Immunology Chapter 3, Lecture 4 Richard L. Myers, Ph.D. Department of Biology Southwest Missouri State Temple Hall 227 Telephone: 417-836-5307 Email: rlm967f@mail.smsu.edu

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 lymphocytes a1-thymosin, b4 -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

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

Bone marrow with adipose tissue

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

Spleen

Assignment Begin reading Chapter 4, Antigens Study question 2 (pg 105)