Anatomy of the Immune System

Gross Anatomy

Surface Barriers  Skin and mucous membranes  Sebaceous glands secrete fatty acids – antibacterial/antifungal  Respiratory tract  Mucous traps bacteria, cilia allows expulsion  Cough reflex  Hairs in the nose  GIT  Stomach acid  Salivary enzymes  Enzymes and macrophages of small intestine  Vagina – low pH due to lactobacilli

Bone Marrow  Active: Red bone marrow  Site of hematopoiesis – 100% in healthy adults  Children: liver and spleen  Bone marrow destruction, fibrosis  B cell development

Thymus  T cell maturation (thymocytes)  Bilobed, lobulated  Asymmetrical – larger right  Fibrous septa (lobules)  Larger in children  Superior mediastinum  Posterior to sternum, anterior to pericardium and heart  Internal thoracic artery  Cortex atrophies during adolescence

 Cortex  Thymocytes  Lymphocytes at various stages of development  Epithelioreticular cells  Medulla  Fewer cells  Mature thymocytes  Hassal corpuscles  Epithelioreticular cells  Outer capsule

Spleen  Blood-borne antigens  Filtration  Left Hypochondrium, posterior to ribs 9-11  Splenic artery + left gastroepiploic  Splenic vein – drains to portal  Prenatal – hematopoiesis  Thin fibroelastic capsule covered by thin layer of preitoneum  Except at hilum  Expand

White Pulp  Surrounds arterioles – lymphocytes + mononuclear cells  Central artery – runs longitudinally through PALS filtration bed  Surrounded by periarteriolar lymphoid sheath (PALS) – T Lymphocytes  Outer margins – lymphoid follicles – activated B cells, dendritic cells, macrophages – germinal centre – maturation to secondary follicles  Loss of plasma

Marginal Zone  Separates the white pulp from red pulp  Finely meshed filtration bed  Contain memory B cells, T Helper cells  Removes damaged red cells and parasites

Red Pulp  Meshwork – splenic cords of Biliroth  Splenic sinuses  Erythrocytes, macrophages, dendritic cells  Blood flows from aterioles through filtration bed into sinuses  RBCs must pass through slits – rigidity  Pass macrophages + dendritic cells  Ingest erythrocytes, store platelets, remove infectious agents (plasmodium)  Antigen presenting cells  Stasis and congestion - splenomegaly

Lymph Nodes  Tissue antigens

 Capsule – thin connective tissue  Cortex – B Cells  Germinal centres – mature into secondary follicles  Paracortex – T-cells  Medullary cords – Plasma cells  Lymph drains through sinuses