Immuno Chapter 7: Humoral Immune Responses
Humoral immunity – uses antibodies to fight extracellular pathogens
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It’s better than cell-mediated immunity, at fighting microbes with capsules rich in
polysaccharides and lipids, and against polysaccharide and lipid toxins
o This is because B cells can respond to more than just protein antigens (they can respond
to lipid and carb antigens as well), unlike cell mediated
Naïve B lymphocytes express 2 classes of membrane-bound antibodies (IgM and IgD) that act as
receptors for antigens
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These naïve B cells are activated by antigens and other signals
Activated B cells proliferate into antigen-specific cells (called clonal expansion) and differentiate
into effector cells called plasma cells (which secrete antibodies)
The secreted antibodies are specific for the same antigen the naïve B cell membrane receptor
recognized to trigger the response
So humoral immunity amplifies: one naïve B cell proliferates into many plasma cells, each
making many antibodies specific to that antigen
o This allows us to keep up with the rapidly proliferating microbe
During differentiation, some B cells may start to make antibodies of a different heavy chain
isotype (class), called heavy chain isotype (class) switching
o The different isotype (class) mediates different effector functions, and are specialized to
fight different types of microbes
Repeated exposure to a protein antigen causes production of antibodies with increasing affinity
for the antigen – called affinity maturation
o This causes making of antibodies with improved ability to bind and neutralize microbes
and their toxins
Antibody responses to different antigens are classified as T-dependent or T-independent
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Protein antigens are processed in antigen-presenting cells (APCs), and recognized by T helper
lymphocytes (TH cells)
o TH cells activate B cells, and induce heavy chain isotype switching and affinity maturation
o Without TH cells helping, protein antigens elicit weak or no antibody responses
o So antibody responses to protein antigens are called T-dependent
Polysaccharides, lipids, and other nonprotein antigens will stimulate antibody making without
any help from T cells, so antibody responses to these antigens are called T-independent
o T-independent antigens show little heavy chain isotype switching & affinity maturation
Follicular B cells – reside in follicles of lymph organs
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Most B cells are follicular B cells
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Follicular B cells are mostly T-dependent, class-switched, and high affinity antibody responses to
protein antigens, and give rise to long-lived plasma cells
Marginal zone B cells – found in marginal zones of the spleen white pulp, they respond to carb antigens
in the blood
B-1 B cells – respond to nonprotein antigens in the mucosal tissues and peritoneum
Marginal zone and B-1 B cells have antigen receptors that have limited diversity in what they recognize,
and do mostly IgM responses, which lack most of the features of T-dependent antibody responses
Less antibody is produced after the first encounter with an antigen (primary response) than in future
encounters (secondary response)
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With protein antigens, secondary responses also show increased heavy chain class switching and
affinity maturation, because repeated stimulation by the protein antigen ↑ the # of TH cells
Figure 7-3 page 134 – table of differences between primary and secondary antibody response
Humoral immune responses are initiated when antigen-specific B cells in the spleen, lymph nodes, and
mucosal lymph tissues, recognize antigens
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Some microbe antigens get transported to and concentrated in B cell-rich follicles and marginal
zones of peripheral lymph organs
In lymph nodes, macrophage lining the subcapsular sinus may capture antigens and display
them to B cells in nearby follicles
B cells specific for an antigen use their immunoglobulin (Ig) receptors to recognize the antigen
o This triggers B cell activation
o A second signal is also needed
When a B cell sees it’s antigen, it’s membrane Ig receptors cluster together, triggering signals that are
transduced by receptor-associated signaling molecules
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Two or more receptor molecules need cross-linked to transduce the signal, which happens when
they see more than one antigen molecule, or one antigen molecule with multiple epitopes
Nonprotein antigens often have many identical epitopes in each molecule, allowing them to
bind to many Ig receptors on a B cell at the same time
Signals initiated by antigen receptor cross-linking get transduced by receptor-associated
proteins
o IgM and IgD have very variable antigen binding regions, but can’t transduce signals
 So the Ig receptors attach to 2 proteins called Igα and Igβ, to form the B cell
receptor (BCR) complex
o The cytoplasmic domain of Igα and Igβ have immunoreceptor tyrosine-based activation
motifs (ITAMs)
o So when antigen receptor clusters form, the tyrosines in the ITAMs get phosphorylated
by kinases associated with the BCR complex
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The new phosphotyrosines then become docking sites for adapter proteins, which then
also get phosphorylated, allowing them to recruit signaling molecules
The signal is then sent to activate transcription factors that will turn on genes that code for
products involved in B cell proliferation & differentiation – page 136 – pic of signal transduction
B cells also have a receptor for a complement protein, to be the second signal to activate the B cell
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When the complement system is activated by a microbe, the microbe gets coated with
breakdown products of C3 complement protein (the most abundant complement protein)
o Includes breakdown product C3d, which B cells have a receptor for, called type 2
complement receptor (CR2 or CD21)
So an antigen on the microbe is the 1st signal, & complement (from innate immunity) is the 2nd
signal
Microbe products that engage B cell toll-like receptors (TLRs) can also trigger activation
o Dendritic cells are B cells with lots of TLRs
Activated B cells proliferate into antigen-specific clones, and may start to make and release IgM
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This response is greatest when the antigen is multivalent, cross-links many antigen receptors,
and activates complement strongly
o T-independent antigens (nonproteins) trigger strong responses this way
Most protein antigens don’t have multiple identical epitopes, so they can’t cross-link that many
receptors on B cells, and therefore don’t cause much B cell proliferation and activation
But protein antigens do cause changes in B cells that enhance their ability to interact with TH
cells
o B cell activation leads to increased expression of
 B7 costimulators - provide second signals for T cell activation and help amplify
TH responses
 Receptors for cytokines – cytokines are what TH cells release
Activated B cells decrease their expression of receptors for chemokines made by lymph follicles
o The chemokines keep the B cell in the follicle, so lack of receptors for them allow
activated B cells to migrate out of the follicles and to where the TH cells are
For a protein antigen to stimulate an antibody response, B cells and TH cells must come together in a
lymph organ, and interact to stimulate the B cell to proliferate and differentiate
TH cells that have been activated to differentiate into effector cells, interact with antigen-stimulated B
cells at the edges of lymph follicles in peripheral lymph organs
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Naïve CD4+ TH cells are stimulated to proliferate and differentiate into cytokine-producing
effector cells, when they recognize antigens on APCs (mainly dendritic cells) in lymph follicles
The antigen is displayed by class 2 major histocompatibility complex (MHC2) on APCs in T cell
rich zones of peripheral lymph organs
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CD4+ T cells can differentiate into effector cells that can make different cytokines (TH1, TH2, and
TH17)
Some of the T cells enter circulation, find microbe antigens, and get rid of the microbes through
cell-mediated immunity
Other TH cells migrate to the edges of lymph follicles at the same time as antigen-stimulated B
cells in the follicles start to leave it
o This migration towards each other depends on chemokines
o Once activated, T cells decrease expression of chemokine receptor CCR7, which
recognizes chemokines made in T cell zones, and instead increase expression of
chemokine receptor CXCR5, which promotes migration in B cell follicles
o B cells when activated, do the opposite: they decrease CXCR5 and increase CCR7
expression
B cells that bind protein antigens endocytose the antigens, process them in endosomes, and display
MHC2 peptides for CD4+ TH cells to recognize
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The antigen protein is processed into a peptide in the endosome vesicle, and the peptide then
binds to the MHC2
So B cells are APCs to TH cells
B cells and T cells recognize different epitopes of the same protein antigen
B cells can activate already differentiated effector T cells, but can’t initiate a response from a
naïve T cell
TH cells that recognize antigen presented by B cells, activate the B cells by expressing CD40 ligand
(CD40L) and by secreting cytokines
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CD40L on activated TH cells binds to CD40 expressed on B cells
o This triggers signals to the B cell to proliferate and make and secrete antibodies
o This need for CD40L-CD40 binding, ensures that only T and B cells in direct contact will
interact
At the same time, cytokines made by the TH cell bind to cytokine receptors on B cells, and trigger
more B cell proliferation and Ig making
TH cell signals also stimulate heavy chain isotype (class) switching and affinity maturation
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Usually seen in T-dependent protein antigens
Different antibody isotypes do different jobs, so class switching broadens the things humoral
immunity can do
Ex: bacteria and viruses can be opsonized by antibodies to be phagocytosed by neutrophils and
macrophage
o This is best done by IgG isotypes, which bind Fc receptors on phagocytes, specific for the
gamma (γ) heavy chain
Ex: helminthes are best eliminated by eosinophils, so you need antibodies eosinopils will bind to
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That antibody is IgE, because eosinophils have receptors for the Fc part of the epsilon (ε)
heavy chain
In both examples though, all naïve B cells specific for them express IgM & IgD antigen receptors
IgM activate complement
IgG work with phagocytes, activate complement, and cross the placenta to help the fetus
IgE fight helminthes and work in mast cell degranulation of allergy (immediate hypersensitivity)
IgA work in mucosal immunity
Heavy chain isotype switching is induced by CD40L and cytokines
o No CD40L-CD40, means B cells only can secrete IgM, and won’t be able to class switch
 X-linked hyper-IgM syndrome – mutation to CD40L gene on the X chromosome
causes nonfunctional CD40L, leading to most antibodies in serum being IgM
 Both humoral and cell mediated immunity are impaired
o Cytokines determine what class the heavy chain will switch to
 IgM making B cells have in their Ig heavy chain locus, a VDJ gene next to the first
constant region – this constant region is a CU (constant mew)
 VDJ RNA was spliced to Cu RNA, which is then translated into the u
heavy chain
 The u heavy chain then combines with a light chain to form an IgM
antibody, the first antibody made by B cells
 CD40 and cytokine receptors stimulate transcription in a constant region
downstream Cu
 Each constant region has a nucleotide sequence called a switch region
 When a downstream constant region becomes transcriptionally active,
the switch region of Cu recombines with the switch region of that
downstream constant region
o The DNA in between is deleted
 The enzyme activation-induced deaminase (AID) is needed to do this,
because it makes nucleotides able to be cleaved, and therefore be
recombined
o CD40 signals induce expression of AID
 This is all called switch recombination
o It brings the rearranged VDJ next to a downstream C region,
causing a B cells that starts making a new heavy chain isotype
that has the same specificity as the original B cell
 The specificity is determined by the rearranged VDJ
 TH cell cytokines determine which heavy chain isotype is made, by
influencing which heavy chain constant region gene participates in
switch recombination
o Ex: IFN-γ stimulates making of opsonizing antibodies
 IFN-γ is the main cytokine of TH1 cells
 IFN-γ also activates phagocytes
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 Many bacteria and viruses stimulate TH1 responses
o Ex: Il-4 stimulates switch to IgE, which works in getting rid of
helminthes, and working with eosinophils
 Il-4 is the main cytokine of TH2 cells
 TH2 cells also activate eosinophils with Il-5
o The site of the immune response also helps influence what heavy chain is made
 Ex: IgA is the major isotype made in mucosal lymphoid tissues
o Visual of heavy chain isotype switching – page 143
Affinity maturation – affinity to antigen increases with prolonged or repeated exposure to that
antigen
o Persistent or recurrent infections will show increased ability of antibodies to bind to a
microbe or antigen
o This increase is due to point mutations in the variable (V) regions, especially in the
antigen-binding hypervariable regions
o Affinity maturation only happens in responses to TH cell- dependent protein antigens
o Affinity maturation happens in the germinal centers of lymph follicles
 Some of the progeny of activated B cells enter lymph follicles and form germinal
centers
 In the germinal centers, B cells proliferate rapidly
 In culture, the germinal center stains lighter due to proliferation
o During proliferation, the Ig genes of B cells undergo many point mutations
 AID works in affinity maturation as well by changing nucleotides and making
them susceptible to things causing the mutations
o The mutation rate in Ig genes is way faster than most other genes, so it’s called somatic
hypermutation
o Germinal center B cells die by apoptosis unless they recognize an antigen, or are helped
by a T cell
o At the same time somatic hypermutation is happening, the original antibody secreted in
the earlier immune response now binds the antigen
 Antigen-antibody complexes can activate complement
 Antigen-antibody complexes are displayed by follicular dendritic cells found in
the germinal center
 Follicular dendritic cells have receptors for the Fc parts of antibodies
and complement
 B cells in germinal centers can bind the antigen on the follicular dendritic cell
o B cells can also bind free antigen ,process it, and present its peptides to germinal ecnter
Th cells, which then provide survival signals
o As the immune response to the antigen develops, especially with repeated antigen
exposure, the amount of antibody made increases
 This decreases the amount of available antigen
o The B cells selected to survive then must be able to bind antigen at lower and lower
concentrations, so these selected cells have receptors with higher and higher affinity
Mature naïve B cells recognize antigen in lymph follicles, and migrate out to find TH cells at the edges of
follicles
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This interface of the B cell-rich zones, and the T-cell rich zones, is where B cell proliferation and
differentiation into antibody-secreting cells starts
The plasma cells made from this, reside in lymph organs, usually outside the B cell-rich follicles
The antibodies the plasma cells secrete, enter the blood
Heavy chain isotype switching is initiated outside the follicles
Affinity maturation and more isotype switching happens in germinal centers formed in follicles
All of this stuff may be seen within a week after exposure to an antigen
Plasma cells that leave the germinal center migrate to bone marrow, where they can stay for
months to years, continuing ot make antibodies even after the antigen is eliminated
o It’s thought more than half the antibodies in the blood of an adult are from these longlived plasma cells – so circulating antibodies reflect each individual’s history of antigen
exposure
o These antibodies provide immediate protection if the antigen reenters the body
Some of the activated B cells, often progeny of isotype-switched high affinity B cells, don’t
differentiate into antibody secretors, and instead become memory cells
o They survive for months to years, and don’t secrete antibodies, but are ready to
respond quickly if the antigen is reintroduced
Antibody responses to T-independent (nonprotein) antigens:
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Nonprotein antigens elicit antibody responses without help from TH cells
o Nonprotein antigens can’t bind to MHC molecules, so they can’t be seen by T cells
Many bacteria have capsules rich in polysaccharides, so defense against them is mediated
mainly by antibodies that bind the polysaccharides and target the bacteria for phagocytosis
It’s thought the multivalent arrays of the same epitope on nonprotein antigens can cross-link
many antigen receptors on a B cell, strongly enough to stimulate proliferation and
differentiation without T cell help
Marginal zone B cells in the spleen are the major contributors to T-independent antibody
responses to blood-borne antigens
Once antibody-secreting cells and memory cells are produced, some survive a while, but most die by
apoptosis
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This is part of the normal physiologic decline of the immune system as we age
B cells also have an Fc receptor called FcγRII, which receognizes Fc tails of IgG antibodies in
antigen-antibody complexes
o Once activated, FcyRII receptor delivers inhibitory signals that shut off antigen-receptor
triggered signals, therefore ending B cell responses
o This is called antibody feedback, where antibody bound to antigen inhibits further
antibody making
IV IgG (IVIG) is given for inflammatory conditions, to turn off the immune