Identify three different organs associated with immunity. Discuss their role in protecting the body from infection.
The immune system is like a police force. It patrols everywhere, and if it finds a disturbance, it calls for back-up. In this way, it is different from other systems in that it has to be able to react in any part of the body. The immune system provides two levels of defense: innate and adaptive immunity. This discussion will begin with a brief description of the organs and tissues associated with the immune system and then focus on the cells that provide innate and adaptive immunity.
Organs and tissues important to the proper functioning of the immune system include the thymus and bone marrow, lymph nodes and vessels, spleen, and skin.
If the immune system is a police force, the bone marrow is the police academy because this is where the different types of immune system cells are created. All cells of the immune system are created in the bone marrow from a common type of starting cell, called a stem cell. These stem cells later develop into specific cell types, including red blood cells, platelets (important for blood clotting), and white blood cells (important for immune responses).
The cell generation and differentiation process occurs every day for as long as we live. As a result, in the same way that the red blood cells in our blood are replenished after an injury or blood donation, our immune system cells are constantly replenished.
Some of the stem cells will become a type of immune system cell called a lymphocyte. Two types of lymphocytes comprise the adaptive immune system — B cells and T cells. B cells mature in the bone marrow (hence the name “B cell”). Cells that eventually become T cells travel from the bone marrow to the thymus by way of our bloodstream where they mature (hence the name “T cell”). The thymus is located just above the heart behind the sternum, or breastbone.
Lymph nodes are tissues full of immune cells. These nodes are located strategically throughout the body. Some are better known than others. For example, many people are familiar with tonsils and adenoids in the neck, but may not be aware of Peyer’s patches, which are lymph nodes that line the intestine. Numerous unnamed lymph nodes also exist throughout the body; in fact, virtually every corner of our body has some group of lymph nodes associated with it. Lymph nodes tend to be most prevalent in areas near body openings, such as the digestive tract and the genital region, because this is where pathogens most often enter the body.
If the immune system is a police force, lymph nodes are their stations. Once a pathogen is detected, nearby lymph nodes, often referred to as draining lymph nodes, become hives of activity, where cell activation, chemical signaling, and expansion of the number of immune system cells occur. The result is that the nodes increase in size and the surrounding areas may become tender as the enlarged nodes take up more space than usual. “Swollen glands” in the neck are an example that most of us have experienced. But, the same thing can occur anywhere lymph nodes are activated.
Two vessel systems are critical to the immune function of lymph nodes:
The spleen is the largest internal organ of the immune system, and as such, it contains a large number of immune system cells. Indeed, about 25 percent of the blood that comes from the heart flows through the spleen on every beat. As blood circulates through the spleen, it is filtered to detect pathogens. As pathogens are detected, immune system cells are activated and increase in number to neutralize the pathogen.
The spleen is particularly important in protecting people from bacterial infections, such as meningococcus and pneumococcus. So, while people can live without a spleen, it is important for them to be up to date on vaccines that protect against these infections because they are at greater risk of suffering from them.
Sometimes the skin is described as the largest organ of the immune system because it covers the entire body. People may not think about the skin as being part of this system, but the reality is that skin serves as an important physical barrier from many of the disease-causing agents that we come into contact with on a daily basis.