2. HUMORAL IMMUNITY
• RECALL: mediated by antibodies AND functions to neutralize and eliminate
extracellular microbes and microbial toxins
• e.g. microbes with capsules rich in polysaccharides and lipid toxins
• B-cells respond to and produce antibodies specific for many types of molecules
(vs T cells = only protein antigens)
• WHAT ARE WE TO KNOW: process and mechanisms of B-cell activation and
antibody production
• How are receptor-expressing B-lymphocytes activated and converted to
antibody-secreting cells
• How is the process of B-cell activation regulated so that the most useful
types of antibodies are produced in response to different types of microbes
Thursday, July 19, 2012
5. TD and TI ANTIBODY RESPONSES
• RECALL: based on the requirement for T-cell help
• T-dependent antibody response: Protein antigens are processed in antigen-
presenting cells and recognized by helper T-lymphocytes, which play an important
role in B cell activation PLUS powerfuI inducers of heavy chain class switching
and affinity maturation
• In the absence of T cell help, protein antigens elicit weak or no
antibody responses
• T-independent antibody response: Polysaccharides, lipids, and other nonprotein
antigens stimulate antibody production without the involvement of helper T cells
• Antibodies show relatively little heavy chain class switching and
affinity maturation
Thursday, July 19, 2012
8. So how do antigens
stimulate B-
lymphocytes?
Thursday, July 19, 2012
9. ANTIGEN RECEPTOR-MEDIATED SIGNAL
TRANSDUCTION IN B-LYMPHOCYTES
• RECOGNITION &
INITIATION: antigen-specific
B-cells in the lymphoid
follicles of the spleen, lymph
nodes and mucosal lymphoid
tissues recognize antigens
• membrane-bound Ig
receptors recognize
antigen in their “native
conformation” (without a
need for processing)
• SIGNAL TRANSDUCTION:
Antigen-induced clustering of
membrane Ig receptors
trigger biochemical signals
that are transduced by
receptor-associated signaling
molecules
Thursday, July 19, 2012
10. THR ROLE OF COMPLEMENT PROTEINS IN
B-CELL ACTIVATION: C3d (“second signals”)
B-cells express a receptor for a
protein of the complement
system that provides signals for
the activation of the cells
(NOTE: C3d = analogous to co-stimulators of T-cells)
Thursday, July 19, 2012
11. FUNCTIONAL CONSEQUENCES OF Ig-
MEDIATED B-CELL ACTIVATION
Note: antigen stimulation induces the
early phase of the humoral immune
response
= response is reatest when:
g
a) antigen is multivalent
b) antigen cross-links many antigen
receptors
c) antigen activates complement
strongly
= typical of polysaccharides and
other TI antigens
Thursday, July 19, 2012
12. The Function of
Helper
T-lymphocytes in
Humoral Immune
Responses to
Protein Antigens
Thursday, July 19, 2012
13. T-CELLS IN B-CELLS FUNCTIONING
•Efficiency: protein antigens elicit excellent
antibody responses within 3-7 days of antigen
exposure
•Helper T-cells that have been activated to
differentiate into effector cells interact with
antigen-stimulated B-lymphocytes at the edges of
lymphoid follicles in the peripheral lymphoid
organs
Thursday, July 19, 2012
17. ISOTYPE SWITCHING
Helper T-cells stimulate the progeny of IgM+ IgD
expressing B-lymphocytes to produce
antibodies of different heavy chain classes
(isotypes) initiated by CD40L-mediated signals
and other cytokines
Thursday, July 19, 2012
18. CLINICAL IMPORTANCE OF CLASS
SWITCHING:
•In the absence of CD40 or CD40L, B-cells secrete only
IgM and fail to switch to other isotypes
• X-linked hyper-IgM syndrome: inactivating mutations in
the CD40L gene, located in the X chromosome
• much of the serum antibody is IgM
• defective heavy chain class switching
• Patients with defective CMI versus intracellular
microbes
Thursday, July 19, 2012
19. MECHANISM OF HEAVY CHAIN CLASS
SWITCHING
WHAT YOU NEED TO
KNOW:
IgM-producing B-cells, which
have not undergone switching
contain a rearranged VDJ gene
adjacent to the first constant
region cluster (Cu) in their
heavy chain locus
Splicing of VDJ RNA to Cu
RNA = heavy chain mRNA
heavy chain mRNA =
translated to u heavy chain
= combines with a light
chain producing IgM
***IgM antibody (1st
antibody produced)
Thursday, July 19, 2012
20. MECHANISM OF HEAVY CHAIN CLASS
SWITCHING
WHAT YOU NEED TO
KNOW:
a) constant region
downstream of Cu: where
stimulation of transcription via
signals from CD40 and
cytokine receptors occurs
b) switch region: conserved
region within the constant
region except for C-theta
SWITCH
RECOMBINATION: 3’ of
Cu recombines with 5’ of
switch region = all intervening
DNA is deleted
Thursday, July 19, 2012
21. MECHANISM OF HEAVY CHAIN CLASS
SWITCHING
WHAT YOU NEED TO
KNOW:
RESULT OF SWITH
RECOMBINATION
B-cells begins to
produce a new heavy
chain class (depending
on C region of the
antibody)
- IgG, IgE
Same specificity as the
original B-cell
Thursday, July 19, 2012
22. CYTOKINES & HEAVY CHAIN CLASSES
• Cytokines produced by helper T-cells determine which heavy chain
class is produced by influencing which heavy chain constant region
gene participates in switch recombination (T-cells as master
controllers of immune responses)
• e.g. OPSONIZING ANTIBODIES: effect of IFN-g on B-cells complement
the actions of this cytokine on phagocytes
• their role: bind to phagocyte Fc receptors & promote phagocytosis
• stimulated by: IFN-g of TH1 cells
• e.g. IgE
• stimulated by IL-5, a TH2 cytokine
Thursday, July 19, 2012
23. SITE OF IMMUNE RESPONSE & HEAVY
CHAIN CLASSES
• nature of antibody class may also be
influenced by site of immune
response
• e.g. IgA: major isotype
produced in mucosal lymphoid
tissues
• RATIONALE: mucosal tissues contain
large number of B-cells able to switch
to IgA and helper T-cells whose
cytokines stimylate switching to IgA
• NOTE: IgA is the principal antibody
class that can be actively secreted
through mucosal epithelia
Thursday, July 19, 2012
24. AFFINITY MATURATION
• process by which the affinity of antibodies
produced in response to a protein antigen
increases with prolonged or repeated
exposure to that antigen
• advantage: ability of antibodies to bind
to a microbe or microbial antigen
increases if the infection is persistent or
recurrent
• occurs in the germinal centers of lymphoid
follicles
• result of somatic hypermutation of Ig genes in
dividing B-cells followed by the selection of
high-affinity B-cells by antigen displayed by
follicular dendritic cells (FDC)
Thursday, July 19, 2012
25. STAGES OF TD ANTIGENS HUMORAL IMMUNE
RESPONSE (note different anatomic compartments)
Thursday, July 19, 2012
26. REGULATION: ANTIBODY FEEDBACK
MECHANISM
SECRETED
ANTIBODIES FORM
IMMUNE
COMPLEXES WITH
RESIDUAL ANTIGEN
AND SHUT-OFF B-
CELL ACTIVATION BY
ENGAGING AN
INHIBITORY Fc
RECEPTOR ON B-
CELLS
Thursday, July 19, 2012
29. SIGNIFICANCE OF HUMORAL IMMUNITY
• RECALL: mediated by antibodies and important for protection against extracellular
microbes and their toxins = PREVENTION
• only antibodies can mediate this function
• by blocking the ability of microbes to bind to and infect host cells
• by binding to microbial toxins and prevent them from damaging host cells
Thursday, July 19, 2012
30. SIGNIFICANCE OF HUMORAL IMMUNITY
• RECALL: mediated by antibodies and important for protection against extracellular
microbes and their toxins = PREVENTION
• only antibodies can mediate this function
• by blocking the ability of microbes to bind to and infect host cells
• by binding to microbial toxins and prevent them from damaging host cells
• ADDITIONAL FUNCTIONS: eliminate microbes, toxins and infected cells from the body
• NOTE: even if they cannot KILL intracellular microbes, they can bind to these
microbes even before they can enter the host cells and thus preventing infections
Thursday, July 19, 2012
31. SIGNIFICANCE OF HUMORAL IMMUNITY
• RECALL: mediated by antibodies and important for protection against extracellular
microbes and their toxins = PREVENTION
• only antibodies can mediate this function
• by blocking the ability of microbes to bind to and infect host cells
• by binding to microbial toxins and prevent them from damaging host cells
• ADDITIONAL FUNCTIONS: eliminate microbes, toxins and infected cells from the body
• NOTE: even if they cannot KILL intracellular microbes, they can bind to these
microbes even before they can enter the host cells and thus preventing infections
• most VACCINES: STIMULATES ANTIBODY PRODUCTION
Thursday, July 19, 2012
32. PROPERTIES OF ANTIBODIES THAT
DETERMINE EFFECTOR FUNCTIONS
• antibodies may function distant from their sites of
production (so that they perform their functions
throughout the body)
• protective antibodies are produced during the first response
and in larger amounts during subsequent responses
• antibodies use their antigen-binding region (Fab) to bind to
and block the harmful effects of microbes and toxins while
their Fc region is used to activate diverse mechanisms
that elimante these antigens and toxins
• heavy chain class switching and affinity maturation enhance
the protective functions of antibodies
Thursday, July 19, 2012
33. IMPORTANT TO KNOW
•What are the mechanisms used by circulating antibodies
to combat different types of infectious agents and their
toxins?
•What is the role of complement system in defense against
microbes?
•How do antibodies combat microbes that enter via the
GIT and RT?
•How do antibodies protect the fetus and newborn from
infections?
Thursday, July 19, 2012
36. NEUTRALIZATION OF MICROBES & TOXINS
BY ANTIBODIES
Antibodies bind to
and block
(NEUTRALIZE) the
infectivity of microbes
and the interactions of
microbial toxins with
host cells
Neutralization does
not allow an infection
to take hold
Thursday, July 19, 2012
37. ANTIBODY-MEDIATED OPSONIZATION &
PHAGOCYTOSIS
opsonization:
antibodies
coat microbes
and promote
their ingestion
by phagocytes
opsonins:
molecules that
coat microbes
Thursday, July 19, 2012
38. ANTIBODY-DEPENDENT CELLULAR
CYTOTOXICITY (ADCC)
• natural killer cells (NK) and other
leukocytes may bind to antibody-
coated cells and destroy these cells
• important in helminth infections
• helminths are too large to be
phagocytosed
• thick teguments resistant to
substances released by
phagocytes (neutrophils and
macrophages
• IgE and eosinophil tandem
Thursday, July 19, 2012
43. REGULATION OF COMPLEMENT ACTIVATION
• mammalian cells express regulatory proteins that inhibit complement activation,
thus preventing complement-mediated damage of host cells (adaptation of
mammals)
• a) decay accelerating factor (DAF); disrupts the binding of Factor B to C3b or
the binding of C4b2a to C3b thus terminating compelemnt activation by both
the alternative and the classical pathways
• b) membrane cofactor protein (MCP): serves as cofactor for the proteolysis
of C3b into inactive fragments (mediated by Factor 1)
• c) Type 1 complement receptor: same functions as MCP
• d) C1 inhibitor (C1 INH): stops complement activation early (at stage of C1)
• ABSENCE OF REGULATION: hypersensitivities; immunodeficiencies, etc
Thursday, July 19, 2012
49. HYPERSENSITIVITY TYPE 1: Anaphylactic
hypersensitivity
IgG antibodies against the Fc portions of IgE that binds to mast cells
has been approved for treatment of certain allergies, as it can block
mast cell sensitization
Thursday, July 19, 2012
54. MUCOSAL IMMUNITY
• immunity when IgA produced in mucosal lymphoid tissues are actively transported
across epithelia and binds to and neutralizes microbes that enter through mucosal
organs
• IgA: 60-70% of 3g of antibody produced by a health adult in his intestines
• ORAL POLIO VACCINE
Thursday, July 19, 2012
