Medical terms, defined.

The withdrawal (flexor, nociceptive) reflex is a polysynaptic spinal reflex in which a noxious stimulus causes ipsilateral flexion (withdrawal) of the limb and, via the crossed extensor reflex, contralateral extension for postural suppor...

Renshaw cells are inhibitory interneurons in the ventral horn of the spinal cord that receive collateral branches from alpha motor neurons and release glycine (and GABA) onto those same motor neurons. This forms a negative feedback (recu...

Gamma motor neurons are lower motor neurons in the ventral horn of the spinal cord that innervate the intrafusal muscle fibers of muscle spindles (not the force-generating extrafusal fibers). They adjust spindle sensitivity by keeping th...

The muscle spindle is a stretch-sensitive proprioceptive receptor embedded in skeletal muscle that detects changes in muscle length and the rate of change in length. It mediates the stretch (myotatic) reflex, such as the patellar tendon...

Cerebrospinal fluid (CSF) is a clear, colorless ultrafiltrate-like fluid produced mainly by the choroid plexus ependymal cells in the ventricles. It cushions and supports the brain, maintains chemical homeostasis, and clears waste. Lumba...

Thrombopoiesis is the production of platelets (thrombocytes) from megakaryocytes in the bone marrow. It is driven primarily by thrombopoietin (TPO), made constitutively by the liver, and acts via the c-Mpl receptor on megakaryocyte proge...

A pluripotent stem cell can self-renew indefinitely and differentiate into cells of all three germ layers (ectoderm, mesoderm, endoderm), but NOT extraembryonic tissues (e.g., placenta/trophoblast). Examples: embryonic stem cells (from t...

Granulopoiesis is the production of granulocytes (neutrophils, eosinophils, basophils) in the bone marrow from myeloid progenitors. It is driven mainly by G-CSF (neutrophils), GM-CSF and IL-3, and IL-5 (eosinophils). Clinically important...

Hemoglobin A2 (HbA2) is a minor adult hemoglobin (α2δ2) making up about 1.5–3.5% of total hemoglobin in normal adults. It is elevated (>3.5%) in beta-thalassemia trait and minor, and normal or decreased in alpha-thalassemia and iron defi...

The lymphoid progenitor (common lymphoid progenitor, CLP) is a bone marrow–derived multipotent hematopoietic cell downstream of the hematopoietic stem cell that gives rise to T cells, B cells, and NK cells (and some dendritic cells). Its...

Pappenheimer bodies are basophilic, iron-containing granules (siderotic granules) seen within RBCs on Wright-Giemsa stain, which stain positive with Prussian blue. They represent iron-laden mitochondria or ferritin aggregates/phagolysoso...

A target cell (codocyte) is an RBC with a central dark spot surrounded by a pale ring and an outer dark ring, resembling a bullseye. It reflects an increased surface-area-to-volume ratio and is classically seen in HALT: Hemoglobin C dise...

A dacrocyte (teardrop cell) is an RBC with a single elongated or pointed end, resembling a teardrop. It classically indicates myelofibrosis with extramedullary hematopoiesis, but also appears in thalassemia, myelophthisic anemia, and oth...

Helmet cells are a type of schistocyte (fragmented RBC) with a straight or concave edge and one or two horn-like projections, resembling a helmet. They indicate mechanical (intravascular) RBC fragmentation, as in microangiopathic hemolyt...

Lymphopoiesis is the production and differentiation of lymphocytes (B cells, T cells, NK cells) from hematopoietic stem cells via the common lymphoid progenitor (CLP). B-cell development occurs in the bone marrow; T-cell development begi...

An elliptocyte is an elliptical or oval-shaped red blood cell seen on peripheral smear. Classically associated with hereditary elliptocytosis (defects in RBC cytoskeleton proteins such as spectrin or protein 4.1), but also seen in iron d...

Burr cells (echinocytes) are RBCs with 10–30 evenly spaced, short, blunt, regular projections and retained central pallor. They are classically seen in uremia (end-stage renal disease), and also in pyruvate kinase deficiency and as an ar...

Spur cells (acanthocytes) are RBCs with 2–20 irregularly spaced, variable-length, thorn-like projections and no central pallor. They are classically seen in severe liver disease (cirrhosis, with hemolytic "spur cell anemia"), abetalipopr...

The direct Coombs test (direct antiglobulin test, DAT) detects antibodies and/or complement already bound to the surface of a patient's RBCs in vivo. A positive result indicates immune-mediated hemolysis, such as warm or cold autoimmune...

Hemoglobin S (HbS) is an abnormal hemoglobin caused by a point mutation in the β-globin gene (GAG→GTG on chromosome 11), substituting valine for glutamic acid at position 6 (Glu6Val). Deoxygenated HbS polymerizes, causing RBC sickling, c...

Haptoglobin is an acute-phase alpha-2 globulin produced by the liver that binds free hemoglobin released during hemolysis. The Hb-haptoglobin complex is cleared by the reticuloendothelial system (macrophages via CD163), so serum haptoglo...

Tissue plasminogen activator (tPA; alteplase, reteplase, tenecteplase) is a serine protease that converts plasminogen to plasmin, which degrades fibrin clots (fibrinolysis). Used for acute ischemic stroke (within 3–4.5 h of onset), STEMI...

The indirect Coombs test (indirect antiglobulin test) detects free circulating IgG antibodies in a patient's SERUM against RBC antigens. Patient serum is incubated with reagent RBCs, then Coombs reagent (anti-human globulin) is added; ag...

A fibrin clot is the stable, cross-linked mesh of fibrin polymer that reinforces the platelet plug in secondary hemostasis. It forms when thrombin cleaves fibrinogen to fibrin monomers, which polymerize and are covalently cross-linked by...

Regulatory T cells (Tregs) are CD4+ T cells expressing CD25 (IL-2 receptor α) and the transcription factor FOXP3 that suppress immune responses and maintain self-tolerance. They act via IL-10, TGF-β, IL-35, CTLA-4, and IL-2 consumption....

MHC class I (HLA-A, -B, -C in humans) is expressed on all nucleated cells and platelets (not RBCs). It presents endogenous (intracellular) peptides to CD8+ cytotoxic T cells, and its loss/downregulation triggers NK cell killing. It is a...

Cytotoxic T cells (CD8+ T cells) recognize antigen presented on MHC class I and kill virus-infected, tumor, and foreign (graft) cells by inducing apoptosis via perforin/granzyme and Fas-FasL. They are central to defense against intracell...

Th2 cells are CD4+ helper T cells driven by IL-4 and the transcription factor GATA3 that secrete IL-4, IL-5, and IL-13. They coordinate humoral and allergic immunity: B-cell class switching to IgE/IgG4, eosinophil activation, and defense...

Tregs are CD4+ T cells expressing CD25 (IL-2Rα) and the transcription factor FOXP3 that suppress immune responses and maintain self-tolerance. Loss-of-function FOXP3 mutations cause IPEX syndrome (immune dysregulation, polyendocrinopathy...

CD8 T cells (cytotoxic T lymphocytes, CTLs) recognize antigen presented on MHC class I (on all nucleated cells) and kill virus-infected, tumor, and allograft cells. They kill via perforin/granzyme-induced apoptosis and Fas–FasL interacti...

The B-cell receptor (BCR) is a membrane-bound immunoglobulin (IgM and IgD on naive B cells) complexed with the signaling heterodimer Igα/Igβ (CD79a/CD79b). It recognizes native, unprocessed antigen, triggers B-cell activation, and, after...

Th1 cells are CD4+ T helper cells that differentiate in response to IL-12 (from macrophages/dendritic cells) and IFN-γ, driven by the transcription factor T-bet. They secrete IFN-γ (and IL-2, TNF-β), activating macrophages and promoting...

CD4 T cells (T helper cells) are lymphocytes that recognize peptide antigen presented on MHC class II and coordinate adaptive immunity by secreting cytokines. They are the primary target of HIV (via CD4 and CCR5/CXCR4), and AIDS is defin...

Th17 cells are CD4+ helper T cells that secrete IL-17, IL-21, and IL-22 and are essential for neutrophil recruitment and defense against extracellular bacteria and fungi (e.g., Candida, S. aureus). They differentiate in response to TGF-β...

A naive T cell is a mature T lymphocyte that has exited the thymus but has not yet encountered its cognate antigen. It circulates through blood and secondary lymphoid organs (via L-selectin/CCR7), is CD45RA+, CCR7+, CD62L+, and requires...

Memory T cells are long-lived antigen-experienced T lymphocytes (CD4+ or CD8+) that persist after an infection or vaccination resolves. They mount faster, stronger secondary responses on re-exposure to the same antigen, and they underlie...

Effector T cells are activated, differentiated T lymphocytes (CD4+ helper subsets and CD8+ cytotoxic T cells) that carry out the immune response against antigen. They arise from naive T cells after APC priming (signal 1: TCR-MHC/peptide;...

A naive B cell is a mature B lymphocyte that has exited the bone marrow and expresses surface IgM and IgD but has not yet encountered its cognate antigen. It circulates through blood and secondary lymphoid organs (follicles of lymph node...

Natural killer (NK) cells are CD56+/CD16+ (CD3−) lymphocytes of the innate immune system that kill virus-infected and tumor cells without prior sensitization. They detect cells with downregulated MHC I ("missing self") and induce apoptos...

Memory B cells are long-lived, antigen-experienced B lymphocytes generated during the germinal center reaction that persist after infection or vaccination. On re-exposure to the same antigen, they mount a faster, stronger, higher-affinit...

Microglia are the resident macrophages of the CNS, derived from yolk sac (mesodermal) progenitors rather than neuroectoderm. They perform phagocytosis, antigen presentation, synaptic pruning, and cytokine release. On histology, they appe...

Natural killer (NK) cells are large granular lymphocytes of the innate immune system that kill virally infected cells and tumor cells without prior sensitization. They are identified by CD16 and CD56 (and lack CD3), and they kill cells w...

PAMPs are conserved molecular structures shared by broad classes of microbes (e.g., LPS, peptidoglycan, flagellin, unmethylated CpG DNA, dsRNA) that are recognized by pattern recognition receptors (PRRs) such as Toll-like receptors on in...

Toll-like receptors (TLRs) are pattern recognition receptors (PRRs) of the innate immune system that recognize pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs). Activation triggers NF-κB (an...

DAMPs (Damage-Associated Molecular Patterns) are endogenous molecules released from damaged or necrotic cells that activate innate immune pattern recognition receptors (e.g., TLRs, NLRs, RAGE), triggering sterile inflammation. Examples i...

Pattern recognition receptors (PRRs) are germline-encoded innate immune receptors on/in sentinel cells (macrophages, dendritic cells, neutrophils, epithelial cells) that detect conserved microbial structures (PAMPs) and host danger signa...

Damage-associated molecular patterns (DAMPs) are endogenous molecules released from damaged or necrotic cells that signal tissue injury and activate innate immunity via pattern recognition receptors (e.g., TLRs, NLRP3 inflammasome, RAGE)...

The classical pathway is the complement activation pathway triggered by IgG or IgM antigen-antibody complexes binding C1q. It leads to C3 convertase (C4b2a) formation, then C5 convertase, and finally the membrane attack complex (MAC, C5b...

C3a is a small (~9 kDa) anaphylatoxin released when C3 is cleaved by C3 convertase (C3bBb in the alternative pathway, C4b2a in the classical and lectin pathways). It binds C3aR on mast cells, basophils, and other cells. This triggers deg...

C5a is a potent anaphylatoxin and neutrophil chemotactic fragment released when C5 is cleaved by C5 convertase (C4b2a3b or C3bBb3b) during complement activation. It recruits neutrophils, increases vascular permeability, and triggers mast...