Immunology Terms Starting With O
Immunology Glossary: O
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Opsonization
/ op-soh-nih-ZAY-shun / · Greek opsonein (to prepare food for) + -ization
Opsonization is the process by which antibodies, complement proteins, or other soluble molecules coat the surface of a pathogen or particle, marking it for enhanced recognition and ingestion by phagocytes bearing Fc receptors or complement receptors.
IgG antibodies bind pathogens through their Fab variable regions and simultaneously engage Fc? receptors on macrophages and neutrophils through their Fc constant region, triggering phagocytosis. Complement fragment C3b deposited on microbial surfaces bridges the pathogen to complement receptor 1 on phagocytes, providing a second independent opsonization pathway.
Together, antibody and complement opsonization can increase the rate of phagocytosis by more than 1,000-fold compared to unopsonized targets. Encapsulated bacteria such as Streptococcus pneumoniae are virtually impossible for phagocytes to engulf without opsonization because the polysaccharide capsule physically prevents direct receptor contact with the bacterial surface.
Collectins, a family of soluble pattern recognition proteins that includes mannose-binding lectin, opsonize bacteria and fungi by depositing on their surfaces and engaging receptors on phagocytes, providing a form of opsonization that operates before antibodies are produced and requires no prior exposure to the pathogen.
Immune System Fun Facts →Phagocytes recognize and engulf every microbe equally well on their own. Many pathogens, particularly encapsulated bacteria, resist direct phagocytosis and require opsonization to be cleared efficiently.
When Streptococcus pneumoniae enters the bloodstream of an immunized host, circulating IgG antibodies bind the bacterial capsule within seconds and recruit complement, coating the surface with both IgG and C3b. Splenic macrophages bearing Fc? receptors and complement receptor 1 then clear opsonized pneumococci roughly 100 times faster than unopsonized cells, explaining why asplenic individuals face dramatically elevated risk from this organism.
Oral Tolerance
/ OR-al TAH-ler-ants / · From Latin oralis, meaning of the mouth, and tolerantia, meaning endurance
Oral Tolerance is the specific suppression of systemic immune responses to antigens encountered through the gastrointestinal tract, preventing inflammatory reactions to dietary proteins and commensal bacteria.
Oral tolerance develops when antigens absorbed through the gut mucosa induce regulatory T cells and anergic lymphocytes rather than inflammatory effector responses. Low doses of oral antigen typically induce active suppression through CD4-positive regulatory T cells producing IL-10 and TGF-?, while high doses cause clonal deletion or anergy of antigen-reactive T cells. Dendritic cells in Peyer’s patches and mesenteric lymph nodes present antigens in tolerogenic contexts by expressing retinoic acid and TGF-?, imprinting regulatory rather than inflammatory T cell fates.
Feeding mice ovalbumin before systemic immunization reduces subsequent antibody responses by 90 percent or more, a benchmark result that established the gut as an active site of peripheral tolerance induction. Failure of oral tolerance contributes to food allergies and inflammatory bowel disease, conditions in which dietary or microbial antigens drive chronic mucosal inflammation.
Regulatory T cells induced in the gut by oral antigen can migrate to distant sites and suppress immune responses to the same antigen encountered elsewhere in the body, a phenomenon called "bystander suppression" that researchers have proposed as the mechanism underlying early clinical trials in which multiple sclerosis patients consumed myelin proteins to reduce neurological inflammation.
Oral tolerance means the immune system ignores everything that enters the gut. The gut mounts active regulatory responses, generating antigen-specific regulatory T cells that spread systemic suppression to harmless food proteins rather than simply failing to detect them.
In the LEAP trial, published in 2015 by Gideon Lack and colleagues, early introduction of peanut protein to high-risk infants between 4 and 11 months of age reduced peanut allergy development by 81 percent compared to avoidance by age five. This result demonstrated by age 5 that oral antigen exposure during a critical developmental window actively programs lasting immune tolerance rather than merely delaying sensitization.
How To Become An Immunologist? →OX40 Ligand
/ OKS-for-tee LY-gand / · OX40 named after the original antibody clone designation from Oxford laboratory; ligand from Latin ligare, meaning to bind
OX40 Ligand is a costimulatory protein expressed on antigen-presenting cells that binds the OX40 receptor on activated T cells to enhance their survival, proliferation, and cytokine production during adaptive immune responses.
OX40 ligand belongs to the tumor necrosis factor superfamily and is expressed on activated dendritic cells, B cells, and macrophages in response to inflammatory signals, with expression rising 10- to 50-fold on dendritic cells following TLR stimulation. When OX40L engages OX40 on T cells, it delivers signals that prolong T cell survival by upregulating anti-apoptotic proteins Bcl-2 and Bcl-xL and enhances cytokine production including IL-2, IFN-?, and IL-4. This interaction is most consequential 24 to 72 hours after initial T cell activation, providing a late-stage costimulatory signal that determines whether activated T cells undergo clonal expansion or apoptosis.
OX40-OX40L blockade in rodent autoimmune models reduces disease severity by 60 to 80 percent, while agonist antibodies boost anti-tumor immunity by expanding effector and memory T cell populations. Signaling through OX40 also suppresses regulatory T cell activity, shifting the balance between effector and suppressive responses toward inflammation.
Human T cell leukemia virus type 1 (HTLV-1) constitutively activates OX40 signaling in infected T cells by encoding the Tax protein, which drives continuous OX40L expression on neighboring cells and contributes to the uncontrolled T cell proliferation seen in adult T cell leukemia, linking this costimulatory pathway directly to viral oncogenesis.
OX40 ligand works the same way as the early costimulatory molecules CD80 and CD86. OX40L acts 24 to 72 hours after initial priming, a later window than CD80/CD86, and its primary impact is on memory T cell survival and numbers rather than on the initial decision to activate naive T cells.
In clinical trials of OX40 agonist antibodies for melanoma, treated patients showed enhanced tumor-specific T cell responses and objective tumor regression in 15 to 30 percent of cases. Combination with PD-1 checkpoint blockade increased response rates further, suggesting that late-stage costimulation through OX40 and release of inhibitory signals act on distinct and complementary steps of the anti-tumor T cell response.