Which term describes foreign particles that enter a host and stimulate the bodys immune response?

The immune system is designed to defend the body against foreign or dangerous invaders. Such invaders include

• Cancer cells

• Transplanted organs and tissues

To defend the body against these invaders, the immune system must be able to distinguish between

• What belongs in the body (self)

• What does not (nonself or foreign)

Antigens are any substances that the immune system can recognize and that can thus stimulate an immune response. If antigens are perceived as dangerous (for example, if they can cause disease), they can stimulate an immune response in the body. Antigens may be contained within or on bacteria, viruses, other microorganisms, parasites, or cancer cells. Antigens may also exist on their own—for example, as food molecules or pollen.

• Recognizing a potentially harmful foreign antigen

• Activating and mobilizing forces to defend against it

• Attacking it

• Controlling and ending the attack

If the immune system malfunctions and mistakes self for nonself, it may attack the body’s own tissues, causing an autoimmune disorder Autoimmune Disorders An autoimmune disorder is a malfunction of the body's immune system that causes the body to attack its own tissues. What triggers an autoimmune disorder is not known. Symptoms vary depending... read more , such as rheumatoid arthritis Rheumatoid Arthritis (RA) Rheumatoid arthritis is an inflammatory arthritis in which joints, usually including those of the hands and feet, are inflamed, resulting in swelling, pain, and often destruction of joints.... read more

Systemic Lupus Erythematosus (SLE) Systemic lupus erythematosus is a chronic autoimmune inflammatory connective tissue disorder that can involve joints, kidneys, skin, mucous membranes, and blood vessel walls. Problems in the... read more

Disorders of the immune system occur when

The immune system has many components:

Antigens Innate Immunity are any substance that the immune system can recognize and that can thus stimulate an immune response.

Cells are the smallest unit of a living organism, composed of a nucleus and cytoplasm surrounded by a membrane.

Chemotaxis is the process of by which a chemical substance attracts cells to a particular site.

Helper T cells are white blood cells that help B cells produce antibodies against foreign antigens, help killer T cells become active, and stimulate macrophages, enabling them to ingest infected or abnormal cells more efficiently.

Histocompatibility (literally, compatibility of tissue) is determined by human leukocyte antigens (self-identification molecules). Histocompatibility is used to determine whether a transplanted tissue or organ will be accepted by the recipient.

Human leukocyte antigens (HLA) are a group of identification molecules located on the surface of all cells in a combination that is almost unique for each person, thereby enabling the body to distinguish self from nonself. This group of identification molecules is also called the major histocompatibility complex.

An immune complex is an antibody attached to an antigen.

An immune response is the reaction of the immune system to an antigen.

Immunoglobulin is another name for antibody.

Interleukin is a type of messenger (cytokine) secreted by some white blood cells to affect other white blood cells.

Killer (cytotoxic) T cells are T cells that attach to infected cells and cancer cells and kill them.

Leukocyte is another name for a white blood cell, such as a monocyte, a neutrophil, an eosinophil, a basophil, or a lymphocyte (a B cell or T cell).

The lymphatic system Lymphatic System: Helping Defend Against Infection

Major histocompatibility complex (MHC) is a synonym for human leukocyte antigens.

Mast cells are cells in tissues that release histamine and other substances involved in inflammatory and allergic reactions.

A molecule is a group of atoms chemically combined to form a unique substance.

Natural killer cells are a type of white blood cell that can recognize and kill abnormal cells, such as certain infected cells and cancer cells, without having to first learn that the cells are abnormal.

Phagocytes are a type of cell that ingests and kills or destroys invading microorganisms, other cells, and cell fragments. Phagocytes include neutrophils and macrophages.

Phagocytosis is the process of a cell engulfing and ingesting an invading microorganism, another cell, or a cell fragment.

A receptor is a molecule on a cell’s surface or inside the cell that can identify specific molecules, which fit precisely in it—as a key fits in its lock.

Regulatory (suppressor) T cells are white blood cells that help end an immune response.

The body has a series of defenses. Defenses include

• Physical barriers

• White blood cells

• Molecules such as antibodies and complement proteins

• Lymphoid organs

The first line of defense against invaders is mechanical or physical barriers:

• The skin

• The cornea of the eyes

• Membranes lining the respiratory, digestive, urinary, and reproductive tracts

As long as these barriers remain unbroken, many invaders cannot enter the body. If a barrier is broken—for example, if extensive burns damage the skin—the risk of infection is increased.

In addition, the barriers are defended by secretions containing enzymes that can destroy bacteria. Examples are sweat, tears in the eyes, mucus in the respiratory and digestive tracts, and secretions in the vagina.

The next line of defense involves white blood cells (leukocytes) that travel through the bloodstream and into tissues, searching for and attacking microorganisms and other invaders.

This defense has two parts:

• Innate immunity

• Acquired immunity

These substances are not contained in cells but are dissolved in a body fluid, such as plasma (the liquid part of blood).

Some of these molecules, including some cytokines, promote inflammation.

Inflammation occurs because these molecules attract immune system cells to the affected tissue. To help get these cells to the tissue, the body sends more blood to the tissue. To carry more blood to the tissue, blood vessels expand and become more porous, allowing more fluids and cells to leave blood vessels and enter the tissue. Inflammation thus tends to cause redness, warmth, and swelling. The purpose of inflammation is to contain the infection so that it does not spread. Then other substances produced by the immune system help the inflammation resolve and damaged tissues heal. Although inflammation may be bothersome, it indicates that the immune system is doing its job. However, excessive or long-term (chronic) inflammation can be harmful.

The immune system includes several organs in addition to cells dispersed throughout the body. These organs are classified as primary or secondary lymphoid organs.

The primary lymphoid organs are the sites where white blood cells are produced and/or multiply:

• The bone marrow produces all the different types of white blood cells, including neutrophils, eosinophils, basophils, monocytes, B cells, and the cells that develop into T cells (T cell precursors).

• In the thymus, T cells multiply and are trained to recognize foreign antigens and to ignore the body’s own antigens. T cells are critical for acquired immunity.

When needed to defend the body, the white blood cells are mobilized, mainly from the bone marrow. They then move into the bloodstream and travel to wherever they are needed.

Lymphatic System: Helping Defend Against Infection

The secondary lymphoid organs include the

• Spleen

• Lymph nodes

• Tonsils

• Appendix

• Peyer patches in the small intestine

These organs trap microorganisms and other foreign substances and provide a place for mature cells of the immune system to collect, interact with each other and with the foreign substances, and generate a specific immune response.

Lymph nodes are one of the first places that cancer cells can spread. Thus, doctors often evaluate lymph nodes to determine whether a cancer has spread. Cancer cells in a lymph node can cause the node to swell. Lymph nodes can also swell after an infection because acquired immune responses to infections are generated in lymph nodes. Sometimes lymph nodes swell because bacteria that are carried to a lymph node are not killed and cause an infection in the lymph node (lymphadenitis).

A successful immune response to invaders requires

• Recognition

• Activation and mobilization

• Regulation

• Resolution

To be able to destroy invaders, the immune system must first recognize them. That is, the immune system must be able to distinguish what is nonself (foreign) from what is self. The immune system can make this distinction because all cells have identification molecules (antigens) on their surface. Microorganisms are recognized because the identification molecules on their surface are foreign.

In people, the most important self-identification molecules are called

• Human leukocyte antigens (HLA), or the major histocompatibility complex (MHC)

HLA molecules are called antigens because if transplanted, as in a kidney or skin graft, they can provoke an immune response in another person (normally, they do not provoke an immune response in the person who has them). Each person has an almost unique combination of HLAs. Each person’s immune system normally recognizes this unique combination as self. A cell with molecules on its surface that are not identical to those on the body’s own cells is identified as being foreign. The immune system then attacks that cell. Such a cell may be a cell from transplanted tissue or one of the body’s cells that has been infected by an invading microorganism or altered by cancer. (HLA molecules are what doctors try to match when a person needs an organ transplant.)

Some white blood cells—B cells (B lymphocytes)—can recognize invaders directly. But others—T cells (T lymphocytes)—need help from cells called antigen-presenting cells:

• Antigen-presenting cells ingest an invader and break it into fragments.

• The antigen-presenting cell then combines antigen fragments from the invader with the cell's own HLA molecules.

• The combination of antigen fragments and HLA molecules is moved to the cell’s surface.

• A T cell with a matching receptor on its surface can attach to part of the HLA molecule presenting the antigen fragment, as a key fits into a lock.

• The T cell is then activated and begins fighting the invaders that have that antigen.

How T Cells Recognize Antigens

T cells are part of the immune surveillance system. They travel through the bloodstream and lymphatic system. When they reach the lymph nodes or another secondary lymphoid organ, they look for foreign substances (antigens) in the body. However, before they can fully recognize and respond to a foreign antigen, the antigen must be processed and presented to the T cell by another white blood cell, called an antigen-presenting cell. Antigen-presenting cells consist of dendritic cells (which are the most effective), macrophages, and B cells.

White blood cells are activated when they recognize invaders. For example, when the antigen-presenting cell presents antigen fragments bound to HLA to a T cell, the T cell attaches to the fragments and is activated. B cells can be activated directly by invaders. Once activated, white blood cells ingest or kill the invader or do both. Usually, more than one type of white blood cell is needed to kill an invader.

Immune cells, such as macrophages and activated T cells, release substances that attract other immune cells to the trouble spot, thus mobilizing defenses. The invader itself may release substances that attract immune cells.

Resolution involves confining the invader and eliminating it from the body. After the invader is eliminated, most white blood cells self-destruct and are ingested. Those that are spared are called memory cells. The body retains memory cells, which are part of acquired immunity, to remember specific invaders and respond more vigorously to them at the next encounter.

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