Immune/Lymphatics II Flashcards
Capsule of thymus
- blood vessels
- efferent lymphatics present
- afferent lymphatics not present (therefore lymph does not circulate through thymus)
- extends trabeculae (septa) into parenchyma
Trabeculae (septa) of thymus
- delicate connective tissue
- divide thymus into incomplete lobules
Lobules of thymus
each composed of:
- outer (dark staining w/ HE) cortex populated by epithelial reticular cells that secrete thymosin, T cells
and an inner (lighter staining) medulla that is specialized to allow entry channel into blood stream of mature lymphocytes
also has Hassall’s corpuscles (highly keratinized) that produce cytokine thymic stromal lymphopoeitin that stims thymic dendritic cells needed for maturation of single positive T cells
Double negative T cells
- lack cell surface molecules typical of mature T cells
- enter cortex from blood vessels
- proliferate in subcapsular space
Double positive T cells
- move to outer cortex
- confronted with epithelial cells with cell surface MHC I and II for clonal selection
- express both CD4 and CD8 coreceptors and TCR receptors
Single positive T cells
- move to inner cortex
- express TCR receptors and either CD4 or CD8 coreceptors
Blood-thymus barrier
- located in thymic cortex
- prevents antigens in the blood from reaching developing T cells in thymic cortex
- leaky during fetal life to allow for development of immunologic tolerance to self-antigen
Morphology of spleen
- no lymph sinuses
- no afferent lymph vessels
- covered by peritoneum except at hilus
- mesothelium-lined connective tissue capsule contains smooth muscle fibers and sends trabeculae into parenchyma
- blood vessels enter and leave at hilus
- divided into red pulp and white pulp
Blood filtering functions of spleen
- only lymphatic organ specialized to filter blood
- stores and removes worn out RBCs
- recylcles iron
- converts hemoglobin to bilirubin
- blood formation in fetus
Immunologic functions of spleen
- screens foreign material in blood
- produces lymphocytes and plasma cells
- removal leads to overwhelming bacterial infections in infants, children, and young adults
White pulp
- elongated, branched strands always assoc w/arteries
- zones of diffuse lymph tissue and germinal center
- site of clonal expansion of antigen-stimmed lymphocytes
- B cell area contains only 2* follicles w/off center arteriole
- T cells found in areas surrounding central artery near center of white pulp (forms the periarterial lymphatic sheath PALS)
- reticular fibers assoc w/fixed macrophages and support splenic pulp
Marginal zone
- forms sinusoidal interface between red and white pulp
- has abundance of antigen-presenting cells
- lymphocytes first encounter antigens here
- activated T-helper cells activate B cells here
Red pulp
- surrounds white pulp (composes 80% of spleen)
- functions to filter blood
- contains large # of RBCs
- billroth cords form red pulp parenchyma; contains blood cells, plasma cells, and antigen presenting cells; terminal caps open directly into substance of cords (open circulation); macrophages destroy worn out RBCs
- endothelial-lined venous sinusoids w/discontinuous basement membrane is storage site for healthy RBCs
Arterial vascularization of spleen
- splenic artery enters hilus
- trabecular arteries branch off
- central arteries: adventitia loosens and becomes mesh-like reticulum infiltrated w/lymphocytes (enlarged areas are splenic nodules)
- after caps form (supplying white pulp), central arteries lose white pulp investment and enter red pulp to form a penicillus
- terminal cap drains into intercellular spaces (open system) or venous sinuses (closed system) lined with reticuloendothelial cells
Venous vascularization of spleen
- venous sinuses lined with reticuloendothelial cells
- drains into pulp veins which unite w/trabecular veins forming splenic vein that exits at hilus
