1. T and B Cell Development
Lymphoid System
T and B Cell Development
Prof. Dr. Zahid Shakoor
MBBS, Dip. Med. Immunology (UK), Ph D (London)
King Saud University

2. Blood Cells Blood cell precursors originate in yolk sac and liver
In postnatal life the stem cells reside in the bone marrow
Stem cells differentiate into cells of erythroid, myeloid or lymphoid series

3. Primary Lymphoid Organs

4. Secondary Lymphoid Organs

5. T-Lymphocyte Differentiation
Lymphoid series comprise of two main lymphocyte populations T cells and B cells
T cell precursors differentiate into mature T cells in thymus
Stem cells lack antigen receptors and CD3, CD4, CD8 surface markers
During their passage through thymus they differentiate into T cells expressing these markers

6. T-Lymphocyte Differentiation
Stem cells initially do not express CD4 and CD8 (Double Negative)
Later they express both (Double Positive)
Double positive cell will differentiate into CD4 positive cell if it contacts a cell bearing Class II MHC protein, but will differentiate into CD8 positive cell if it contacts a cell bearing Class I MHC proteins

7. T-Lymphocyte Differentiation
Thymic Education
Involves two processes
CD4 and CD8 positive bearing receptors for “self” proteins are killed (clonal deletion). This type of removal is called “negative selection”- tolerance to self proteins
CD4 and CD8 positive cells bearing antigen receptors that do not react with self MHC proteins are also killed this process is called “positive selection”

8. Mature Lymphocyte

9. T-Lymphocytes Effector Functions CD4 and CD8 types of cells
These are carried out by cytotoxic (CD8 positive) cells which kill virus infected cells, tumor cells and allografts
CD4 and CD8 types of cells
T cell precursors differentiate under the influence of thymic hormones (thymocins and thymopoetins) into T cell subpopulations ie, CD3, CD4 and CD8.

10. T-Lymphocytes
All T cells have CD3 proteins on their cell surface in association with T cell receptor, that transmits information that T cell receptor is occupied
Mature T cells have either CD4 or CD8 proteins but not both

11. Functions of Helper Lymphocytes
CD4 Lymphocytes (Helper Type)
Functions
Help B cells to develop into antibody producing plasma cells
Help CD8 cells to become activated cytotoxic T cells
Help macrophages in delayed type of hypersensitivity

12. CD4 Positive Lymphocytes
Main Functions of Helper T cells
Main Functions
Cytokine Mediating That Function
Activates the antigen specific helper T cell to produce a clone of these cells
IL-2
Activates cytotoxic T cells
Activates B cells
IL-4 and IL-5
Activates Macrophages
Interferon Gamma

13. CD8 positive cells About 35% of peripheral blood T cells
Perform cytotoxic functions
They kill virus-infected cell, tumor and allograft cells
Perforins
Programmed cell death (apoptosis)

14. B cells Origin During embryogenesis – fetal liver
Migrate to bone marrow – final destination
They do not require thymus for maturation
Maturation of B cells involves two phases:
Antigen independent phase consists of stem cells, pre-B cells and B cells
Antigen dependent phase consists of activated B cells and plasma cells

15. B cells B cells display surface IgM which serves as antigen receptor
Surface IgD on some B cells also serves as and antigen receptor
Pre B cells are found in bone marrow and mature B cells are found circulating in bloodstream

16. B cells B cells constitute about 30% of circulating small lymphocyte
Their life span is short ie, days or weeks
Location: lymph nodes – germinal centers, spleen – white pulp and Peyer’s patches

17. B cells Clonal selection
Antigen reacts with surface IgM and IgD on the best fit basis
After antigen binds B cell is stimulated to proliferate to form a clone of cells
These selected B cells later differentiate into antibody producing plasma cells that secrete antibody specific to that antigen

18. Thank you