Download 02. Lymphoid System lecture2010-10-01 03:421.2 MB

Lymphoid System
T and B Cell Development
Prof. Dr. Zahid Shakoor
MBBS, Dip. Med. Immunology (UK), Ph D (London)
King Saud University
Blood Cells
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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
Primary Lymphoid Organs
Secondary Lymphoid Organs
T-Lymphocyte Differentiation
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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
T-Lymphocyte Differentiation
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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
T-Lymphocyte Differentiation
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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”
Mature Lymphocyte
T-Lymphocytes
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Effector Functions
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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.
T-Lymphocytes
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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
Functions of Helper Lymphocytes
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CD4 Lymphocytes (Helper Type)
Functions
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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
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
IL-2
Activates B cells
IL-4 and IL-5
Activates Macrophages
Interferon Gamma
CD8 positive cells
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About 35% of peripheral blood T cells
Perform cytotoxic functions
They kill virus-infected cell, tumor and
allograft cells
Perforins
Programmed cell death (apoptosis)
B cells
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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:
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Antigen independent phase consists of stem cells,
pre-B cells and B cells
Antigen dependent phase consists of activated B
cells and plasma cells
B cells
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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
B cells
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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
B cells
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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
Thank you