4. Structure and Function of Immune System - Secondary Lymphoid Organs

List the secondary lymphoid organs.

• Spleen

• Lymph nodes

• Mucosa associated lymphoid tissues

Which secondary lymphoid organs are the most highly organized ones?

Lymph nodes and spleen

Less-organized lymphoid tissues are collectively called

mucosal associated lymphoid tissue (MALT)

MALT includes

• Peyer’s patches (in small intestine)

• Tonsils

• Appendix

• Numerous lymphoid follicles within lamina propria of intestines

• Mucous membranes of upper airways (bronchi associated lymphoid tissues (BALT))

• Lymphoid tissues in genital tract

Secondary lymphoid organs are richly supplied with _____ vessels and _______ vessels, that facilitate movement of ____________, __________, and _________ cells into and out of these organs. Specialized regions of the vasculature called _____ ____________ ________ permit the movement of cells between the _____ and the ________ or _______ through which they are passing.

blood vessels

lymphatic vessels

lymphocytes

monocytes

dendritic

high endothelial venules

blood

tissues

organs

What processes does the leukocyte-rich nature of secondary lymphoid tissues facilitate?

• Cellular interaction

• Exchange regulatory signals

• Undergo further development

• Proliferation before re-entering circulation

What is the largest secondary lymphoid organ?

Spleen

What is the role of the spleen?

• It clears particulate matter/blood borne antigens/pathogens through the stimulation of T and B cells

• The spleen is the major site of immune responses to blood-born antigens

Where is the spleen situated?

Below the diaphragm, on the left side of the abdomen

How much does the adult spleen measure and weigh?

Measures 5 inches in length and weighs 150g

What are the 2 compartments the spleen is divided into?

Central white pulp and outer red pulp

Describe the capsule of the spleen.

Spleen is surrounded by capsule that extends a number of projections (trabeculae) into interior to form the compartmentalized structure

Describe the white pulp of the spleen.

• Central densely populated area which contains T and B cells

• Has 2 parts:

  1. Periarteriolar lymphoid sheath (PALS)

  2. Marginal zone

Describe the periarteriolar lymphoid sheath (PALS) of the white pulp of the spleen.

• Rich in T cells

• Surrounds the branches of the splenic artery

Describe the marginal zone of the white pulp of the spleen.

• Located peripheral to PALS

• Populated by B cells, lymphoid follicles (primary and secondary), and macrophages

Describe the red pulp of the spleen.

• Area that surrounds the sinusoids

• Filled with RBC’s

• Older and defective RBCs are destroyed here

Describe what happens where there are defects in the spleen and what they are caused by.

• As the spleen is the site of destruction for most microbes, functional or structural abnormalities of spleen or splenectomy, esp in children, often leads to increased incidence of bacterial sepsis

• Caused primarily by capsulated bacteria e.g.

  • Streptococcus pneumoniae

  • Neisseria meningitidis

  • Haemophilus influenzae

Describe the shape, arrangement and location of lymph nodes.

• Small, bean-shaped organs

• Occur in clusters or chains

• Distributed along the length of lymphatic vessels

Describe the role of lymph nodes.

• Sites where immune responses are mounted to antigens in lymph

• Act as physiological barriers; they filter the microbial antigens carried to lymph node by activating the T and B cells

• Are the first organized lymphoid structure to encounter antigens that enter through tissue spaces

• If any particular antigen is brought in the lymph, it will be trapped by phagocytic and dendritic cells

Lymph nodes are divided into 3 parts. List them.

1. Cortex (B cell area)

2. Medulla (B cell area)

3. Paracortex (T cell area)

Describe the cortex of lymph nodes.

• Surrounded by capsule and intervened by trabeculae

• Contains lymphoid follicles that are composed of mainly B cells and few special types of dendritic cells (called follicular dendritic cells)

What are the two main types of lymphoid follicles?

1. Primary lymphoid follicles

2. Secondary lymphoid follicles

Describe primary lymphoid follicles.

• Found before the antigenic stimulus

• Smaller in size

• Mainly contain resting B cells

Describe secondary lymphoid follicles and what occurs in them.

• Following contact with an antigen, resting B cells in primary follicles start dividing and become activated

• The activated B cells differentiate rapidly into plasma cells (produce antibodies) and memory B cells (which remember the antigenic structure and become activated on subsequent antigenic exposure)

• Follicles become larger in size and are called secondary lymphoid follicles

Secondary follicles have two areas. Name and describe them.

• Central area is called germinal center; contains dividing B cells of various stages

  • Germinal center has 2 zones: light and dark zones; site where activation of B cells takes place

• Peripheral zone is called mantle area; contains activated B cells

Describe the paracortical area of the lymph node.

• Present in between cortex and medulla

• Is the T-cell area of lymph node

• Rich in naive T-cells

• Also contains macrophages and interdigitating dendritic cells, which trap the antigens and present to T-cells

Describe the medulla of the lymph node.

• Innermost area of lymph node

• Rich in B cells; mainly plasma cells

• After infection, lymph leaving a node is enriched with antibodies newly secreted by medullary plasma cells and also higher conc. of lymphocytes

Describe mucosal associated lymphoid tissue (MALT).

• Mucous membranes lining intestine, respiratory and urogenital tract (total surface area of about 400 m²) are major entry sites for most pathogens

• Defense mechanisms needed in mucosal sites to prevent microbial entry

• Group of lymphoid tissues lining mucosal sites are collectively known as MALT

Structurally, MALT may be arranged in 2 types. Describe them.

1. Loose clusters of lymphoid cells - usually found in lamina propria of intestinal villi

2. Lymphoid tissues arranged as organized structures - such as tonsils, appendix, and Peyer’s patches

Describe MALT in intestinal mucosa and describe its layers.

• Lymphoid tissues lining intestinal mucosa are best studied in MALT

• Present in different layers in intestinal wall: submucosa, lamina propria, epithelial layer

1. Submucosa: contains Peyer’s Patches (PP is a nodule of 20-40 lymphoid follicles)

2. Lamina propria: contains loose clusters of lymphocytes (B-cells, plasma cells, T-helper cells) and macrophages

3. Epithelial layer: contains few specialized lymphocytes called intraepithelial lymphocytes (IELs) and modified epithelial cells called M-cells

Describe M-cells and their role.

• Specialized flattened epithelial cells

• Do not have microvilli

• Have deep invaginations or pockets in basolateral side; contain B-cells, T-cells, and macrophages

• M-cells act as portal of entry of a number of microbes such as Salmonella, Shigella, Vibrio, and Polio virus

• Invading microbes are taken up by M-cells, then transported in a vesicle, and delivered to the basolateral pockets