Immune system

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The immune system is the one that prevents or limits infection or fights foreign body. An example of this principle is found in immune-compromised people, including those with genetic immune disorders, immune-debilitating infections like HIV, and even pregnant women, who are susceptible to a range of microbes that typically do not cause infection in healthy individuals.

Immune system

The immune system can distinguish between normal, healthy cells and unhealthy cells by recognizing a variety of "danger" cues called danger-associated molecular patterns (DAMPs). Cells may be unhealthy because of infection or because of cellular damage caused by non-infectious agents like sunburn or cancer. Infectious microbes such as viruses and bacteria release another set of signals recognized by the immune system called pathogen-associated molecular patterns (PAMPs).

When the immune system first recognizes these signals, it responds to address the problem. If an immune response cannot be activated when there is sufficient need, problems arise, like infection. On the other hand, when an immune response is activated without a real threat or is not turned off once the danger passes, different problems arise, such as allergic reactions and autoimmune disease.

The immune system is complex and pervasive. There are numerous cell types that either circulate throughout the body or reside in a particular tissue. Each cell type plays a unique role, with different ways of recognizing problems, communicating with other cells, and performing their functions. By understanding all the details behind this network, researchers may optimize immune responses to confront specific issues, ranging from infections to cancer.

Location

All immune cells come from precursors in the bone marrow and develop into mature cells through a series of changes that can occur in different parts of the body.

  • Skin: The skin is usually the first line of defense against microbes. Skin cells produce and secrete important antimicrobial proteins, and immune cells can be found in specific layers of skin.
  • Bone marrow: The bone marrow contains stems cells that can develop into a variety of cell types. The common myeloid progenitor stem cell in the bone marrow is the precursor to innate immune cells—neutrophils, eosinophils, basophils, mast cells, monocytes, dendritic cells, and macrophages—that are important first-line responders to infection. The common lymphoid progenitor stem cell leads to adaptive immune cells—B cells and T cells—that are responsible for mounting responses to specific microbes based on previous encounters (immunological memory). Natural killer (NK) cells also are derived from the common lymphoid progenitor and share features of both innate and adaptive immune cells, as they provide immediate defenses like innate cells but also may be retained as memory cells like adaptive cells. B, T, and NK cells also are called lymphocytes.
  • Blood stream: Immune cells constantly circulate throughout the bloodstream, patrolling for problems. When blood tests are used to monitor white blood cells, another term for immune cells, a snapshot of the immune system is taken. If a cell type is either scarce or overabundant in the bloodstream, this may reflect a problem.
  • Thymus: T cells mature in the thymus, a small organ located in the upper chest.
  • Lymphatic system: The lymphatic system is a network of vessels and tissues composed of lymph, an extracellular fluid, and lymphoid organs, such as lymph nodes. The lymphatic system is a conduit for travel and communication between tissues and the bloodstream. Immune cells are carried through the lymphatic system and converge in lymph nodes, which are found throughout the body.
  • Lymph nodes are a communication hub where immune cells sample information brought in from the body. For instance, if adaptive immune cells in the lymph node recognize pieces of a microbe brought in from a distant area, they will activate, replicate, and leave the lymph node to circulate and address the pathogen. Thus, doctors may check patients for swollen lymph nodes, which may indicate an active immune response.
  • Spleen: The spleen is an organ located behind the stomach. While it is not directly connected to the lymphatic system, it is important for processing information from the bloodstream. Immune cells are enriched in specific areas of the spleen, and upon recognizing blood-borne pathogens, they will activate and respond accordingly.
  • Mucosal tissue: Mucosal surfaces are prime entry points for pathogens, and specialized immune hubs are strategically located in mucosal tissues like the respiratory tract and gut. For instance, Peyer's patches are important areas in the small intestine where immune cells can access samples from the gastrointestinal tract.​

Glossary of the immune system

immunity that has memory and occurs after exposure to an antigen either from a pathogen or a vaccination

attraction of molecular complementarity between antigen and antibody molecules

immune reaction that results from immediate hypersensitivities in which an antibody-mediated immune response occurs within minutes of exposure to a harmless antigen

protein that is produced by plasma cells after stimulation by an antigen; also known as an immunoglobulin

foreign or “non-self” protein that triggers the immune response

immune cell that detects, engulfs, and informs the adaptive immune response about an infection by presenting the processed antigen on the cell surface

antibody that incorrectly marks “self” components as foreign and stimulates the immune response

inappropriate immune response to host cells or self-antigens

type of hypersensitivity to self antigens

total binding strength of a multivalent antibody with antigen

lymphocyte that matures in the bone marrow and differentiates into antibody-secreting plasma cells

leukocyte that releases chemicals usually involved in the inflammatory response

adaptive immune response that is carried out by T cells

activation of B cells corresponding to one specific BCR variant and the dramatic proliferation of that variant

array of approximately 20 soluble proteins of the innate immune system that enhance phagocytosis, bore holes in pathogens, and recruit lymphocytes; enhances the adaptive response when antibodies are produced

binding of an antibody to an epitope corresponding to an antigen that is different from the one the antibody was raised against

chemical messenger that regulates cell differentiation, proliferation, gene expression, and cell trafficking to effect immune responses

adaptive immune cell that directly kills infected cells via perforin and granzymes, and releases cytokines to enhance the immune response

immune cell that processes antigen material and presents it on the surface of other cells to induce an immune response

lymphocyte that has differentiated, such as a B cell, plasma cell, or cytotoxic T lymphocyte

leukocyte that responds to parasites and is involved in the allergic response

small component of an antigen that is specifically recognized by antibodies, B cells, and T cells; the antigenic determinant

protease that enters target cells through perforin and induces apoptosis in the target cells; used by NK cells and killer T cells

cell of the adaptive immune system that binds APCs via MHC II molecules and stimulates B cells or secretes cytokines to initiate the immune response

an organism that is invaded by a pathogen or parasite

adaptive immune response that is controlled by activated B cells and antibodies

spectrum of maladaptive immune responses toward harmless foreign particles or self antigens; occurs after tissue sensitization and includes immediate-type (allergy), delayed-type, and autoimmunity

acquired ability to prevent an unnecessary or harmful immune response to a detected foreign body known not to cause disease or to self-antigens

failure, insufficiency, or delay at any level of the immune system, which may be acquired or inherited

localized redness, swelling, heat, and pain that results from the movement of leukocytes and fluid through opened capillaries to a site of infection

immunity that occurs naturally because of genetic factors or physiology, and is not induced by infection or vaccination

cytokine that inhibits viral replication and modulates the immune response

watery fluid that bathes tissues and organs with protective white blood cells and does not contain erythrocytes

leukocyte that is histologically identifiable by its large nuclei; it is a small cell with very little cytoplasm

large phagocytic cell that engulfs foreign particles and pathogens

protein found on the surface of all nucleated cells (I) or specifically on antigen-presenting cells (II) that signals to immune cells whether the cell is healthy/normal or is infected/cancerous; it provides the appropriate template into which antigens can be loaded for recognition by lymphocytes

leukocyte that produces inflammatory molecules, such as histamine, in response to large pathogens and allergens

antigen-specific B or T lymphocyte that does not differentiate into effector cells during the primary immune response but that can immediately become an effector cell upon reexposure to the same pathogen

type of white blood cell that circulates in the blood and lymph and differentiates into macrophages after it moves into infected tissue

collection of lymphatic tissue that combines with epithelial tissue lining the mucosa throughout the body

lymphocyte that can kill cells infected with viruses or tumor cells

phagocytic leukocyte that engulfs and digests pathogens

process that enhances phagocytosis using proteins to indicate the presence of a pathogen to phagocytic cells

transfer of antibodies from one individual to another to provide temporary protection against pathogens

an agent, usually a microorganism, that causes disease in the organisms that it invades

carbohydrate, polypeptide, and nucleic acid “signature” that is expressed by viruses, bacteria, and parasites but differs from molecules on host cells

molecule on macrophages and dendritic cells that binds molecular signatures of pathogens and promotes pathogen engulfment and destruction

destructive protein that creates a pore in the target cell; used by NK cells and killer T cells

immune cell that secrets antibodies; these cells arise from B cells that were stimulated by antigens

specialized lymphocyte that suppresses local inflammation and inhibits the secretion of cytokines, antibodies, and other stimulatory immune factors; involved in immune tolerance

lymphocyte that matures in the thymus gland; one of the main cells involved in the adaptive immune system


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