ImmunologyChapter 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 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

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)