1500+ medical terms, defined.

β2-microglobulin is a small protein component of MHC class I molecules, found on the surface of all nucleated cells. It is filtered by the renal glomerulus and reabsorbed/catabolized by the proximal tubule, making serum levels useful as...

Tenofovir is a nucleotide reverse transcriptase inhibitor (NRTI) used in the treatment and prevention (PrEP) of HIV infection and in the treatment of chronic hepatitis B. It works by inhibiting viral reverse transcriptase, causing chain...

Hookworms (Ancylostoma duodenale and Necator americanus) are soil-transmitted helminths that cause iron-deficiency anemia and protein malnutrition via chronic intestinal blood loss. Infection occurs when filariform larvae penetrate skin...

The invariant chain (Ii, CD74) is a chaperone protein that binds nascent MHC class II molecules in the endoplasmic reticulum, preventing premature peptide loading. It occupies the peptide-binding groove until the complex reaches the endo...

Erythroblastosis fetalis (hemolytic disease of the newborn) is fetal/neonatal hemolytic anemia caused by maternal IgG antibodies against fetal RBC antigens, most classically Rh(D) incompatibility. It occurs when an Rh-negative mother is...

Lewis antibodies (anti-Le^a and anti-Le^b) are naturally occurring alloantibodies directed against Lewis blood group antigens, which are soluble antigens adsorbed onto RBC membranes rather than intrinsic RBC proteins. They are typically...

Hemolytic disease of the newborn (HDN) is a condition caused by maternal-fetal blood group incompatibility, most classically Rh(D) incompatibility, where maternal IgG antibodies cross the placenta and destroy fetal RBCs. This leads to fe...

Alloimmunization is the development of an immune response (antibody production) against antigens from a genetically different member of the same species, most commonly seen with blood group antigens (Rh, Kell, Duffy) or HLA antigens afte...

Exchange transfusion is a procedure where a patient's blood is removed and replaced with donor blood, used to rapidly remove harmful substances (bilirubin, abnormal RBCs, antibodies) while maintaining blood volume. It is most commonly us...

Achlorhydria refers to the absence or marked reduction of gastric acid (HCl) secretion by parietal cells. It is classically associated with chronic atrophic gastritis (autoimmune or H. pylori-related), long-term PPI use, and gastrinoma-r...

Non-UV phototherapy refers to light-based treatments that do not use ultraviolet wavelengths, most notably visible blue light phototherapy used for neonatal hyperbilirubinemia. It works by converting unconjugated bilirubin in the skin in...

Endotoxins are lipopolysaccharides (LPS) found in the outer membrane of gram-negative bacteria, released upon bacterial cell lysis or death. They trigger a potent systemic inflammatory response via TLR4/CD14 signaling, often causing feve...

Chorioretinitis is inflammation of the choroid and retina, commonly caused by congenital infections (TORCH), reactivated toxoplasmosis, CMV in immunocompromised patients, or syphilis. It presents with blurred vision, floaters, and charac...

Nucleoside/Nucleotide Reverse Transcriptase Inhibitors (NRTIs) are antiretroviral drugs used to treat HIV by inhibiting the reverse transcriptase enzyme, blocking conversion of viral RNA to DNA. Examples include tenofovir, emtricitabine,...

Howell-Jolly bodies are basophilic nuclear remnants found within circulating erythrocytes, normally removed by a functioning spleen. Their presence on peripheral blood smear is a classic sign of asplenia (functional or anatomic) or splen...

Emtricitabine (FTC) is a nucleoside reverse transcriptase inhibitor (NRTI) used in combination antiretroviral therapy (ART) for HIV treatment and pre-exposure prophylaxis (PrEP). It is often combined with tenofovir in fixed-dose formulat...

Capsular polysaccharides are thick, mucoid layers surrounding certain bacteria that serve as major virulence factors by inhibiting phagocytosis. They are important antigens used in vaccines (e.g., pneumococcal, meningococcal, Hib vaccine...

Partial Thromboplastin Time (PTT), specifically activated PTT (aPTT), measures the intrinsic and common coagulation pathways. It evaluates factors XII, XI, IX, VIII, X, V, II (prothrombin), and fibrinogen. It is the standard test used to...

Prothrombin Time (PT) is a laboratory test that measures the time it takes for blood to clot via the extrinsic and common coagulation pathways. It is used to assess factors I, II, V, VII, and X, and is commonly reported with the Internat...

Type II Hypersensitivity Reaction (HSR) is an antibody-mediated immune response where IgG or IgM antibodies bind directly to antigens on the surface of cells or tissues, leading to cell destruction via complement activation, antibody-dep...

Downey type II cells are a type of atypical (reactive) lymphocyte, also known as Downey cells, classically seen in the peripheral blood smear of patients with infectious mononucleosis caused by Epstein-Barr virus (EBV). They are reactive...

Merozoites are an invasive, asexual reproductive stage of Plasmodium (and Babesia) parasites that infect and lyse red blood cells. They are produced by schizogony and are responsible for the cyclical clinical symptoms of malaria. Each me...

Borrelia is a genus of spirochete bacteria transmitted by arthropod vectors, most notably Borrelia burgdorferi (and related species) causing Lyme disease via Ixodes ticks, and Borrelia recurrentis/hermsii causing relapsing fever via lice...

A schizont is a developmental stage of Plasmodium (and other apicomplexan parasites like Babesia) that forms during asexual reproduction (schizogony) within the host's red blood cells or liver cells. It represents a multinucleated cell p...

Gametocytes are sexual-stage forms of Plasmodium (malaria) parasites that circulate in human blood but cause no direct disease themselves; they are essential for transmission because they are ingested by Anopheles mosquitoes to continue...

Lethal factor is one of three components of anthrax toxin produced by Bacillus anthracis, acting as a zinc-dependent metalloprotease that cleaves MAPKK (MEK) proteins. Combined with protective antigen, it disrupts cell signaling, leading...

DAF (Decay-Accelerating Factor, CD55) is a GPI-anchored membrane protein that protects host cells from complement-mediated lysis by accelerating the decay of C3 and C5 convertases. Its deficiency, along with CD59, underlies paroxysmal no...

MIRL (Membrane Inhibitor of Reactive Lysis), also known as CD59, is a GPI-anchored complement regulatory protein found on host cell membranes. It protects cells from complement-mediated lysis by inhibiting the formation of the membrane a...

PD-1 (Programmed Death-1) is an inhibitory immune checkpoint receptor expressed on activated T cells, B cells, and NK cells. It binds PD-L1/PD-L2 on tumor or antigen-presenting cells, delivering an inhibitory signal that dampens T-cell a...

Th17 cells are a subset of CD4+ helper T cells that produce IL-17, IL-21, and IL-22, playing a key role in mucosal immunity against extracellular bacteria and fungi, and are implicated in autoimmune diseases. They differentiate from naiv...

INR (International Normalized Ratio) is a standardized measure of the extrinsic/common coagulation pathway, calculated from the prothrombin time (PT). It is primarily used to monitor patients on warfarin therapy and assess liver syntheti...

Factor XIII (fibrin stabilizing factor) is a transglutaminase enzyme that cross-links fibrin monomers via covalent bonds, stabilizing the clot at the final step of the coagulation cascade. It is activated by thrombin in the presence of c...

Bronchiolitis obliterans (BO) is a rare, irreversible obstructive lung disease caused by inflammation and fibrosis of the small airways (bronchioles), leading to their narrowing or complete obliteration. It presents with progressive dysp...

Fibrin is the insoluble protein that forms the structural meshwork of a blood clot, generated from fibrinogen via thrombin cleavage in the final common pathway of the coagulation cascade. It is then cross-linked by factor XIIIa to form a...

Factor VII is a vitamin K-dependent clotting factor that, once activated to VIIa, binds tissue factor (TF) released from damaged subendothelial tissue to initiate the extrinsic coagulation pathway. It has the shortest half-life of all cl...

Factor XIIIa is the activated form of factor XIII, a transglutaminase enzyme that cross-links fibrin monomers to stabilize the clot. It acts as the final step in the coagulation cascade, converting soluble fibrin into an insoluble, mecha...

Factor III, also known as tissue factor (thromboplastin), is a membrane-bound glycoprotein that initiates the extrinsic pathway of the coagulation cascade. It is released by damaged endothelial cells and subendothelial tissue upon vascul...

Factor VIIIa is the activated cofactor form of factor VIII, which works with factor IXa in the intrinsic coagulation pathway to form the 'tenase complex,' dramatically accelerating activation of factor X to Xa. Deficiency or dysfunction...

Factor Va is the activated form of coagulation factor V, which serves as a critical cofactor for factor Xa in the prothrombinase complex. This complex (Factor Xa + Factor Va + Ca2+ + phospholipid) catalyzes the conversion of prothrombin...

P2Y12 inhibitors are antiplatelet drugs that block the P2Y12 ADP receptor on platelets, preventing ADP-mediated platelet activation and aggregation. Key drugs include clopidogrel, prasugrel, ticagrelor, and cangrelor. They are used in ac...

Protein S is a vitamin K-dependent plasma glycoprotein that acts as a cofactor for activated Protein C (APC), enhancing its ability to degrade clotting factors Va and VIIIa. Deficiency (inherited or acquired) leads to a hypercoagulable s...

Granulocytopenia is an abnormally low number of granulocytes (neutrophils, eosinophils, basophils) in the blood, most clinically relevant when it refers to neutropenia (ANC <1500/µL). It significantly increases risk of severe bacterial a...

Antiplatelet drugs inhibit platelet aggregation, reducing arterial thrombus formation. They are used for secondary prevention of cardiovascular events (MI, stroke) and in patients with coronary stents or peripheral arterial disease. Key...

COX-1 (cyclooxygenase-1) is a constitutively expressed enzyme that converts arachidonic acid into prostaglandins and thromboxane A2, playing a key role in gastric mucosal protection, platelet aggregation, and renal blood flow maintenance...

Cyclooxygenase-2 (COX-2) is an inducible enzyme that converts arachidonic acid to prostaglandins, primarily at sites of inflammation. Unlike COX-1, it is not constitutively expressed in gastric mucosa or platelets, making COX-2 selective...

IgG is the most abundant immunoglobulin isotype in serum, providing long-term humoral immunity. It is the only antibody class that crosses the placenta, protecting the fetus/newborn. IgG mediates opsonization, complement fixation (via cl...

Acetaldehyde dehydrogenase (ALDH2) is a mitochondrial enzyme that converts acetaldehyde to acetate in the second step of ethanol metabolism. It is the target of the drug disulfiram, which inhibits ALDH2 to cause aversive reactions to alc...

Zosyn is the brand name for piperacillin-tazobactam, a broad-spectrum beta-lactam/beta-lactamase inhibitor combination antibiotic. It covers gram-positive, gram-negative (including Pseudomonas aeruginosa), and anaerobic organisms, making...

Piperacillin-tazobactam is an extended-spectrum beta-lactam/beta-lactamase inhibitor combination antibiotic with broad coverage including Pseudomonas aeruginosa, gram-positive, gram-negative, and anaerobic organisms. It is commonly used...

Herceptin (trastuzumab) is a humanized monoclonal antibody targeting the HER2/neu (ErbB2) receptor, used in treatment of HER2-positive breast and gastric cancers. It is notable for its unique cardiotoxicity profile (reversible dilated ca...