Chapter 9

Question 1

Hematopoietic Stem Cells

Answer 1

A common lymphoid precursor (CLP) of lymphocytic lineage cells either differentiates within the thymus (T-Cell Lineage) or remains in the bone marrow (B-Cell Lineage).

Question 2

Prothymocytes

Answer 2

T Cell precursors

• Migrate from the bone marrow to the thymus attracted by thymic molecules

Question 3

Thymocytes

Answer 3

Prothymocytes entering at the cortical region, now called thymocytes, lack TCRs, CD3, CD4, & C8 surface molecules

Question 4

Cortex of Thymus

Answer 4

Dense outer region

Question 5

Medulla of Thymus

Answer 5

Looser inner region

Question 6

Epithelial Reticular Cells

Answer 6

An inclusive term that includes several cell types such as dendritic cells, macrophages, & epithelial cells that serve as ‘instructors’ for the thymocytes as they complete their education.

Question 7

Hassall’s Corpuscles

Answer 7

Are structures found in the medulla of the human thymus, formed from eosinophilic type VI epithelial reticular cells arranged concentrically.

Question 8

Medullary Post Capillary Venules

Answer 8

Important for the egress of thymic graduates or T cells from the thymus

Question 9

Double Negative (DN) Cells

Answer 9

Do not express CD4 or CD8 molecules (or TCR or CD3 complex molecules)

Question 10

Double Positive (DP) Cells

Answer 10

Express both CD4 & CD8 surface molecules

Question 11

Positive Selection

Answer 11

A process where DP thymocytes die within 3 to 4 days unless they recongize & bind to MHC or to peptide + MHC (pMHC)

Question 12

Single Positive (SP) Cells

Answer 12

• DP cells whose CD8 molecules have engaged pMHCI then cease the expression of CD4 molecules & becomes SP CD8⁺ cells.
• Likewise, those that are bound to pMHC II cease expression of CD8, becoming SP CD4⁺ cells.

Question 13

Negative Selection

Answer 13

Survivors of positive selection then run a second gauntlet called negative selection when they arrive at the corticomedullary junction. There, they meet & interact with a second set of epithelial reticular cells (antigen-presenting cells such as dendritic cells & macrophages)

Question 14

T Cells

Answer 14

Thymocytes that pass both positive & negative selection tests “graduate” from the thymus, entering the circulation through the medullary postcapillary venules as T Cells.

Question 15

Common Lymphoid Progenitor (CLP)

Answer 15

The CLP produces only T or B cells.

Question 16

Pre-Pro-B Cell

Answer 16

The earliest identifiable cell committed to the B-Cell lineage

• Begins to express Ig-alpha & Ig-beta BCR accessory molecules

Question 17

Pro-B Cell

Answer 17

Immunoglobulin DJ gene joining & cytoplasmic expression of Surrogate Light Chain (SLC) occurs at the early Pro-B Cell stage.

Question 18

Early Pre-B Cell

Answer 18

Characterized by the surface expression of pseudo-IgM (rearranged µ heavy chains plus SLC) & is accompanied by a burst of cellular proliferation

Question 19

Late Pre-B Cell

Answer 19

Immunoglobulin light chain kappa or lambda genes rearrange, & their products replace the SLCs.

Question 20

Immature B Cells

Answer 20

Expresss µ heavy chains plus kappa or lambda light chains on their cell surfaces

Question 21

Mature B Cells

Answer 21

Coexpress IgM & IgD on their cell surfaces

Question 22

B-2 B Cells

Answer 22

Conventional B Cells

• Widely distributed throughout the bosy, require interaction with T cells for their activation & proliferation, & are continually replaced from the bone marrow throughout adult life.

Question 23

B-1 B Cells

Answer 23

• Arise from the fetal liver by the eight gestational week.
• They might represent a transitional type of lymphocyte that bridges the innate & adaptive immune systems.

Question 24

Natural Antibodies

Answer 24

It is thought that most, if not all, natural antibodies are of B-1 B Cell origin

• e.g. IgMs directed against A & B blood groups that exist in the absence of known immunization