Organization (levels) of immunocytes Diffuse cells Follicle organ Patch

PERIPHERAL LYMPHOID ORGANS Sites of lymphocyte activation and differentiation Spleen Lymph nodes Epithelial cell – associated lymphoid tissues Skin-associated lymphoid tissue (SALT) Mucosa-associated lymphoid tissue (MALT) Gut-associated lymphoid tissue (GALT) Bronchial tract-associated lymphoid tissue (BALT)

Lymph nodes are communication centers within the immune system They form a unified, interconnected system via circulation of blood and lymph

Fluid from tissues is collected by venules and lymphatic capillaries

Cortex Trabecula Follicles Medullary cords Medullary sinuses Capsule

5. Medullary cords (Macrophage & plasma cell area) Medullary sinus 6. Efferent lymphatic vessel Artery Vein 4. Germinal centre (site of intense B cell proliferation) 3. Secondary lymphoid follicle Paracortical (T cell) area 2. Primary Lymphoid follicle (B cell area) 1. Afferent lymphatic vessel. Lymph, Ag, & cells with captured Ag drained from tissues enters here Lymph node

Collagen capsule Marginal sinus Paracortex Cortex Trabecula High endothelial venule (HEV) Afferent lymph Efferent lymph Primary follicle B CELLS Secondory follicle Germinal Center (GC) B CELLS T CELLS STRUCTURE OF LYMPH NODES Plasma cell Memory B cell Mature,naive B cell

Subcapsular sinus Valve Capsule

LymphaticArtery and vein Blood & Lymphatic vessels

Spleen

STRUCTURE OF THE SPLEEN NO LYMPHOID CIRCULATION Filtration of blood borne antigens

Red pulp Marginal zone Marginal sinus B cell corona Germinal centre Periarteriolar lymphocytic sheath (PALS) – T cell area Central arteriole Spleen white pulp Transverse section

MALT  Most pathogens attack mucosal epithelia  Covered by mucus containing glycoproteins, proteoglycans, enzymes  Antimicrobial peptides protect against physical damage and infection  Thin dynamic layers covering large areas  Well protected by the immune system  More lymphocytes are assigned to the mucosal surfaces than to the rest of the body

Follicle associated endothelium

NAIVE T CELLS REACH LYMPH NODES THROUGH EFFERENT LYMPHATICS AND BLOOD CIRCULATION Vér Nyirok

Naive B lymphocytes enter lymph nodes via HEV B cells are reqruited to HEV from the blood by CCL21 chemokine secreted by stromal cells CCL21 and CCL19 chemokines attract B lymphocytes to the lymph node HOMING OF B LYMPHOCYTES IN LYMPH NODES

SULPHATED L-SELECTIN LIGANDS

Gut-associated lymphoid tissues Peyer’s patches follicles forming a dome structure M-cells: microfold cells --- no glycocalyx – antigen uptake

Transport of antigens via M cells

Dendritic cells of the lamina propria outside Peyer’s patches capture antigens by samlpeing the gut lumen directly

Peyer’s patches

Dome area Villi GC

Intraepithelial Lymphocytes Large number of Intraepithelial Lymphocytes are present in gut epthelia