The immune system is composed of an interdependent set of organs, cells, and the proteins and other chemicals that they produce. Following is a brief glossary to explain what these components are and what they do.

Organs of the immune system

Skin and mucous membranes: The skin is a critical barrier that helps to prevent invasion by germs and foreign substances. The various mucous membranes that come into contact with the outside world also serve to protect against invasion. Saliva, tears and mucus contain antibacterial substances. The small hairs or cilia that line the trachea (windpipe) beat upwards to move foreign matter that has been inhaled back out of the respiratory tract. Germ-killing cells line the mucous membranes and skin and prevent germs from establishing themselves as infections.

Lymphatic system: These are vessels that carry lymph, a clear, watery fluid, which bathes cells of the body and carries nutrients to them. After having supplied the cells with needed substances, lymph collects again in the lymphatics. Lymph also collects and conveys immune cells and foreign substances through the lymphatics to clear them from the body. Lymphatics run side by side with the blood vessels. Liquid lymph that has been cleared of foreign substances re-enters the circulation as plasma.

Lymph nodes: These are small oval structures are found along the lymphatics.
Immune cells collect in them and they filter out foreign substances. In inflammation and infection, they enlarge, and are often called "swollen glands."

Thymus: The thymus is an organ in the chest, under the breastbone. It produces immune cells called T cells (see below) and is important in their maturation. The thymus is critical for the development of the immune system of infants. It has important functions in adult immunity, but adults can live without a thymus.

Spleen: The spleen is an organ in the upper left quadrant of the abdomen that filters out foreign cells. It is particularly important in protecting against infection with bacteria that have capsules, such as Pneumococcus, a common cause of bacterial pneumonia. While we can live without spleens, we are at higher risk of developing certain infections.

Bone marrow: The bone marrow is the home of blood stem cells, the cells that give rise to the various circulating blood cells. The marrow produces red blood cells, that carry oxygen to our tissues and platelets, small cells critical for proper blood clotting. The marrow also produces white blood cells, of which there are several different kinds. White blood cells are the critical immune cells of the body.

Cells of the immune system

The various white blood cells important in immunity include:

Granulocytes: These constitute about half of your circulating white blood cells. They contain granules, or little spots visible under a microscope. Types of granulocytes are:

	-	Neutrophils: These are the most common white cells we have. They live less than a day, and their job is to react immediately to the presence of foreign matter. They locate the foreign matter through chemotaxis, a response to the release of chemicals in the presence of foreign substances. They will surround a germ or foreign body (such as a splinter), engulf it, and release other chemicals such as hydrogen peroxide to destroy it. Pus is simply a large volume of dead neutrophils and foreign matter.
	-	Eosinophils: The numbers of circulating eosinophils rise in the presence of certain parasites.
	-	Basophils: These carry the chemical histamine, the source of the woes associated with allergies (hence the value of antihistamines). They participate in inflammation.

Lymphocytes: These white blood cells are called into action to defeat bacterial and viral infections. There are two major categories of lymphocytes, B lymphocytes and T lymphocytes (often simply called B cells and T cells).

	-	B cells: B cells mature in the bone marrow. B cells "recognize" specific germs, either bacteria or viruses, and in the presence of those specific germs, they reproduce rapidly, as identical clones. These B cells mature into plasma cells, which are responsible for producing antibodies (see below), infection-fighting proteins also specific to the particular germs that have invaded the body.
	-	T cells: T cells also come in subcategories. The two main types are the killer (or cytotoxic) T cells, which can kill tumor cells and virus-infected cells, and the helper T cells, which provide help to the killer T cells, to B cells and to macrophages (see below). Some T cells also serve as regulators to stop immune responses before they go too far.

Monocytes: These are produced in the bone marrow and released into the circulation. They make their way to various organs in the body and mature into macrophages. Macrophages "eat" foreign matter such as bacteria, parasites, inhaled fibers and dusts and anything else they find in organs that does not belong there, including pus.

Proteins and chemicals of the immune system

The various immune cells make a variety of proteins and chemicals that are also important components of the immune and inflammatory responses. These include:

Antibody or immunoglobulin: Antibodies are proteins that are shaped like the letter Y. There are five major forms of antibody or immunoglobulin (abbreviated Ig): IgG, IgD, IgE, IgA, and IgM. The base of the Y-shaped protein is the same for all molecules within one of these five categories, and the very tips of the arms of the Y vary, and can stick to the foreign invader at which they are aimed.



	- IgG: There are four different subcategories of IgG, the major form of immunoglobulin. It coats germs and makes them easier for germ-eating cells to find.
	- IgD: This immunoglobulin is found on the surface of B cells and is involved in activating them to participate in the immune response.
	- IgE: This immunoglobulin participates in the allergic response.
	- IgA: IgA molecules generally come in pairs and are found in body secretions, such as saliva. They are also found inside the stomach and intestines.
	- IgM: IgM molecules gather in clusters and are involved in killing bacteria.

Cytokines: These are chemicals that are secreted by T and B cells. They are chemical messengers that recruit other immune cells to where they are needed to fight infection and even cancers. Because they communicate between white blood cells, also called leukocytes, cytokines are sometimes called interleukins.

Complement proteins: These proteins are manufactured in the liver. They circulate in the bloodstream and are activated by the presence of antibodies (thus they complement their activity). They act to puncture and thus burst or lyse foreign cells.

Antigens: Antigens are the chemicals on the surfaces of foreign cells that are recognized by the immune system. Antibodies are created to specific antigens. B cells have antigen-receptor sites on their surfaces, which are proteins that recognize those antigens. T cells have more sophisticated antigen-sensing systems, and do not recognize "naked" antigen, recognizing instead antigen that has been processed by other immune cells.

Immune-related diseases

There are a few diseases that are related to disorders within the immune system. Among these are:

Autoimmune disease: The loss of the recognition of some body tissue as "self" is part of autoimmune disease. Antibodies and T cells attack the body's own tissue. Some examples of this include some forms of diabetes, rheumatoid arthritis and lupus.

Immune complex disease: Normally, the clumps of antibodies and antigens that form as we fight infection or invasion are cleared from the circulation and destroyed. If that clearance does not occur, those clumps can lodge in tissues or small blood vessels and cause damage.

AIDS: The human immunodeficiency virus or HIV destroys helper T cells, limiting the body's ability to fight infection.

Cancer and its treatments: Cancer is sometimes a disease that arises because of a failure of the immune system to destroy a cancerous growth. Chemotherapy and radiation can destroy or weaken the cells and organs of the immune system as an inadvertent side effect, and this can reduce our resistance to infection.

Characterization of immune response

Scientists characterize the immune response based on which aspects of it they are studying. Those aspects include:

Innate immunity: This is the nonspecific part of immunity, or the generalized response the body has to the presence of an invader.

Adaptive or acquired immunity: This refers to the very specific response that comes in the presence of specific antigens. This includes the recruitment of B cells that make the antibodies that are aimed at those antigens and activated T cells that also recognize the specific invader.

Humoral immunity: This describes the production of antibodies by B cells.

Cellular immunity: This refers to the immunity that is controlled by T cells.

Mucosal immunity: This is a term that refers to the immune adaptations found within the mucous membranes (mouth, intestines).

Allergic reactivity: This is the immune system's response to allergens, substances such as pollen or foods that incite the inflammatory response.