Immune System Anatomy and Organization

Vocabulary flashcards covering the structural hierarchy, primary and secondary lymphoid organs, histology, leukocyte differentiation, and functional pathways of the immune system.

Immune System

A collection of barriers, cells, and soluble proteins that interact and communicate with each other via complex pathways to defend the body against infection.

Pneumocystis jirovecii

An opportunistic fungal pathogen (formerly called Pneumocystis carinii) normally found in healthy human lungs that causes severe pneumonia in individuals with suppressed immune systems, notably recognized in June 1981 at the onset of the AIDS crisis.

Barrier Defenses

The instantaneous first temporal phase of immune defense, composed of physical and chemical barriers such as the skin and mucous membranes that prevent pathogen entry.

Innate Immunity

A rapid, non-specific general resistance system that acts as the first active line of cellular defense, lacks immunological memory, and utilizes skin, mucous membranes, epithelial cells, and phagocytes.

Adaptive Immunity

A slower, pathogen-specific line of defense mediated by T lymphocytes and B lymphocytes that develops following antigen exposure, involves cell-mediated and humoral mechanisms, and forms memory cells.

Hematopoiesis

The continuous process throughout adulthood by which blood and immune cells develop and differentiate in the bone marrow from hematopoietic stem cells (HSCs) to replenish aging or consumed cells.

Hematopoietic Stem Cells (HSCs)

Multipotent stem cells residing in the bone marrow that give rise to both the myeloid and lymphoid lineages of blood cells.

Common Myeloid Progenitor (CMP)

A bone marrow stem cell descendant that differentiates into erythrocytes, megakaryocytes (platelets), granulocytes (neutrophils, basophils, eosinophils), and monocytes/macrophages.

Common Lymphoid Progenitor (CLP)

A bone marrow progenitor cell that differentiates into natural killer (NK) cells, T lymphocytes, and B lymphocytes.

Lymphocytes

The second most common type of white blood cell ($20\text{--}50\%$ of leukocytes) and the smallest in size, characterized histologically by a round, densely stained nucleus and a thin margin of pale, non-granular cytoplasm.

Cytotoxic T-lymphocytes (CTLs)

Effector T cells (killer T lymphocytes) responsible for cell-mediated immunity that directly recognize cells presenting foreign antigens and destroy them.

Helper T-lymphocytes ($T_H$ cells)

Regulatory T lymphocytes that secrete soluble factors (cytokines) to stimulate and activate B lymphocytes, cytotoxic T lymphocytes, and macrophages.

Suppressor T cells ($T_S$ cells)

A subtype of T lymphocytes that dampens or shuts down immune responses and inhibits the activity of B lymphocytes.

B-lymphocytes

Humoral immune cells that develop and mature in the bone marrow, possess variable lifespans, and differentiate into plasma cells that secrete circulating antibodies.

Natural Killer (NK) Cells

Large granular null lymphocytes of the innate immune system that attack cancer cells, foreign cells, and virus-infected cells without prior sensitization.

Primary Lymphoid Organs

The anatomical sites—specifically the red bone marrow and thymus gland—where lymphocytes proliferate and reach immunologic maturity.

Red Bone Marrow

Active hematopoietic tissue situated inside bones where blood cell lineages are generated and where B lymphocytes undergo maturation.

Yellow Bone Marrow

Quiescent marrow tissue dominated by adipocytes that serves as an energy reserve, replacing substantial portions of red bone marrow in long bones during adulthood.

Thymocyte

An immature T lymphocyte that leaves the red bone marrow and migrates to the thymus to undergo differentiation and selection.

Thymus Gland

A bilobed primary lymphoid organ encapsulated by connective tissue and situated in the anterior thoracic cavity between the sternum and the aorta of the heart.

Thymic Cortex

The densely packed outer zone of the thymus, staining darker due to dense concentrations of developing small T lymphocytes alongside epithelial reticular cells, macrophages, and the blood–thymus barrier capillaries.

Thymic Medulla

The pale, lightly staining inner region of the thymus that contains larger T lymphocytes, epithelial reticular cells, and concentrically laminated Hassall's corpuscles.

Hassall's Corpuscles

Isolated, concentric, eosinophilic whorls of closely packed epithelioreticular cells with flattened nuclei, found characteristically within the thymic medulla.

Thymic Involution

The age-dependent shrinkage and replacement of functional thymic parenchyma by adipose tissue, causing a marked decrease in the de novo output of naive T lymphocytes.

Secondary Lymphoid Organs

Tissues and organs—including lymph nodes, the spleen, and lymphoid nodules—where mature, naive lymphocytes encounter foreign antigens and initiate adaptive immune responses.

Lymph

Extracellular fluid that escapes from blood capillaries into tissue spaces and is collected by lymphatic vessels for unidirectional transport back into the venous system toward the heart.

Right Lymphatic Duct

The lymphatic duct that drains lymph exclusively from the upper right quadrant of the body: the right side of the head and neck, right arm, and right thorax.

Thoracic Duct

The main lymphatic trunk (left lymphatic duct) that drains lymph from the left side of the head and neck, left arm, left thorax, entire abdomen, and both lower limbs.

Afferent Lymphatic Vessel

A vessel that conducts lymph from peripheral tissues through the capsule into the subscapular sinus of a lymph node.

Efferent Lymphatic Vessel

A vessel located at the hilum through which filtered lymph and activated lymphocytes leave a lymph node.

Germinal Center

The pale-staining central zone of a lymphoid follicle where activated B lymphocytes rapidly proliferate and differentiate into plasma cells.

Spleen

The largest secondary lymphoid organ, located in the left hypochondrium posterior-lateral to the stomach, responsible for recycling red blood cells and mounting immune responses against blood-borne pathogens.

Splenic Red Pulp

The vascular compartment of the spleen containing splenic cords and venous sinuses lined by macrophages that phagocytose aged erythrocytes and filter particulate matter from the blood.

Splenic White Pulp

The lymphoid tissue of the spleen, composed of central arterioles enclosed by T-cell-rich periarteriolar lymphatic sheaths (PALS) and attached B-cell follicles with germinal centers.

Periarteriolar Lymphatic Sheath (PALS)

A sleeve-like cylindrical arrangement of mature T lymphocytes enclosing the central arterioles within the splenic white pulp.

Erythrocyte Recycling

The breakdown of aged red blood cells ($\approx 120\text{ days}$ lifespan) by macrophages in the spleen, liver, and bone marrow into iron, amino acids from globin, and bilirubin.

Splenic Marginal Zone

The histological transition boundary between the white pulp and red pulp of the spleen that traps antigen and facilitates lymphocyte transit.

Tonsils

Aggregations of unencapsulated lymphoid tissue situated in the pharyngeal mucosa, comprising the pharyngeal, palatine, and lingual tonsils.

Mucosa-Associated Lymphoid Tissue (MALT)

Diffuse and aggregated unencapsulated lymphoid tissue embedded in the mucosa of mucosal tracts exposed to external antigens, prominently including the gastrointestinal tract.

Peyer's Patches

Specialized follicular aggregates of mucosa-associated lymphoid tissue situated in the lamina propria and submucosa of the ileum that sample and defend against ingested antigens.

Bronchus-Associated Lymphoid Tissue (BALT)

Unencapsulated lymphoid nodule aggregates situated under the epithelium at branch points of bronchi and between bronchi and pulmonary arteries, providing mucosal defense against inhaled antigens.

Appendix

A tubular cecal projection containing dense collections of submucosal lymphatic nodules, lined by simple columnar epithelium and goblet cells without intestinal villi.

Antigen-Presenting Cells (APCs)

Specialized immune cells, such as dendritic cells, that engulf pathogens at epithelial barriers, process their antigens, and migrate via lymphatics to regional lymph nodes to activate naive T cells.