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.
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.
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