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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

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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)