The Immune Response Cycle

By HelloLife

The immune system is one of the more dynamic and complex systems within the body. It is made up of specialized cells, chemicals and hormones which allow the identification, isolation, and destruction of almost any foreign substance.

A Few Important Terms

Antigen: any foreign substance or organism that elicits an immune response (sliver, bacteria, virus, etc)
Antibody: a small protein released by B cells, bind to an antigens in order to draw attention to them and prevent them from binding to anything (such as a cell).

The Cells of Immunity

Before I explain how the immune response works, it's important to understand at least some of the cells involved in it.

Resident Macrophages: cells which exist all over the body, but each cell patrols a certain area, ingesting foreign particles or dead cells. Highly populated in skin.
Neutrophils: type of white blood cell which circulates around the body, leaving blood vessels to enter tissue at infected sites, also ingests and destroys foreign particles.
Natural Killer cell: patrols body for diseased cells (cancer, viruses, intracellular bacteria), injecting toxic chemicals to kill infected cells.
Dendritic cells: circulate throughout body, ingesting foreign objects and presenting them to T cells in lymph nodes.
T cells: spend their time in lymph nodes, receiving information about antigens from antigen-presenting cells like B cells and dendritic cells. Organize immune responses by instructing other cells how to react.
B cells: also spend their time in lymph nodes, sampling antigens which enter the lymph nodes through the lymphatic vessels. Present antigens to T cells, are sole producers of antibodies.

Alright, now that we"ve gotten all that terminology out of the way we can get to the nitty-gritty of the immune response.

Innate Immunity

There are two divisions of immunity, one which runs all the time (innate) and one which is activated when more serious infections occur (adaptive). Innate immunity involves cells like resident macrophages, neutrophils, natural killer cells, and others. Other parts of innate immunity include things like stomach acid, mucus, and normal flora (harmless bacteria which live all over the body, preventing the colonization of more harmful microorganisms). An example of an innate immune response would be the reaction to a sliver lodged in the skin. The area usually becomes red, warm, and swollen as blood vessels surrounding the sliver become leaky through the process of inflammation. This attracts resident macrophages, neutrophils, and other helpful proteins to the area in an attempt to destroy the invading wood particles.

Adaptive Immunity

This form of immunity is needed when innate responses aren't enough. Viruses often cause adaptive immunity to respond. A dendritic cell patrolling the mucus membranes of the nose could come upon a virus particle, ingest it, and then travel through the blood to a lymph node. The dendritic cell would then present the digested virus to a T cell. When a T cell recognizes the virus, it instructs a B cell to produce antibodies against it. The B cell will then make multiple copies of itself, most of which will release antibodies which circulate through the blood, targeting the infecting virus. Some of the copied B cells will become memory B cells, which remain relatively inactive but can live nearly as long as the body. If the body ever encounters that virus again, memory B cells will be around the recognize it, allowing the virus to be surrounded and destroyed before it has a chance to cause harm. This memory is the key to adaptive immunity. All of this specialized activity requires a lot of energy, which explains why you feel wiped out when you get sick. Keeping the immune system strong requires proper nutrition, minimal stress, and plenty of sleep.

Sources:

http://www.cancerhelp.org.uk/help/default.asp?page=118#acquired

http://www.howstuffworks.com/immune-system.htm