Immunology Terms Starting With I

Immediate Hypersensitivity is a rapid IgE-mediated allergic reaction that occurs within minutes of allergen exposure when allergen cross-links IgE antibodies bound to mast cells or basophils, triggering release of histamine and other mediators that cause symptoms ranging from itching and swelling to life-threatening anaphylaxis.

During initial allergen exposure, the immune system produces allergen-specific IgE antibodies that bind to high-affinity FcepsilonRI receptors on mast cells and basophils, a process called sensitization that causes no symptoms. Subsequent exposure cross-links these surface-bound IgE molecules, activating the cell within seconds and releasing preformed mediators such as histamine, tryptase, and heparin within minutes, followed by newly synthesized prostaglandins and leukotrienes over the next 30 to 60 minutes. Histamine binds H1 receptors on smooth muscle and endothelial cells, causing bronchoconstriction, vasodilation, and increased vascular permeability that produce the classic symptoms of allergy.

Anaphylaxis, the most severe form, can cause fatal cardiovascular collapse within 15 minutes if epinephrine is not administered promptly. Worldwide, peanut, tree nut, shellfish, and insect venom are among the most common triggers of severe immediate hypersensitivity reactions.

Immune Tolerance is the state of antigen-specific unresponsiveness in which the immune system does not mount a destructive response against defined antigens, including self-tissues, commensal microorganisms, and harmless environmental substances.

Central tolerance is established in the thymus for T cells and the bone marrow for B cells, where developing lymphocytes that bind self-antigens too strongly are deleted or, in the thymus, redirected into the regulatory T cell lineage. Peripheral tolerance mechanisms, including anergy, regulatory T cell suppression, and activation-induced cell death, prevent self-reactive lymphocytes that escape central selection from causing tissue damage. Regulatory T cells expressing the transcription factor Foxp3 are indispensable for peripheral tolerance; mice and humans with Foxp3 mutations develop fatal multi-organ autoimmunity within weeks of birth.

Breakdown of tolerance to self-antigens underlies diseases such as type 1 diabetes and multiple sclerosis, while failure of tolerance to commensal gut bacteria drives inflammatory bowel disease. Transplant immunologists exploit tolerance mechanisms to reduce rejection, and cancer immunotherapies deliberately disrupt tumor-induced tolerance to restore anti-tumor responses.

Immunoglobulin is a glycoprotein produced by B cells and plasma cells that consists of two heavy chains and two light chains joined by disulfide bonds, forming a structure that binds specific antigens and mediates immune effector functions either as a secreted antibody or as a membrane-bound B cell receptor.

Immunoglobulins are divided into five major classes based on their heavy chain constant region: IgG, IgA, IgM, IgE, and IgD. IgG is the most abundant antibody in serum, constitutes roughly 75 percent of total serum immunoglobulin in healthy adults, and crosses the placenta to confer passive immunity on the fetus. IgA dominates at mucosal surfaces and is secreted as a dimer into saliva, breast milk, and intestinal fluid, while IgM is typically the first antibody produced in a primary immune response and circulates as a pentamer with ten antigen-binding sites.

The enormous diversity of immunoglobulin antigen-binding sites is generated by V(D)J recombination during B cell development in the bone marrow, theoretically producing more than 10 to the 11th power distinct specificities before any antigen exposure. IgE, present at the lowest serum concentration of all classes, binds with exceptionally high affinity to FcepsilonRI receptors on mast cells and drives both allergic reactions and anti-parasite immunity.

Immunological Memory is the capacity of the adaptive immune system to respond more rapidly and with greater magnitude to a previously encountered antigen, mediated by long-lived memory B cells and T cells generated during an earlier infection, vaccination, or immune exposure.

Memory cells differ from naive lymphocytes in several measurable ways: they have a lower activation threshold, faster proliferation kinetics, altered homing receptors that direct them to peripheral tissues, and in B cells, higher-affinity antibody genes produced through affinity maturation in germinal centers. These long-lived T cells persist for decades in humans, maintained by low-level homeostatic proliferation driven by cytokines including IL-7 and IL-15, without requiring repeated antigen exposure. Studies of survivors of the 1918 influenza pandemic found circulating memory B cells and serum antibodies specific for the H1N1 hemagglutinin still detectable more than 90 years after infection.

The durability of immunological memory forms the biological basis of vaccination and explains why infections such as measles confer lifelong protection after a single natural exposure. Long-lived plasma cells residing in bone marrow niches secrete antibodies continuously for years, providing a standing level of circulating protection even before memory B cells are recalled.

Innate Immunity is the non-specific, rapidly responding first line of immune defense that recognizes broad molecular patterns associated with pathogens and damaged tissue through germline-encoded receptors, without requiring prior antigen exposure.

Innate immune components include physical barriers such as skin and mucus, antimicrobial peptides, complement proteins, phagocytes, natural killer cells, and pattern recognition receptors such as Toll-like receptors that detect conserved pathogen-associated molecular patterns. This response activates within minutes to hours of infection and provides immediate containment while the adaptive immune response develops over one to two weeks. Innate immune signals also direct adaptive immunity by determining the type of T cell response most appropriate for the detected pathogen, a process mediated largely through cytokines released by dendritic cells and macrophages.

In humans, at least 10 distinct Toll-like receptors have been characterized, each recognizing different microbial signatures ranging from bacterial lipopolysaccharide to viral double-stranded RNA.

Interleukin is a cytokine originally defined as a chemical messenger between leukocytes that regulates immune cell growth, differentiation, activation, and migration, with over 40 distinct interleukins now characterized.

Interleukins are produced by lymphocytes, macrophages, dendritic cells, epithelial cells, and stromal cells, and act through specific receptors on target cells to alter gene expression. Key interleukins include IL-1 and IL-6, which drive inflammation and fever; IL-2, which promotes T cell proliferation; IL-4 and IL-13, which drive allergic responses; IL-12, which promotes Th1 differentiation; and IL-10, which suppresses immune responses. Therapeutic antibodies targeting specific interleukins or their receptors have transformed treatment of autoimmune diseases including rheumatoid arthritis, psoriasis, and inflammatory bowel disease.

IL-6 receptor blockade with tocilizumab, approved by the FDA in 2010, was among the first such targeted therapies to reach widespread clinical use.

Interleukin-9 is a pleiotropic cytokine produced primarily by Th9 cells, mast cells, and innate lymphoid cells that promotes mast cell proliferation, mucus production, and airway inflammation, with documented roles in allergic disease and anti-parasite immunity.

IL-9 signals through the IL-9 receptor, a heterodimer of IL-9R? and the common gamma chain, activating JAK1 and JAK3 kinases and downstream STAT1 and STAT3 transcription factors. In the lung, IL-9 amplifies mast cell numbers and promotes goblet cell mucus secretion, contributing to asthma pathology in mouse models.

Th9 cells, the primary lymphocyte source of IL-9, were characterized as a distinct T helper subset around 2008 by two independent research groups, and their differentiation requires TGF-beta combined with IL-4. Emerging evidence also supports IL-9 roles in anti-tumor immunity, where mast cell activation downstream of IL-9 can enhance immune surveillance in certain solid tumor models.