THE LYMPHOID SYSTEM Kristina C. Erasmo, M.D.

Defense System of the Body Non-Immune Immune

Non-Immune Defense System INFLAMMATION Mediated by: Neutrophils Eosinophils Basophils NK cells Monocytes and macrophages

Immune Defense System Lymphocytes – principal effector cells

Lymphoid System Refers to the tissues and organs that participate in the immune defense system Components are distributed in various areas of the body

LYMPHOID SYSTEM Non-Encapsulated Lymphoid Tissue Encapsulated Lymphoid Organs Tonsils

LYMPHOID SYSTEM Non-Encapsulated Lymphoid Tissue Diffuse lymphoid tissue Lymphoid nodules (lymphoid follicles) Encapsulated Lymphoid Organs Lymph Node Thymus Spleen

LYMPHOID SYSTEM Tonsils Palatine Pharyngeal Lingual Tubal

LYMPHOID SYSTEM Non-Encapsulated Lymphoid Tissue Diffuse lymphoid tissue Lymphoid nodules (lymphoid follicles)

Diffuse Lymphoid Tissue Histologic features: Stroma composed of network of reticular fibers and reticular cells Spaces occupied primarily by lymphocytes Some macrophages found

Diffuse Lymphoid Tissue Location: Practically in all CT in the body Most prominent in lamina propria and submucosa of GIT, respiratory tract

Diffuse Lymphoid Tissue (Stomach)

Diffuse Lymphoid Tissue Types: Loose lymphoid tissue – lymphocytes far apart Dense lymphoid tissue – lymphocytes tightly packed

Lymphoid Nodule LYMPHOID FOLLICLE Lymphoid tissue where clustered lymphocytes form discrete masses or lumps Usually interspersed in areas of diffuse lymphoid tissue

Lymphoid Nodule Location: Lamina propria of GIT and respiratory tracts Spleen Lymph nodes Tonsils

Lymphoid Nodule Types based on arrangement: Solitary nodule – occur singly Aggregates – Peyer’s patches (submucosa of the ileum)

Peyer’s Patch

Peyer’s Patches

Lymphoid Nodule Types based on structure: Primary nodule – cells evenly distributed throughout nodule, no mitotic figures seen Secondary nodule (germinal centers) – 2 distinct regions

Lymphoid Nodule Secondary nodule Germinal center (reaction center) – pale, central Forms the core of the nodule B lymphocytes proliferate and differentiate following exposure to antigen Many mitotic figures Very few T lymphocytes Corona – darker, peripheral

Secondary Lymphoid Follicle

Secondary Lymphoid Follicle (Lymph Node)

LYMPHOID SYSTEM Encapsulated Lymphoid Organs Lymph Node Thymus Spleen

Lymph Node Encapsulated, bean-shaped organ Location: Popliteal, inguinal, axillary regions Sides of neck Along abdominal vessels In mesentery

Lymph Node

Lymph Node Hilus – the indented area of the lymph node, where blood vessels enter and leave the organ Efferent lymphatic vessels leave the lymph node on the hilar area Afferent lymphatic vessels enter the lymph node on the convex side

Lymph Node

Histologic Features of Lymph Node Capsule – dense connective tissue covering Trabeculae – incompletely subdivide the organ into compartments Reticular cells and fibers – supporting meshwork

Lymph Node

Histologic Features of Lymph Node Parenchyma: Cortex – outer Outer portion – composed of lymphoid nodules (primary and secondary), mostly B lymphocytes Inner – dense lymphoid tissue, no lymphoid nodules, mostly T lymphocytes Medulla – inner, paler-staining, contain medullary cords

Lymph Node

Lymph Node

Spleen Largest lymphoid organ (7 x 12 cm) Found in the upper left quadrant of the abdominal cavity Hilus – where splenic vessels enter and leave the spleen

Spleen Functions: Contain macrophages – destroy foreign substances, microorganisms, abnormal cells in blood Removes and destroys old RBCs and platelets from circulating blood Recycles iron contain in the RBCs

Spleen Functions: Storage area for blood Where lymphocytes proliferate and differentiate into different types after being stimulated

Histologic Features of Spleen Enveloped by peritoneum that blends with its capsule (mesothelium) Trabeculae – divide into compartments Reticular cells and fibers – supporting framework

Spleen

Histologic Features of Spleen Splenic pulp – parenchyma of the spleen White pulp Red pulp Marginal zone

Histologic Features of Spleen White pulp Consists of lymphoid nodules embedded in dense lymphoid tissue Lymphoid nodules – B lymphocytes Dense lymphoid tissue – T lymphocytes

Histologic Features of Spleen Red pulp Bulk of the parenchyma Consists of large blood-filled sinusoids with strands of lymphoid tissue in-between (Billroth’s cords, splenic cords)

Histologic Features of Spleen Marginal zone Poorly-delineated transitional area between red and white pulp Contains lymphocytes and plasma cells

Spleen

Red Pulp (Spleen)

Thymus Located in the superior mediastinum Composed of two pyramidal lobes fused together

Thymus Where T cell stem cells proliferate, differentiate, and transform into immunologically competent T lymphocytes Devoid of B lymphocytes

Histologic Features of Thymus Capsule – thin loose connective tissue Trabeculae Reticular cells – supporting meshwork

Histologic Features of Thymus Cortex – peripheral, darker-staining region Medulla – central, lighter-staining region Hassall’s corpuscles (Hassall’s bodies, thymic corpuscles) Composed of hyaline core surrounded by layers of flattened epithelioid cells

Thymus

Thymus

Tonsils Lymphoid organs that form a ring (Waldeyer’s ring) underneath the epithelium around the entrance to the respiratory and digestive passages

Tonsils Histologic feature: Dense lymphoid tissue embedded with lymphoid nodules Types: Palatine, lingual, pharyngeal, tubal tonsils

Palatine Tonsils Located in the lateral aspect o the oropharynx (one on each side) Covered by stratified squamous non-keratinized epithelium Tonsillar crypts – deep invaginations of the epithelium (contain dead epithelial cells, lymphocytes, etc)

Lingual Tonsils Several discrete masses of lymphoid tissue located in the dorsum of the tongue Covered by stratified squamous non-keratinized epithelium May have single broad deep crypt into which ducts of mucus-secreting glands open

Pharyngeal Tonsils Central area of the posterior and superior nasopharyngeal wall Covered by ciliated pseudostratified columnar epithelium (“respiratory epithelium”) Some areas covered with stratified squamous non-keratinized epithelium Does not form crypts but form shallow folds Adenoids – enlarged pharyngeal tonsils

Tubal Tonsils Masses of lymphoid tissue in the nasopharynx near the openings of the eustachian tube Some believe they are simple extensions of the pharyngeal tonsils Covered by respiratory epithelium