Dr. Jack L. Haar Department of Anatomy and Neurobiology Lymphoid System I & II Dr. Jack L. Haar Department of Anatomy and Neurobiology

Overview of Immune System Functions Provides immune surveillance and defense Provides immune tolerance (distinguishes self from non-self) Absorbs lipids Maintains fluid balance by returning tissue fluid and lymphocytes to the blood

Overview continued Concept of immunity Immune system provides a way to recognize “self” from “non-self” An individual mounts an immune response to foreign material If person survives an attack by foreign material (infection), immunity may result, meaning the foreign material is “remembered” Thus immunity has specificity and memory

Overview continued Terminology Antigen: a foreign (non-self) substance. Could be bacterial, tumor cells, transplanted cells or virus-infected cells Antibody: circulating protein in blood plasma (immunoglobulin) that interacts with a specific antigen Humoral immunity: antibodies against antigen circulating in blood stream; produced by plasma cells derived from B lymphocytes Cellular (cell-mediated) immunity: Immunocompetent cells contact, react against, and destroy antigen; mediated by T lymphocytes

Major lymphoid organs and tissues

Cells of the Immune response 20 – 50% WBC’s are lymphocytes

A. 35% of circulating lymphocytes are B lymphocytes or B cells A. 35% of circulating lymphocytes are B lymphocytes or B cells Become Plasma cells or B memory cells B. 65% of circulating lymphocytes are T lymphocytes or T cells Become Helper, Killer, or T memory cells

Immunoblasts Form from T or B lymphocytes when stimulated by Antigen Antigen -presenting cells Monocyte-macrophage derived. Process antigen to present to lymphocytes Includes macrophages, epidermal Langerhans cells, dendritic cells of lymphoid organs, and epithelial cells of thymus

Lines of Defense

IV. Thymus Located in mediastinum

Thymus undergoes involution with age Is influenced by adrenal cortical steroids and radiation Thymus is a bilobed organ

Stroma formed from thymic epithelial cells with bundles of tonofibrils and linked by desmosomes Thymocytes develop from HSC from yolk sac (mesoblastic), fetal liver (hepatic), and bone marrow (myeloid) phases of development Thymus is not exposed to external environment

Histology Dense irregular CT capsule Septa form lobules Cortex and medulla Stroma formed from thymic epithelial cells

Cortex Thymic epithelial cells Large numbers of thymocytes (lymphocytes) Macrophages

Medulla Thymocytes are larger and fewer in number than in cortex Only 5% of thymocytes are in the medulla Hassall’s corpuscles form from the stromal cells Keratohyaline granules

Vessels enter and leave through the capsule and follow the septa Blood thymus barrier

Function of the thymus Thymus is seeded with HSC’s Thymocytes proliferate in cortex Self-reactive thymocytes are eliminated Non-self reactive cells migrate to medulla Cells enter blood stream and migrate to secondary lymphoid organs They occupy T-dependant areas of lymphoid organs where they nest, divide, and mature The thymus is an endocrine organ and responds to many hormones, viz. ACTH, GH, and sex hormones Thymectomy at birth impairs immune function

Bursa of Fabricius Functions in birds as primary lymphoid organ giving rise to B-lymphocytes

Bone marrow Functions in mammals as primary lymphoid organ giving rise to B-lymphocytes

Lines of Defense

Outer Line of Defense - MALT

MALT GI tract Respiratory tract Genito-urinary tract Diffuse lymphatic tissue Lamina propria Reticular fibers Lymphocytes Primary lymphatic nodule

Secondary Lymphatic nodule Germinal center B-dependant spherical area Cap of tightly packed small lymphocytes Not encapsulated by C.T.

Germinal center Oval pale-staining area Contains dividing cells and macrophages

Solitary nodules Not encapsulated by C.T. May be primary or secondary

High endothelial venules Associated with diffuse lymphatic tissue Allow lymphocytes to escape from vessel

Locations of aggregates of nodules Peyer’s patches Typhoid nodules Appendix

Typhoid Fever - a bacterial illness caused by Salmonella typhi

Appendix

Tonsils 1. Palatine 2. Lingual 3. Pharyngeal

Dr. Jack L. Haar Department of Anatomy and Neurobiology Lymphoid System II Dr. Jack L. Haar Department of Anatomy and Neurobiology

Lines of Defense

Inner Line of Defense

Lymphatic Vessels Lymphatic capillaries Lacteals for lipid Valves direct flow Asymmetrical system Edema

Lymphatic filariasis, also known as elephantiasis, is best known from dramatic photos of people with grossly enlarged or swollen arms and legs. The disease is caused by parasitic worms, including Wuchereria bancrofti, Brugia malayi, and B. timori, all transmitted by mosquitoes. Lymphatic filariasis currently affects 120 million people worldwide, and 40 million of these people have serious disease.

Lymph node Histology Distributed along vessels Filter lymph Produce lymphocytes Dense CT capsule Afferent lymphatics on convex surface Reticular fiber framework Subcapsular sinus Lymphoid nodule in cortex

Hilus Blood vessels enter and leave Efferent lymphatics leave node

Cortex of lymph node Lymphoid nodules and diffuse lymphatic tissue Lymphatic sinuses Subcapsular Cortical

Medulla of lymph node Medullary cords Medullary sinuses

Blood vessels follow trabeculae into node Blood vessels follow trabeculae into node HEV with cuboidal endothelium Lymphocytes pass from blood into lymph node

B and T-dependant areas Outer cortex and medullary cords are B-dependant Inner cortex (paracortical area) is T-dependant

Cells found in a lymph node Lymphocytes of all sizes Plasma cells Macrophages (dendritic or reticular cells Antigen presenting cells

Functional significance of Lymphatic Tissue Lymph flowing through lymph vessels is filtered by at least one lymph node Macrophages in lymph node remove 99% of antigen in lymph 1% of antigen passes through a lymphoid nodule The antigen stimulates formation of germinal centers Long-lived lymphocytes present in blood re-enter lymph nodes via HEV to stimulate immune response Tumor cells may be trapped in lymph nodes

Lines of Defense

Third Line of Defense

Spleen Filters blood Largest lymphoid organ Non-vital organ Dense CT capsule and trabeculae Covered by mesothelium Reticular fiber framework

White pulp Periarterial lymphoid sheath (PALS): T-dependant Germinal centers: B-dependant Marginal zone Red pulp Splenic (Billroth) cords Venous sinusoids, arterioles, venules

Central artery (arteriole) with PALS and Germinal center

Central artery (arteriole) with PALS and Germinal center

Red pulp: Splenic cords and venous sinusoids

Circulation of blood through the spleen Splenic artery Trabecular arteries Central artery (arteriole) Pulp artery Penicillar arteriole Venous sinuses Veins Trabecular veins Splenic vein

Functions of the Spleen Graveyard of RBC’s Filters blood to remove bacteria, viruses and inert particles Blood cell production Lymphocytes in the white pulp Plasma cells and monocytes in the red pulp Erythrocytes in fetus and with certain infections