Lecture 18

Question 1

Distinguish between passive and active immunity.

Answer 1

Passive Immunity: temporary immunity due to donated antibodies (this occurs early in life when maternal antibodies are transferred to the fetus)

Active Immunity: Long-lasting/ permanent immunity due to self exposure to antigens resulting in memory T cells and B cells specific for those antigens

Question 2

differentiate between primary and secondary immune tissue/organs. List the organs for each and explain the process of each.

Answer 2

Primary immune organs: Thymus and bone marrow

Precursor cells mature into immunocompetent cells and each cell is programmed to recognize a specific antigen.

Secondary immune organs: lymph nodes, spleen, tonsils

Trapped antigens stimulate clonal expansions of mature T and B cells

Question 3

Describe distinguishing characteristics of a primary lymph follicle and compare with a secondary lymph follicle.

Answer 3

Primary lymph follicle: spherical, tightly packed accumulations of virgin B cells and dendritic reticular cells that have not been exposed to antigens

Secondary lymph follicle: Are not present at birth and are derived from primary follicles that have been exposed to nonself antigens

Question 4

Describe the function of the MHC (Major histocompatibility complex) and compare the two subdivisions of the MHC.

Answer 4

The main function of MHC gene products is the presentation of antigenic peptides to T cells

Subdivisions
MHC I: expressed on the surface of all cells except trophoblast and RBCs

MHC II: expressed on the surface of B cells and antigen-presenting cells

Question 5

describe CD4+ T cells and their functions

Answer 5

CD4+ T cells: helper cells that recognize antigens bound to MHC class II molecules

Assist CD8+ T cell and B cell differentiation

Question 6

Describe CD8+ T cells and their functions

Answer 6

CD8+ T cells: Cytolytic T cells that bind to an antigen presenting cell (they only recognize antigens bound to MHC class I molecules) and release perforins (perforates cell membranes in order to destroy them) and Fas Ligand

Undergo mitosis

They are mediators of cellular immunity

Question 7

Describe CD16+ T cells and their function. be sure to explain what the following cytokines do when CD16+ T cells release them: Interleukin0-2, Interferon-gamma, MAF, and Chemotactic factor.

Answer 7

CD16+ T cells: Natural Killer (NK) cells that release cytokines when activated by tumor cell antigens

Interleukin-2: stimulates proliferation of NK cells

Interferon-gamma: activates NK cells

Macrophage activating factor (MAF): activates macrophages

Chemotactic factor
Tumor necrosis factor (TNF-beta): kills tumor cells directly

Question 8

Outline the interplay among T-helper cells, macrophages, B cells, and the MHC complex in the immune response.

Answer 8

Macrophages phagocytize foreign material and breaks them down into fragments, some of which have “epitopes” (antigenic properties)

These epitopes are then expressed on the surface of macrophage-bound to MHC-II

This MHC-II/antigen complex is then presented to activated helper T cells

The activated T cells then undergoes mitosis where some daughter cells become memory cells and some daughter cells secrete interleukins

This T cell activity attracts B cells, which have access to free antigens

B cells then undergo mitosis where some daughter cells become memory cells and some daughter cells become plasma cells (which secrete appropriate antibodies)

Question 9

Describe the differences between B and T cells when they undergo mitosis while participating in an immune response

Answer 9

The activated T cells then undergoes mitosis where some daughter cells become memory cells and some daughter cells secrete interleukins

B cells then undergo mitosis where some daughter cells become memory cells and some daughter cells become plasma cells (which secrete appropriate antibodies)

Question 10

Describe the components of C1, and their functions that aid in the conduction of the complement cascade

Answer 10

C1 is the first component of the complement cascade and it consists of 3 components (C1q, C1r, and C1s)

C1q: binds to the Fg region of the Ig

C1r: is activated by C1q binding and in turn activates C1s

C1s: is a serine protease

C1s has 2 functions:
cleaves complement protein C4 into C4a and C4b (binds to the pathogen)

Cleaves C2 into C2a and C2b (binds to C4b)

Question 11

Beginning with the function of C1s, describe the complement cascade, ending with the fate of the pathogen

Answer 11

After C1s cleavages occur, C4b, the pathogen it is bound to, and C2b bind to one another to form a complex (this complex is called C3 convertase)

Multiple C3b’s will bind to C3 convertase in order to form C5 convertase (C4b-C2b-C3b complex)

C5 convertase activates C5, which makes it split into C5a and C5b

C5b joins the C5 convertase complex to form the Final opsinization complex

Once the final opsinization complex is formed, C6, C7, C8, and C9 are added to the complex, forming pores in the membrane of the pathogen to kill it.

Question 12

Name the most important opsonin.

Answer 12

C3b is the most important opsonin

Question 13

List the results of the complement cascade.

Answer 13

Activation of the MAC (membrane attack complex) on the pathogen leading to perforations and lysis

Production of opsonins, which are coatings that make the antigens more palatable to phagocytes

Release of chemotactic agents (chemokines) which attract phagocytes (they move via chemotaxis) to the ares of infection or inflammation

Question 14

Distinguish between stroma and parenchyma.

Answer 14

Stroma: consists of mostly reticular fibers and cells including undifferentiated cells and fixed and free macrophages

Parenchyma: consists of cells, mostly lymphocytes, that pack the areas of the lymphoid organ

Question 15

When it comes to lymph nodes, what is the site of lymphocyte entry into the the lymph system?

Answer 15

the medulla of the lymph node is where this occurs

Question 16

What is the main function of the Hilus of a lymph node? what type of vessels enter through the convex side of the node?

Answer 16

The hilus serves as the entry and exit point for vessels

Afferent lymphatic vessels enter through the convex side of the node

Question 17

What is the capsule of a lymph node composed of?

Answer 17

Capsule: composed of dense collagen fibers, some elastic fibers, and smooth muscle fibers

Question 18

Describe the inner and outer portions of the cortex of a lymph node

Answer 18

the outer cortex houses lymph follicles (nodules)

Deep (inner) cortex contains Th cells and macrophages

Question 19

In a Lymph node, where can the following be found?

B cells, follicular dendritic cells, and migrating dendritic cells

Answer 19

In the lymph follicles

Question 20

Compare and contrast the characteristics of primary and secondary lymph follicles

Answer 20

Secondary follicles feature a mantle and germinal center

Primary follicles lack a mantle and germinal center

Question 21

Describe the significance of HEVs.

Answer 21

HEV (high endothelial venules) are the port of entry for circulating differentiated lymphocytes to move into the lymph node

Question 22

Describe the presentation of a lobule in the thymus.

Answer 22

Composed of 2 regions

Cortex (dark staining): features epithelial reticular cells that secrete thymosin, and T cells in various stages of development

Medulla (light staining): features capillary beds that are not sheathed by epithelial cells, which allows them to release mature lymphocytes into the blood stream (slide 53)

Question 23

Describe the capsule and trabeculae of the thymus

Answer 23

Capsule: full of blood vessels and efferent lymphatics only (no afferent lymphatics means lymph does not circulate through the thymus
Extends the trabeculae into the parenchyma

Trabeculae (Septa): composed of delicate CT and divides the thymus into incomplete lobules

Question 24

List source of thymosin.

Answer 24

The epithelial reticular cells that are found in the cortex (dark staining portion) of the thymus lobules

Question 25

Describe the location and role of the epithelial barrier surrounding blood vessels.

Answer 25

This is a structure that is found in the cortex (darker staining part) of thymus lobules

This allows the thymus to maintain lymphopoiesis, while still remaining segregated from antigens in the blood

Question 26

Describe the composition and role of Hassall’s corpuscles.

Answer 26

These are composed of whorls of highly keratinized medullary epithelial cells

They produce cytokine thymic stromal lymphopoietin, which stimulates the thymic dendritic cells needed for the maturation of single positive T cells

Question 27

Describe, in detail, the changes in T cells as they move into the thymus through the subcapsular space, outer cortex, inner cortex, and medulla.

Answer 27

Double negative T cells, which lack the surface molecules that are characteristic of mature T cells, enter the cortex from the blood vessels.
Double negative T cells then proliferate in the subcapsular area

Double negative T cells become double positive T cells when they express CD4 andCD8 coreceptors, as well as TCR receptors AND move to the outer cortex
Double negative T cells are confronted with epithelial cells with cell surface MHC classes I and II for clonal selection (they pick either CD4 or CD8 to express from here on)

Single positive T cells move to the inner cortex where they express TCR receptors and either CD4 or CD8 coreceptors

Clonal deletion is completed in the medulla

Question 28

Describe the location and structure/components of the blood-thymus barrier. (include any changes that may occur in it’s characteristics during devleopment)

Answer 28

It is located in the thymic cortex and prevents antigens in the blood from reaching developing T cells in the thymic cortex
It is leaky during fetal development in order to develop immunologic tolerance to self antigens

Components: endothelium, endothelial basal lamina, perivascular space, basal lamina of reticular cells, reticular cells, and thymic paranchymal cells

Question 29

True or False: The spleen is completely covered by a peritoneum. explain

Answer 29

False

it is covered by peritoneum except at the hilus

Question 30

Describe the pathway of blood and lymph into the spleen

Answer 30

blood vessels enter and exit via the hilus

it has no lymph tissues or afferent lymph vesicles

Question 31

What are the 2 main divisions of spleen tissue?

Answer 31

white and red pulp

Question 32

List functions of the spleen. (2 main ones)

Answer 32

Blood filtering functions (it is the only lymphatic organ that is specialized to filter blood)

Stores and removed worn-out RBCs

Recycles iron

Converts hemoglobin to bilirubin

Blood formation in the fetus

Immunologic functions (Removal of the spleen leads to overwhelming bacterial in infants, children, and young adults)

Screens foreign material in the blood

Produces lymphocytes and plasma cells

Question 33

What is the white pulp of the spleen composed of? the white pulp is the main site of what?

Answer 33

White pulp: composed of zones of diffuse lyphoid tissue and germinal centers

It is the site of clonal expansion of antigen-stimulated lymphcytes

Question 34

In the white pulp tissue of the spleen, describe the B cell and T cell areas

Answer 34

B cell area contains secondary follicles in which central arteriole is off center

T cell area surrounds the central artery near the center of the white pulp
(this forms the periarterial lymphatic sheath (PALS))

Question 35

State the function, arrangement, and composition of the red pulp of the spleen

Answer 35

Red pulp: contains large numbers of RBCs and other blood elements that function to filter the blood
Surrounds the white pulp and makes up about 80% of the spleen

Has billroth cords and venous sinusoids

Question 36

Describe the PALs; where are they located?

Answer 36

PAL (periarterial lymphatic sheath) are located in the white pulp of the spleen, in areas surrounding the central artery near the center of the white pulp

The PALs portion of the white pulp of the spleen are where T cells are found

Question 37

Describe Billroth cords (what type of circulation they conduct and what they house) where are they located.

Answer 37

Billroth cords form the red pulp parenchyma (in the spleen)

They contain various blood cells, plasma cells, and antigen presenting cells

Their terminal capillaries open directly into the substance of the cords (open circulation)

They house macrophages that destroy worn-out or defective RBCs

Question 38

State the pathway of blood flow through the spleen (starting with the main artery of the spleen)

Answer 38

splenic artery (enters the spleen via the hilus)

trabecular arteries

central arteries (feature splenic nodules with a mesh-like reticulum that is infiltrated with lymphocytes)

penicillus

venous sinuses

Question 39

Compare open and closed systems in the vascular drainage pattern.

Answer 39

Open systems occur in the spleen when blood drains into intercellular spaces

Closed systems occur in the spleen where the venous sinuses are lined with reticuloendothelial cells