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ImmunologyChapter 3, Lecture 4
• Richard L. Myers, Ph.D.
• Department of Biology
• Southwest Missouri State
• Temple Hall 227
• Telephone: 417-836-5307
• Email: [email protected]
Organs of the Immune System• Divided on the basis of function
– primary lymphoid organs– secondary lymphoid organs
• Primary (central) lymphoid organs– responsible for maturation of lymphocytes
• bone marrow
• thymus
• Secondary lymphoid organs– trap antigens and allow lymphocytes to respond
• other lymphoid tissue
Primary lymphoid organs• Thymus
– cells entering the thymus are thymocytes– the thymus is a flat, bilobed organ with lobes
divided in lobules separated by trabeculae– each lobule divided into inner (medulla) and
outer (cortex) compartments– thymic hormones cause differentiation and
maturation of T lymphocytesthymosin, -thymosin, thymopoietin and
thymulin
Primary lymphoid organs (cont)– Antigenic diversity of TCR results from
random gene rearrangements in thymus– Most thymocyte progeny undergo programmed
cell death by two-step selection process (99%)– Neonatal thymectomy results in decrease of
circulating T cells and cell mediated immunity• DiGeorge’s syndrome in humans
• nude mice have no thymus
– Thymus reaches its maximal size at puberty• then atrophies
• fat content increases
• next slide shows a cross-section of the thymus
Bone marrow
• Bone marrow– site of B cell maturation– maturation occurs in the “microenvironment”– stromal cells secrete involved cytokines– a selection process eliminates some B cells
• The equivalent in birds is the bursa of Fabricius
• The next slide shows the bone marrow
Lymphatic system• Fluid component of blood is plasma
– without clotting factors called serum
• Plasma leaves the capillaries and moves into the interstitial tissues– some returns, the remainder is called lymph
• This moves into the lymphatic vessels
• Returns to the blood via the thoracic duct
• Antigens move with lymph into lymphoid tissues
Secondary lymphoid organs
• Before antigenic stimulation, resting B cells are organized into a primary follicle
• After antigenic stimulation, the primary follicle become a larger secondary follicle– contains many proliferating B cells which
comprise a germinal center
• Lymph nodes and the spleen have these structures
Lymph nodes• Some lymphatic tissue is composed of
diffuse collections of lymphocytes and macrophages
• Others organized into lymphoid follicles
• Contain lymphocytes, macrophages and dendritic cells in reticular network
• Lymph nodes are divided into cortex, paracortex and medulla
• Medullary plasma cells produce antibodies
Section of enlarged human lymph node (low power view) showing numerous enlarged lymphoid follicles, each composed of a mantle of dark stained small lymphocytes surrounding a pale stained germinal center.
Human lymph node showing two enlarged germinal centers containing IgG producing B cells (white) using fluorescent
anti-human IgG.
Spleen• Functions to filter blood and trap blood-borne
antigens
• Composed of red pulp (macrophages and red blood cells) and white pulp
• White pulp surrounds the arteries and forms the PALS
• Splenectomy predisposes to the individual to several bacterial diseases
• The next slide shows a section of the spleen