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بسم الله الرحمن بسم الله الرحمن الرحيمالرحيم
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 cortex Thymus 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– 100-200 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: arteriole b: capillary
c: post - capillary venule d: muscular venule
Lymph node Lymphatic vessel, lymph node
Lymph node reticular stainCortex of lymph node withlymphoid nodule
Lymph node medulla Lymph node medulla withsinusoid 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 andwhite 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 tonsilPharyngeal tonsil
Peyer’s Patches
• Lymphoid nodules in the lamina propria of the ileum (covered in detail in the digestive tract section)