LYMPHOID SYSTEM Lecture 1 Lecturer Dr. Firdous M. Jaafar Department of Anatomy/Histology section

objectives

• 1- Define lymphoid system, and describe its forms. • 2- Classify its types and list its organs. • 3- Determine the role of lymphoid system in immune

process, and tissue graft. • 4- Define lymphatic nodule. • 5- Study the thymus as a central lymphoid organ, and: • a- Describe the structure of thymus. • b-Define blood-thymus barrier, and state its role in

immunity. • c- List the types of cells in the thymus.

Distribution of lymphoid organs

Lymphoid System • It consists of organs, whose tissues and cells play an

important role in immunity

• Lymphoid tissue is found either in a form of: • Individual structure(reticular tissue),as in lymph

nodes and spleen, which is composed of reticular fibers and fixed cells(reticular cells and macrophages)

• Free cells, as lymphocytes,granulocytes, and mononuclear phagocytic system.

• Antigen-presenting cells, which are found, in addition to lymphoid tissue, in skin.

Reticular tissue

• There are two types of lymphoid organs:

• Central lymphoid organs: as the thymus and bone marrow.

• Peripheral lymphoid organs: as the spleen, lymph node, tonsil, Peyer’s patches, and lymphatic nodules in the wall of digestive,urinary,respiratory, and reproductive systems.

• Tissue graft: • Autograft

• Isograft • Homograft

• heterograft

Lymphatic nodule:

• Small collections of densely-packed lymphocytes, that appear strongly stained by haematoxylin in histological section, due to their basophilic nuclei.

• There is certain lymphoid tissue present in association with body organs:

• MALT(Mucosa Associated Lymphoid Tissue)

• GALT(Gut = = = ); in tonsils, esophagus, Peyers patches, and lymphatic nodules of appendix.

• BALT(Bronchus Associated Lymphoid Tissue); in the sub mucosa of bronchus.

Thymus

Embryonic origin of thymus

Epithelial reticular cells(arrow head)

blood-thymic barrier. • Blood capillary wall, which is of the continuous

type, with no fenestrations, and a very thick basal lamina. This will prevent circulating antigens from reaching T lymphocytes in the cortex.

• Cytoplasm of endothelial cell. • Small amount of connective tissue, with some

macrophages. • Basal lamina of epithelial reticular cell. • Cytoplasm of = = = .

Cortex(dark) and Medulla(light) of thymus

Hassall corpuscle(red arrow)

Electron microscopical picture of Hassall corpuscle