Chapter 9 Flashcards
Hematopoietic Stem Cells
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).
Prothymocytes
T Cell precursors
- Migrate from the bone marrow to the thymus attracted by thymic molecules
Thymocytes
Prothymocytes entering at the cortical region, now called thymocytes, lack TCRs, CD3, CD4, & C8 surface molecules
Cortex of Thymus
Dense outer region
Medulla of Thymus
Looser inner region
Epithelial Reticular Cells
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.
Hassall’s Corpuscles
Are structures found in the medulla of the human thymus, formed from eosinophilic type VI epithelial reticular cells arranged concentrically.
Medullary Post Capillary Venules
Important for the egress of thymic graduates or T cells from the thymus
Double Negative (DN) Cells
Do not express CD4 or CD8 molecules (or TCR or CD3 complex molecules)
Double Positive (DP) Cells
Express both CD4 & CD8 surface molecules
Positive Selection
A process where DP thymocytes die within 3 to 4 days unless they recongize & bind to MHC or to peptide + MHC (pMHC)
Single Positive (SP) Cells
- 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.
Negative Selection
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)
T Cells
Thymocytes that pass both positive & negative selection tests “graduate” from the thymus, entering the circulation through the medullary postcapillary venules as T Cells.
Common Lymphoid Progenitor (CLP)
The CLP produces only T or B cells.
Pre-Pro-B Cell
The earliest identifiable cell committed to the B-Cell lineage
- Begins to express Ig-alpha & Ig-beta BCR accessory molecules
Pro-B Cell
Immunoglobulin DJ gene joining & cytoplasmic expression of Surrogate Light Chain (SLC) occurs at the early Pro-B Cell stage.
Early Pre-B Cell
Characterized by the surface expression of pseudo-IgM (rearranged µ heavy chains plus SLC) & is accompanied by a burst of cellular proliferation
Late Pre-B Cell
Immunoglobulin light chain kappa or lambda genes rearrange, & their products replace the SLCs.
Immature B Cells
Expresss µ heavy chains plus kappa or lambda light chains on their cell surfaces
Mature B Cells
Coexpress IgM & IgD on their cell surfaces
B-2 B Cells
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.
B-1 B Cells
- 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.
Natural Antibodies
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
