B-cells develop and mature in the bone marrow from stem cells through distinct stages marked by specific cell surface markers and patterns of immunoglobulin gene expression. Mature B-cells leave the bone marrow and travel to peripheral lymphoid tissues where they are activated upon encountering antigen to produce plasma cells that secrete antibodies and memory B-cells. B-cell activation involves proliferation, somatic hypermutation, selection, and potential class switching in germinal centers to produce high affinity antibodies and long-lasting immunological memory. This allows for a rapid secondary immune response upon re-exposure to the same antigen.

1. Santosh Yadav
Santosh Yadav
M.Sc. Clinical Microbiology
Dept. of Microbiology
Institute of Medicine
Tribhuvan Univarsity Teaching Hospital, Nepal

2. Santosh Yadav
B-cell development and maturation

3. Santosh Yadav
Introduction
 Derives name from the organ it matures (bursa of
fabricus in birds)
 In human it matures in bone marrow.
 The lymphocyte which arise and develop in bone marrow
is called B-lymphocytes.

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The discovery of B cell immunity
1954 - Bruce Glick
Studies on the function of the bursa of Fabricius, a lymphoid organ in the
cloacal region of the chicken
Bursectomised chickens
were later used in
experiments to raise
antibodies to Salmonella
antigens
None of the
bursectomised
chickens made anti-
Salmonella
antibodies
Bursa was later found to be the organ in which antibody producing cells
developed – antibody producing cells were thereafter called B cells
Mammals do not have a bursa of Fabricius

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 Generation begins in the embryo and continues
throughout life
 Before birth-
 Yolk sac
 Fetal liver
 Fetal bone marrow
 After birth
 Bone marrow

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 A mature and immunocompetent B-cell have three
uniqueproperties:-
It is specific only to one particular antigen.
It produces antibodies specific to the antigen to which it
was primed, and
It generates memory for any similar antigenic attack in
future.

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B-cell Carry many surface marker:
CD19
CD21
CD22
Membranous immunoglobulin
Igα and Igβ heterodimer

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Stages of B cell Development
 Stem cell
 Progenitor B cell (Pro B cell)
 Precursor B cell (Pre B cell)
 Immature B cell
 Mature B cell
Each stage characterized by distinct cell surface
markers and a specific pattern of Ig gene expression.

9. Santosh Yadav
Pro -B-cell
 Is the earliest recognisable B-cell stage.
 It has following surface marker:-
CD -45
Igα and Igᵦ heterodimer :-part of B-cell membrane
receptor in later stage.
CD-19
CD-24
CD-43
C-kit
• The heavy chain gene rearrangement begins in this
stage and its completion signals the end of this stage.

10. Santosh Yadav
Pre-B-cell
 Characterized by the beginning of the translation of
heavy chain hene.
 Essentially require bone marrow stromal cell.
 Mu chain is usually first synthesized.
 No light chains are synthesized.
 Small peptide called Surrogate light chain are
synthesized( not true light chain)
 They complex with mu chain and this complex is
expressed on the cell surface.
 The mu chain and surrogate light chain complex
associate with Igα/Igᵦ hetrodimer to form pre-B-cell
receptor.

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Early
pro-B
Interleukin-7
receptor
Late
pro-B Pre-B
Interleukin-7
Growth factor
Stromal cell

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Immature -B-cell
• Begins with light chain gene.
• Only one type of light chain is expressed(kappa or
lambda)
• Light chain associates with mu chain and so a true IgM
expression occurs on the B-cell Surface.
• This also results in B-cell commited to one particular
antigenic specificity as determined by binding site of
surface IgM.
• True BCR appears.

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Mature B-Cell
 Characterized by simultaneous expression of IgM and Igd
on B-cell surface.
 The mature B-cell then leaves the bone marrow and go to
the peripheral lymphoid tissue where they get activated
on encountering the antigen and produce two types of
effector cells:-
a)Plasma cell:- produce antibody, and
b) Memory cell:- produce secondary immune response after
re-encounter with the same antigen.
• If the mature B-cell cannot encounter antigen, they die
within few days by apoptosis.

15. Santosh Yadav
 So the production of mature B-cell in bone marrow is an
antigen independent phase while activation and effector
cell production is an antigen dependent phase.

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Negative selection of B-cells
 On an average, only 10 percent of total newly cells
produced(5 x 10⁷ cells) i.e. 5 x 10⁶ cells are recruited into
circulation everyday.
 Rest of 90 percent of newly produced B-cells die.
 This loss is due to the negative selection of those
developing B-cells which express self reactive mIgM on
their cell surface.
 Such cells are eliminated by programmed apoptosis.
This process is called negative selection.

18. Santosh Yadav
Development of Humoral immune response
 Humoral immune response refers to the production of
secreted antibodies by plasma cells following antigenic
stimulation of B-cells.
 Involves activation of B-cells by antigen followed by
their proliferation and differentiation in to plasma
cells and memory cell.

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B-cell Activation
Occurs in the peripheral lymphoid organ
Antigen-driven activation and clonal selection of naive B
cells
 Generation of plasma cells and memory B cells
 In absence of antigen-induced activation, naive B cells
die within few weeks by apoptosis

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2 types of Antigens
Thymus cell independent antigen(TI Ag)
 Do not require direct interaction of Ag specific helper T
lymphocytes.
 Require cytokines secreted by TH cell.
 Multivalent nonprotein Ags
 Polysaccharides, lipids, nucleic acids
Thymus cell dependent antigen (TD Ag)
 Require direct interaction with TH cells
 Proteins antigens

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 Multivalent Ag composed of repeated identical antigenic
epitopes
 Maximal cross-linking of the BCR complex on specific B-cells
 Sufficient signalling for activation.
 Bcell co-receptor complex, further augment signaling

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B cell co-receptor complex
3 protein
 CD19
 CR-2/CD 21
 TAPA-1
(Target of antiproliferative
antibody-1)

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T cell dependent antigen response

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Memory
Cells
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Changes in germinal centres
 somatic hypermutation
 Selection
 Class switching

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SOMATIC HYPERMUTATION
 B cells undergo rapid cell division
 Mutation of genes occurs randomly
 Ag binding affinity of mIg on B cells may
increases
 Decrease
 No affinity

29. Santosh Yadav
Selection
Centrocytes compete with each other to
bind Ag on follicular dendritic cells
B cells bearing high-affinity mIg are most
likely to compete
 Those that fail die by apoptosis
 Signal from TH cells for further survival
 Differentiate into Memory B cells or
Plasma cells

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Clonal selection of B cells
 Generates a clone of short-lived activated effector cells and a clone of
long-lived memory cells
Antigen molecules
Antigen
receptor
B cells that
differ in
antigen
specificity
Antibody
molecules
Clone of memory cells
Clone of plasma cells
Antigen molecules
bind to the antigen
receptors of only one
of the three B cells
shown.
The selected B cell
proliferates, forming
a clone of identical
cells bearing
receptors for the
selecting antigen.
Some proliferating
cells develop into
short-lived plasma
cells that secrete
antibodies specific
for the antigen.
Some proliferating cells
develop into long-lived
memory cells that can
respond rapidly upon
subsequent exposure
to the same antigen.
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Class switching or Isotype switching
 Process by which B cell changes class of IG produced while
preserving antigenic specificity
 Allows any given VH domain to associate with the constant
region of any isotype
 Enables antibody specificity to remain constant while the
biological effector activities of the molecule vary
 Interaction between CD40 on the B cell and CD40L on the
TH cell is essential
 Occurs only during active immune response

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Clonal Selection
Only one type of
antibody—and one
type of B cell—
responds to the
antigenic determinant
That cell type
then produces a
large number of
clones
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Plasma cell
 Membrane form Ig changes to secreted form
 The rate of transcription of heavy and light-
chain genes significantly greater

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Memory cell
 Seen in T-dependent immune responses
 Survive for long periods
 High-affinity antigen receptors
 Capable of mounting rapid responses to
subsequent introduction of Ag
 Remain in the lymphoid organ where
generated or recirculates

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Primary and secondary immune response
Primary immune response
 The first contact of an exogenous antigen with an
individual generates a primary humoral response,
characterized by the production of antibody-
secreting plasma cells and memory B cells.
 The kinetics of the primary response, as measured
by serum antibody level, depend on the nature of
the antigen, the route of antigen administration,
the presence or absence of adjuvants, and the
species or strain being immunized.
 Gradual rise in antibody production taking days to
weeks
 Antibody level declines

37. Santosh Yadav
Secondary immune response
 Activation of memory cells by antigen results in a
secondary antibody response that can be distinguished
from the primary response in several ways.
 The secondary response has a shorter lag period,
reaches a greater magnitude, and lasts longer.
 The secondary response is also characterized by
secretion of antibody with a higher affinity for the
antigen,
 And isotypes other than IgM predominate
 Second exposure to same antigen.
 Recognition of antigen is immediate.
 Results in immediate production of protective
antibody, mainly IgG but may see some IgM

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THANK U