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Cells of the immune system -monocytes/macrophages -granulocytes/polymorphonuclear cells - (PMN) -mast cells -interdigitating dendritic cells (DC) -follicular dendritic cells -Lymphocytes NK cells B cells CD4+ and CD8+ T cells

Lymphoid System Basics Cells, tissues and organs that function to protect body from invasion and damage by foreign cells, microbes, viruses and parasites Also known as the immune system Two types of lymphoid tissues: –Encapsulated: connective tissue capsule spleen, thymus, lymph nodes –Unencapsulated (or partly encapsulated) tonsils, Peyer’s patches, lymphoid nodules in GI tract, respiratory tract, urinary & reproductive tracts

2 Types of Lymphoid Organs Central lymphoid organ: where lymphoid precursor cells undergo antigen dependent proliferation and differentiation –T cells in thymus –B cells in bone marrow Peripheral lymphoid organ: where functional lymphocytes go including lymph nodes, spleen, Peyer’s patches, lymphoid nodules of GI and other tracts

Functional categories Primary lymphoid organs: antigen independent; isolated from outside “world” –Bone marrow –Thymus –Bursa of Fabricius (birds) Secondary lymphoid organs: antigen dependent; exposed to outside “world” –MALT (BALT, GALT) –Lymph nodes –Spleen

Peripheral Lymphoid Tissues Lymphocytes contact antigens and divide and differentiate into B cells and T cells Memory cells form that circulate for years to provide extended immunity

Thymus 1 Central lymphoid organ Thin capsule, lobular organization Each lobule has cortex (greater cell density) with many T lymphocytes surrounding lighter medulla Epithelial reticular cells Hassal’s corpuscles (flattened epithelial reticular cells)

Thymus: cortex and medulla

Thymus cortexThymus medulla

Thymus 2 Cortex: many lymphocytes, macrophages, epithelial reticular cells Medulla: more epithelial reticular cells and fewer lymphocytes –mature T lymphocytes leave from here to go to spleen and lymph nodes –Hassal’s corpuscles: concentric layers of epithelial reticular cells, core degenerated; function/significance unknown After puberty thymus undergoes involution and increases in connective tissue and adipocytes

Lymphatic vessels Resemble veins (same 3 layers) Found throughout body except: –Avascular tissues –Central nervous system –Splenic pulp –Bone marrow

Lymph Nodes Number, location and size – clustered primarily in neck, thorax, abdomen and pelvis; few in the extremities –Absent from CNS –Usually not located within other organs –Size of watermelon seed to one-third that size Difficult to palpate

Lymph Nodes Throughout body along lymph vessels Numerous in axilla, groin, cervical area and thoracic/abdominal mesenteries Filter lymph before it returns to vasculature Hilum, concave side, arteries, nerves enter and veins leave Afferent lymph vessels enter convex surface Efferent lymph vessels exit hilum

Lymph Nodes Capsule of dense irregular connective tissue with incomplete septa Reticular fiber network Cortex: subcapsular sinus, peritrabecular sinuses, several primary and secondary (have germinal centers) lymphoid nodules, venules may have thick endothelium

Lymph Nodes Lymphoid nodule germinal centers have mitotic lymphocytes with surrounding B cells Outside nodules is paracortical zone where there are many T cells; endothelium may be thickened Medulla has sinuses which join to form efferent vessels

Lymph Node

Blood Flow Legend: a: arterioleb: capillary c: post - capillary venuled: muscular venule

Lymph node Lymphatic vessel, lymph node

Lymph node reticular stain Cortex of lymph node with lymphoid nodule

Lymph node medulla Lymph node medulla with sinusoid and medullary cords

Spleen 1 Largest lymphatic organ Many macrophages; rbc phagocytosis Capsule of dense irregular connective tissue w/ trabeculae dividing pulp incompletely White pulp with lymphoid nodules Red pulp found between sinusoids has reticular fibers, reticular epithelial cells and macrophages

White Pulp Central arteries with encircling lymphoid tissue T cells form periarterial lymphatic sheaths (PALS) around small arteries Nodules are mostly B cells Reticular epithelial cells & macrophages

Red Pulp Reticular cells with cords of cells between sinuses Cords have macrophages, monocytes, lymphocytes, plasma cells, rbc, granulocytes Sinuses have irregular lumen, incomplete endothelium and basal lamina

Spleen with red pulp and white pulp Spleen red pulp

Spleen white pulp with surrounding red pulp

Functions of Spleen Lymphocyte production in white pulp RBC phagocytosis in red pulp T and B cells involved in immune response Blood storage; small amount in humans

MALT - I Diffuse and solitary lymphoid nodules: a portion of GALT and all of BALT Unencapsulated lymphoid tissue (multiple nodules) –Peyer’s patches: (a portion of GALT) M (Microfold) cells: epithelial cells which transport antigen –Appendix (a portion of GALT)

MALT - II Partially encapsulated lymphoid tissue (multiple nodules) – Tonsils: palatine, lingual and pharyngeal (tonsils are a portion of GALT). –GALT is, therefore, any gut-associated lymphoid tissue whether it takes the form of diffuse LT, solitary lymphoid nodules, Peyer’s patches, the appendix or the tonsils. –GALT + BALT = MALT

Unencapsulated or Incompletely Encapsulated Lymphoid Tissue Lymphoid nodules Tonsils: palatine, pharyngeal, lingual Peyer’s patches

Lymphoid Nodules Nodules of densely packed lymphocytes located in digestive tract, respiratory tract, urinary tract, and reproductive tract Most lymphocytes are B cells

Tonsils Incompletely encapsulated lymphoid nodules Palatine: covered by stratified squamous nonkeratinized epithelium; crypts; underlying connective tissue barrier Pharyngeal: covered by ciliated pseudostratified epithelium, no crypts Lingual: smaller, at base of tongue; covered by stratified squamous nonkeratinized epithelium; one crypt in each nodule

Palatine tonsil Pharyngeal tonsil

Peyer’s Patches Lymphoid nodules in the lamina propria of the ileum (covered in detail in the digestive tract section)