The Immune System
The system by which a lifeform fights off would-be invaders.
Or a system of biological structures and processes within an organism that protects against disease.
So what are the structures and processes involved?
What are our bodies lines of defense?
What causes disease?
How is disease spread?
How does our body fight off diseases?
How do we become immune?
Pathogens are anything that generate a disease. They can be bacteria, viruses, protists, and maybe more (prions?).
Bacteria are tiny living things that have cell walls, so we can fight them with antibiotics (if our immune system needs a boost)
Viruses are extremely tiny, non-living sections of DNA (or RNA) that get injected into our cells. Once there, they live inside our DNA forever. They use our cellular systems to make more of themselves. Antibiotics do not work on them.
Protists are much bigger (though still single celled). They do not have cell walls, so antibiotics do not work on them.
Prions are misshaped proteins that convert functional proteins into their badly shaped form, thus making that protein not work. Nothing we've found works on these.
Pathogens can be spread through direct or indirect transmission.
Direct means that there must be direct contact between one organism who is infected and one who is not. HIV / AIDS and hepatitus are two examples.
Indirect means that the pathogen can survive outside the body in a vector for some amount of time. The common cold and E. coli poisoning are two examples.
Which is rabies? How about malaria? What is the vector?
All living things need to defend themselves from infection. Let's see what systems we have and then look at what other organisms have.
What are your bodies lines of defense?
The first line is to keep the invaders out:
We have skin and all these things to keep them out!
Your body must repair damage to this line of defense. This means that it must clot blood which you may remember from the circulatory system unit.
If an invader (pathogen) gets past your first line of defense, then your body must fight it through what is called your "innate immune response".
This second line of defense is common to plants and all animals and is non-specific.
That means that you have the same response regardless of what has invaded you.
So what are these responses?
Swelling
Vomiting
Diarrhea
Running nose
Watery eyes
Cough
Sneeze
Fever
It doesn't matter what invaded you, you try to expel it and / or cook it out.
Flush that pathogen out!
While your body is engaged in war through the innate system (secondary line of defense), your cells begin the process of studying the invader to make specific weapons against this pathogen. This begins the 3rd line of defense, the "adaptive immune response".
Long story short, the white blood cells study the invader and then make "killer t cells" and antibody making b cells (antibodies are flags that attach to the invader to mark it to be killed).
It is important to know is that your white blood cells divide and make specific weapons against the invader. Your T-cells divide into memory cells and "killer T's" These cells are specific to the one invader, but monopolize all its weaknesses. Your B-cells divide into memory cells and "antibody making B's". These pump out antibodies (little red flags that stick to the invader attracting all our defenses to it).
Note that both immune cells made memory cells. This is so that is you ever get invaded by that same invader you can mount the full attack and squash it in hours instead of days. This grants you immunity to many things you have had before!
There are two types of immunity: active and passive.
Active immunity is when you received the pathogen and mounted your own response to it and have memory cells alive inside you for life.
Passive immunity is when someone gave you the antibodies. This is nice, but very short lived.
Which type of immunity is breast feeding? How about vaccines?
Another vital component to the immune system is the lymph system. All of the immune cells and destroyed invaders need to get cleaned up and dumped back into the blood for re-circulation. The lymph system is the way this happens and the lymph nodes are where the filtering happens.
A few more parts to this unit:
Allergies - when your body mounts an immune response to something benign.
Autoimmune disorder - when your body mounts an immune response to itself.
The immune response is complex, as is life. It works 24: 7 to ensure you are healthy. The things you can do to boost it are:
Eat nutritious foods
Get plenty of sleep
Exercise
Keep balance in life
Keep stress down
Now, which of those systems does a tree have? Does it have a protective barrier? Of course. Does it flush wounds out? Yep. Do they have an adaptive response? Great question. We think so. There is evidence that plants mount different responses to different invaders, we just don't understand it that well yet...