There's a mantra in organized sports that says, defense is king! Animals are no different. In today's world, with germs lurking around every corner, it pays to have a strong defense. I'm talking about the body's natural defense mechanism, the immune system.
Cells of the immune system are found in our bone marrow, lymph nodes, spleen, thymus, tonsils, and in the liver of embryos. When microorganisms— such as bacteria or viruses— invade the body, nonspecific defense mechanisms provide the first line of defense.
These are the primary deterrents which ensure protection from numerous germs. There are physical deterrents (including the skin and nasal hairs), chemical deterrents (enzymes found in perspiration and saliva), and inflammatory reactions. These particular mechanisms are named appropriately because their responses are not specific to any particular pathogen. Think of these as a perimeter alarm system on a house. No matter who trips the motion detectors, the alarm will sound.
In cases where microorganisms get through the primary deterrents, there is a back-up system— the specific defense mechanisms— which consists of two components: the humoral immune system and the cell mediated immune system.
The humoral immune system protects against bacteria and viruses present in the fluids of the body. This system uses white blood cells, called B cells, which have the ability to recognize organisms that don't belong to the body. In other words, if this isn't your house, get out! Intruders are referred to as antigens. B cells produce antibodies that recognize and bind to a specific antigen to identify it as an invader that needs to be terminated.
The cell mediated immune system protects against foreign organisms that have managed to infect body cells. It also protects the body from itself by controlling cancerous cells. White blood cells involved in cell mediated immunity are called T cells. Unlike B cells, T cells are actively involved with the disposal of antigens. They make proteins called T cell receptors that help them recognize a specific antigen. There are three classes of T cells that play specific roles in the destruction of antigens: Cytotoxic T cells (which directly terminate antigens), Helper T cells (which precipitate the production of antibodies by B cells), and Suppressor T cells (which suppress the response of B cells and other T cells).
There are serious consequences when the immune system is compromised. Three known disorders are allergies, severe combined immunodeficiency (T and B cells are not present or functional), and HIV/AIDS (severe decrease in the number of Helper T cells).