The human immune system (or that of any Gnathostomata) is an amazing, adaptable, and abolishing collection of systems aimed almost directly at one goal: to keep the body healthy. Normally, practicing excellent daily hygiene keeps you healthy. However, it doesn't mean that pathogens aren't invading your body already or are about to. You see, even though our first defenses are perfect, we might as well be organism dumpsters. No matter where you go or what you do, there will always be thousands of pathogens on or in you. This is why we carry a deeper immunity method; it's all brought to fruition by white blood cells.

First of all, no pathogen can get into the body without physically making passage. This means that the pathogen requires a break in the skin, or the ability to escape digestion and bodily fluids. Many pathogens cannot escape acidic skin oils or stomach fluid. Once inside, the pathogen has access to nutrients and good living conditions.

The first things to happen are a series of non-specific reactions. If you have ever taken a scrap, you would usually find the site to get red, extended, and warm. This is the act of inflammation. The body is reacting by sending immunity cells to the area to fight pathogens. You may also get a fever. It's the body's way of making conditions worse in the body to denature a pathogen's own enzymes.

If the pathogen is found inside the body, a large leukocyte known as the macrophage will devour it and break it down. The Macrophage will save a little trinket of a molecule known as an antigen. Antigens are surface proteins of pathogen cells that can identify it. The macrophage will go on a long journey to find a T-lymphocyte that knows what the antigen. It may take hours, days, or even weeks before it can be found. After location and communication is established by the Macrophage, the T Cell will divide into two cells: Killer T Cells and Memory T Cells. Killer T cells go off to find any host cells liberated by the pathogen and destroys them. Memory T cells stick around to act fast if the pathogen even returns. This cell-mediated immunity, and it leads to humoral immunity.

If a T cell has reacted to an immune response, it will activate B Cells. B cells create Y-shaped molecules called antibodies. Antibodies have two ends that fit to a specific antigen. B cells will split into Memory B Cells. B cells will continue to make antibodies to clump pathogens together and make them easier to fight off. Memory B cells will remember the recipe for creating specific antibodies if the infection ever returns.

The immune system itself is susceptible to disease. A virus known as the Human Immunodeficiency Virus attacks immunity cells (CD4+ T cells) and uses them to produce more of itself. The HIV virus destroys T cells. Without T cells, your compromised cells will successfully create more of a virus. Also, without T cells, your B cells will not activate and will not create antibodies. Finally, without a properly working immunity system, infections that normally do not occur will take advantage of the absence of defense mechanisms. This is known as an opportunistic infection, and it's the reason people with HIV/AIDS die.

In summary, the methods and checkpoints used by the immune systems are complicated and efficient. Without barriers, all would rise in our bodies. Without immunity cells, nobody would be able to live; period.