Blood, Hemopoiesis & Lymphoid Organs – Vocabulary Review

Hematopoiesis

The regulated process in bone marrow that produces all blood cells from a pluripotent stem cell.

Myeloid lineage

Hematopoietic branch giving rise to granulocytes, monocytes, erythrocytes and megakaryocytes.

Lymphoid lineage

Hematopoietic branch that generates B lymphocytes, T lymphocytes and natural killer (NK) cells.

Plasma

Blood’s fluid extracellular matrix (pH 7.4) containing water, ions, proteins (≈7%) and dissolved molecules.

Albumin

Major plasma protein (≈58%) made by the liver; maintains colloid osmotic pressure and transports substances.

Globulins

Plasma proteins (≈37%) made in the liver; include immunoglobulins and transport proteins.

Fibrinogen

Liver-derived plasma protein (≈4%) that is converted to fibrin during blood coagulation.

Blood smear

Thin film of blood routinely stained with eosin and methylene blue (Wright-Giemsa) for cell identification.

Hemopoietic stem cell (HSC)

Pluripotent bone-marrow stem cell capable of self-renewal and asymmetric division to form all blood cells.

Red bone marrow

Blood-forming marrow rich in hemopoietic cells, sinusoids and reticular stroma.

Yellow bone marrow

Marrow dominated by adipocytes with little hemopoietic activity; may convert to red marrow if needed.

Bone-marrow stroma

Reticular connective tissue framework of marrow composed of stromal cells, type I collagen and ECM proteins.

Erythropoiesis

Sequence of stages that produces erythrocytes from erythroid progenitors under erythropoietin control (~1 week).

Erythropoietin

Kidney-derived growth factor that stimulates erythrocyte production in bone marrow.

Proerythroblast

Large, nucleolated, basophilic first recognizable erythroid precursor.

Basophilic erythroblast

Smaller erythroid cell with intense basophilia due to abundant polyribosomes.

Polychromatophilic erythroblast

Erythroid stage showing mixed basophilic and acidophilic cytoplasm as hemoglobin accumulates.

Orthochromatophilic erythroblast

Late erythroid precursor with acidophilic cytoplasm and pyknotic nucleus.

Reticulocyte

Immature, anucleate RBC containing a few polyribosomes; matures in blood within 1–2 days.

Erythrocyte (RBC)

Anucleate, biconcave cell packed with hemoglobin; transports O2/CO2; lifespan ≈120 days.

Hemoglobin A (HbA)

Normal adult hemoglobin (α2β2) constituting ≈96 % of total Hb.

HbA1c

Glycosylated HbA subtype used to monitor long-term blood glucose control.

Hemoglobin F (HbF)

Fetal hemoglobin (α2γ2); <1 % in adults but predominant prenatally.

Granulopoiesis

Bone-marrow process that forms neutrophils, eosinophils and basophils through myeloblast → band cell stages.

Myeloblast

Earliest granulocyte precursor with fine chromatin, nucleoli and basophilic cytoplasm.

Promyelocyte

Granulocyte precursor containing primary (azurophilic) granules.

Myelocyte

Stage where specific granules appear; last stage capable of division.

Metamyelocyte

Granulocyte precursor with indented (kidney-shaped) nucleus; no mitosis.

Band cell

Immature neutrophil with horseshoe-shaped nucleus; increases during infection.

Neutrophil

Most abundant WBC; multilobed nucleus; phagocytic first responder to bacteria.

Leukocyte

Collective term for white blood cells; normal count 4,500-11,000 /µL.

Agranulocyte

WBC group lacking specific granules: lymphocytes and monocytes.

Megakaryopoiesis

Formation of platelets from megakaryoblasts → megakaryocytes under thrombopoietin.

Thrombopoietin

Hormone (mainly liver-derived) that drives megakaryocyte development and platelet production.

Megakaryocyte

Large, polyploid bone-marrow cell that extends pro-platelets into sinusoids.

Platelet (Thrombocyte)

Small, anucleate cell fragment (150–400 K/µL) that mediates hemostasis and wound repair; lifespan ≈10 days.

Lymphatic system

Network of vessels and organs that returns tissue fluid to blood and mediates immune defense.

Primary lymphoid organs

Sites of lymphocyte formation: bone marrow (B cells) and thymus (T cells).

Secondary lymphoid organs

Sites of lymphocyte activation: lymph nodes, MALT and spleen.

Lymph

Interstitial fluid entering lymphatic capillaries; water, ions, proteins, lipids, lymphocytes and waste.

Thymus

Bilobed mediastinal organ responsible for T-cell maturation and central tolerance; involutes after puberty.

Thymic epithelial cells (TECs)

Specialized epithelial-reticular cells that support and educate developing T cells in thymus.

Thymic (Hassall) corpuscle

Concentric TEC structure found in thymic medulla; role in T-cell tolerance.

Blood-thymus barrier

Multi-layered barrier (endothelium, perivascular space, type I TECs) protecting thymic cortex from antigens.

MALT

Mucosa-associated lymphoid tissue in digestive, respiratory and genitourinary tracts containing up to 70 % of immune cells.

Tonsils

Aggregates of lymphoid tissue at posterior oral cavity/nasopharynx (palatine, lingual, pharyngeal) intercepting inhaled/ingested antigens.

Lymph node

Encapsulated, bean-shaped organ filtering lymph and producing antibodies; organized into cortex, paracortex and medulla.

Cortex (lymph node)

Outer region with lymphoid nodules rich in B cells.

Paracortex

Intermediate lymph-node zone rich in T cells and high-endothelial venules.

Medulla (lymph node)

Inner lymph-node region with medullary cords and sinuses.

Spleen

Largest lymphoid organ; filters blood, destroys old RBCs, mounts immune responses.

White pulp

20 % of splenic parenchyma; lymphoid nodules and periarteriolar lymphoid sheaths surrounding central arterioles.

Red pulp

80 % of spleen; blood-filled sinusoids and splenic cords where RBC removal occurs.

Periarteriolar lymphoid sheath (PALS)

T-cell rich sleeve of white pulp surrounding splenic central arterioles.

Splenic cords (of Billroth)

Reticular tissue in red pulp containing macrophages, RBCs, and immune cells between sinusoids.

Splenic sinusoids

Wide