of antigens are injected, and the cells of the adaptive immune system produce antibodies. In this way, the body “remembers” that particular pathogen, and the antibodies will help the immune system protect against the pathogen if and when it does attempt to invade.
Now that you’ve been introduced to both contestants, what do you think? If both pathogen and immune system were on Survivor, who would win?