Title: Fundamentals of Acquired Immunity
1Fundamentals of Acquired Immunity
2 3- Immunology the study of all aspects of host
defense against infection and of adverse
consequences of immune response - Immunity the state of protection fro infectious
diseases utilizing both innate and acquired
mechanisms
4- Immune response - a specific and complex series
of events throughout the animals body that helps
it defend against disease-causing organisms or
substances - Antigen particular foreign molecules that
stimulate and immune response - Often immunogenic on its own, otherwise requires
a carrier molecule) - Non-self substances to which lymphocytes can
respond
5- Innate Immunity non-specific protective
mechanisms conferring basic resistance to disease
6- general, physical, chemical, and biological
barriers against disease - discussed in previous unit
- also referred to as innate or natural immunity
7II. Different Types of Acquired Immunity
8- Acquired Immunity specific protective
mechanisms displaying specificity, diversity,
memory and self/non-self discrimination - Often dependent on innate immunity for full
activation - Can be acquired actively or passively
9- Naturally acquired active immunity - An
individual comes in contact with an antigen (Ag)
and produces sensitized lymphocytes and/or
antibodies that inactivate the antigen - Ag is encountered naturally
- Immune responses are activated
- Antibodies made
- Lymphocytes sensitized
- Result Ag inactivated or destroyed
- Memory generated (long-lasting immunity)
10- Naturally acquired passive immunity - transfer of
antibodies (Abs) from one individual to another - Abs transferred from a donor to a recipient
(adoptive transfer) - Maternal transfer of IgG Abs to fetus across the
placenta - Maternal transfer of IgA Abs to newborn in
colostrom and breast milk - Newborn has immunity to infections encountered by
mother previously - Short-lived (weeks to months)
11- Artificially acquired active immunity -
deliberate exposure of an individual to a vaccine
with subsequent development of an immune response - Vaccine preparation of attenuated or killed
microbes or inactivated toxins (toxoids)
12- Artificially acquired passive immunity -
deliberate introduction of antibodies into an
individual - Abs made deliberately in another animal or
synthesized using in vitro methods - Antiserum
- Antitoxins
- Antivenom
- Monoclonal antibodies (in vitro)
13- III. Origin, Maturation and Function of
Lymphocytes
14B Lymphocytes
- Birds Bursa of fabricius
- Part of the chicken cloaca where B cells mature
- Absent in mammals
- Humans
- Fetal liver
- Bone marrow stem cells
- Two types
- T-dependent B cells (require T cell help)
- T-independent B cells (do not require T cell help)
15T Lymphocytes
- Originate in the thymus
- DiGeorge Syndrome athymic no T cells
- Specialized anatomicl locations responsible for
educating T cells - Cortex
- Medulla
- Education involves two main processes
- Positive selection
- Negative selection
16- Three types
- TH1 Inflammatory T cells
- Help macrophages (and B cells)
- TH2 Helper T cells
- Help B cells
- Tcyt Cytotoxic T cells (killer T cells)
- Kill target cells using perforin and granzyme
17Natural Killer Cells (NK cells)
- Probably derived from prethymic lymphocytes
- Do not have the characteristics of either B-cells
or T-cells - AKA Large Granular Lymphocytes (LGL)
- AKA Null cells
- Not equivalent to killer T cells but kill their
target cells using same mechanism (perforin and
granzyme) - Participate in ADCC
18 19B Cells and Their Role in Humoral Immunity
- Involvement of Ab in immune responses
- T-dependent
- T-independent
- Protects against pathogens that exist in fluid
spaces - Bacteria and their toxins
- Viruses
- Protozoans
- Helminths
- When B cells begin to produce antibody upon
activation by antigen, they differentiate into
plasma cells
20Cell-Mediated Immunity
21T Cells and Their Role in Cell-Mediated Immunity
- Involvement of T cells subsets
- TH1
- TH2
- Tcyt
- Requirement for direct interaction with infected
cell or foreign cells - Involves complex of T cell receptor (TCR), Major
Histocompatibility Complex molecule (MHC) and
Antigenic peptide fragment
22- Functions
- Lysis of host cells infected by viruses
- Elimination of cancer cells
- Productin of cytokines to assist other cells in
eradicating foreign Ags - Cytokines Molecules secreted by one cell and
affecting a different cell type ? specific
regulatory interactions - Trasplantation rejection
- Due to differences in MHC class I and class II
molecules (alloantigens)
23NK Cells and Their Role in Cell-Mediated Immunity
- Nonspecifically kill tumor cells, virus-infected
cells, and other parasite-infected cells - Play a role in regulating the immune response
- Exhibit antibody dependent cell-mediated
cytotoxicity (ADCC)
24Figure 31.20
25- IV. The Nature of Antigens
26SELF VERSUS FOREIGN
27Self
- Prior to and after birth, the immune system
develops and is able to differentiate between
self proteins and foreign proteins - T cells that are responsive to self Ags are
eliminated early in their developmental pathway - Negative selection
- Apoptosis occurs in the thymus
- Never enter periphery
28- B cells interacting with self Ags either apoptose
or anergize - Apoptosis programmed cell death
- Anergy induction of non-responsive state
29Self-Tolerance
- The process of elimination of self-reactive
lymphocytes is called self tolerance - Leads to removal of what could initiate harmful
autoreactive response Autoimmunity - Is very efficient, but not complete
30FOREIGN
31Immunogenicity
- The term immunogen is often used synonymously
with the term antigen - Careful!!!!
- All immunogens are antigens, but not all antigens
are immunogens - An immunogen is any substance that can mediate an
immune response
32- Types of Antigens (immunogens)
- Antigens recognized by B cells
- Proteins, peptides, glycoproteins,
nucleoproteins, polysaccharides, lipids,
glycolipids, and small chemical groups (haptens) - B cell receptor Antibody (Ab)
- Antigens recognized by T cells
- Peptide in association with MHC molecule
- T cell receptor TCR
33Epitopes
- Epitopes (antigenic determinant sites) are areas
of an antigen that can stimulate production of
specific antibodies and that can combine with
them - Some antigens have more than one site capable of
interacting with Abs
34- Valence - the number of epitopes on an antigen
determines number of antibody molecules an
antigen can combine with at one time - Generally the high the valency, the more
immunogenic the substance - Epitopes (determinants) that are more easily
accessible stimulate better immune response
(projections or NH2/COOH ends)
35Figure 32.3 The number of antigenic determinant
sites (epitopes) on an antigen is its valence.
36- Hapten - a small organic molecule that is not
itself antigenic but that may become antigenic
when bound to a larger carrier molecule - Examples
- Dinitrophenol (DNP)
- Penicillin
37Figure 32.4 Effect of carrier on immunogenicity
of hapten.
38- V. Structure and Function of Antibodies
(Immunoglobulins)
39 40Antibodies
- A group of glycoproteins in the blood, serum, and
tissue fluids of mammals - Produced in response to an antigen and can
combine specifically with that antigen - In the serum, four major classes can be measured
electrophoretically, but there are actually five
classes
41Figure 32.6 Electrophoresis of human serum
showing distribution of serum proteins and four
major classes of immunoglobulins.
42- Immunoglobulin structure
- Multiple antigen-combining sites (usually two
some can form multimeric antibodies with up to
ten combining sites) - Composed of four polypeptide chains (two heavy or
long, and two light or short) that form a
flexible Y with a hinge region - Heavy chain 440 aa, 50-70kDal (GAMED)
- Light chain 220 aa, 25kDal (kappa or lambda)
43- The stalk of the Y (called the Fc) is constant in
amino acid sequence (i.e., the amino acid
sequences of antibodies of the same subclass do
not vary significantly) - Constant regions are relatively invariable
between Igs belonging to the same class (
isotype, IgG, IgA, IgM, IgE, IgD) - Constant regions confer different biological
properties upon the Ig classes (in vivo half
life, anatomical location, interactions with
other molecules)
44- B cells can switch from production of one type of
Ig to another type (isotype) by a mechanism known
as ISOTYPE SWITCHING - Isotype switching is mediated by signals received
during ongoing immune responses (e.g. T cell
cytokines)
45- The arms of the Y have variable regions and
constitute the antigen-binding domains (Fab) - Each Ab has 2 Ag-combining sites (bivalent) made
up of H and L chain regions encoded by the
variable portion of the Ig (VLVH) - Some Abs exist as dimers (IgA) or pentamers (IgM)
and so have 4 or 10 Ag-binding sites, respectively
46- Intrachain disulfide bonds create loops on both
heavy and light chain - Each loop contains 25 aa making up a single
domain - Interchain disulfide bonds hold the heavy and
light chains together - H to H
- H to L
47- Within the Fab segment are hypervariable (or
complementarity determining) regions - these regions are responsible for the diversity
of antibodies
48Figure 32.8 Constant and Variable Domains in
Heavy and Light Chains Dark blue
hypervariable regions in the variable
domains Hypervariable regions are also known as
complementarity determining regions (CDRs).
49Pockets Grooves Extended Surfaces Peptide Ag
Red HIV peptide Orange HEL F(ab) frag. 5
CDRs used Red Ab contact sites Blue
Backbone Yellow Ag
50- There are five types of heavy chains that
determine the five classes (isotypes) of
immunoglobulins (IgG, IgA, IgM, IgD, and IgE) - In IgG there are four subclasses, and in IgA
there are two subclasses
51- Categories of immunoglobulin types
- Isotypes - variations in the constant regions of
heavy chains that are associated with different
classes and subclasses - Allotypes - genetically controlled allelic forms
of the immunoglobulin molecule - Idiotypes - individual-specific immunoglobulin
molecules that differ in the hypervariable
regions of the Fab segments
52- Review of Immunoglobulin Function
- Fab region binds to antigen
- Fc region mediates binding to host tissue, to
various cells of the immune system, to some
phagocytic cells, or to the first component of
the complement system
53- Binding of antibody to an antigen does not
destroy the antigen, but marks (targets) the
antigen for immunological attack or attack by the
nonspecific defense mechanisms that do destroy it - Opsonization - coating a bacterium with
antibodies to stimulate phagocytosis
54Figure 31.15
55- Immunoglobulin classes (Isotypes)
- IgG - monomeric protein, 70 to 75 of Ig pool
- Antibacterial and antiviral
- Enhances opsonization
- Neutralizes toxins
- Only IgG is able to cross placenta (naturally
acquired passive immunity for newborn)
56- Activates the complement system by the classical
pathway - Four subclasses with some differences in function
- IgG1
- IgG2
- IgG3
- IgG4
57- IgM - pentameric protein, 10 of Ig pool
- First antibody made during B-cell maturation
- First antibody secreted into serum during primary
antibody response - Never leaves the bloodstream
- Activates complement by classical pathway
- Enhances phagocytosis of target cells
58- Agglutinates bacteria and foreign red blood cells
- Up to 5 may be hexameric which is better able to
activate the complement system than pentameric IgM
59Figure 32.11
60- IgA - dimeric protein, 15 of Ig pool
- Associated with secretory mucosal surfaces
- Protects gastrointestinal tract, respiratory
tract, and genitourinary tract - Also found in saliva, tears, and breast milk
(protects nursing newborns) - Secretory form (sIgA) helps rid the body of
antibody-antigen complexes by excretion into the
gut lumen and subsequent excretion from the body - Adherence Inhibition
61Figure 32.12
62- IgD - monomeric protein, trace amounts in serum
- Abundant on surface of B cells
- May play a role in B-cell recognition of antigens
- May play a role in B cell anergy
- Does not activate the complement system
- Cannot cross the placenta
63Figure 32.13
64- IgE - monomeric protein, less than 1 of Ig pool
- Skin-sensitizing and anaphylactic antibodies
- When an antigen cross-links two molecules of IgE
on the surface of a mast cell or basophil, it
triggers release of histamine, and it increases
intestinal motility, which helps to eliminate
helminthic parasites
65Figure 32.14
66 67- A number of mechanisms contribute to the
generation of antibody diversity - Somatic recombination (gene shuffling)
- Combinatorial joining (unique H and L chains)
- Somatic Hypermutation
- Isotype switching
68- Somatic Recombination
- Ig genes contain multiple segments of variable
encoding regions of heavy and light chains (VDJ
for heavy VJ for light) - During differentiation of B cells, these genes
are rearranged on the chromosome to form various
combinations - The number of different antibodies possible is
the product of the number of light chains
possible and the number of heavy chains possible
69- Imprecise joining - during combinations, the same
segments can be joined at different nucleotides,
thus increasing the number of codons and the
possible diversity (N and P nucleotides) - Somatic mutations - the V regions of germ-line
DNA are susceptible to a high rate of somatic
mutation following B cell activation - The total diversity produces more than 2x108
different antibody molecules
70Fig. 32.15
71Figure 32.16
72Figure 32.17
73- Specificity of Antibodies
74- The Clonal Selection Theory
75- Because of somatic recombination, combinatorial
joining and somatic mutation, there are a small
number of B cells capable of responding to any
given antigen
76- Each group of cells is derived asexually from a
parent cell and is referred to as a clone - There is a large, diverse population of B-cell
clones that collectively are capable of
responding to many possible antigen - The surface receptor molecules of the B cells
bind to the appropriate antigen
77- The cell is then stimulated to divide and
differentiate into two populations of cells - Plasma cells
- Memory cells
- Plasma cells are protein factories that produce
about 2,000 antibodies per second for their brief
life span (5-7 days)
78Figure 32.18
79- Memory cells, like the original B cells, can
differentiate into plasma cells if they are
stimulated by being bound to the antigen - because there are more memory cells than original
B cells, the secondary (anamnestic) response can
be (and usually is) faster and larger than the
primary response - they have long life spans (years or decades)
80- Sources of Antibodies (pure homogeneous
preparations)
81 82- Procedure
- Inject animal with antigen to stimulate primary
immune response - Initial lag phase of several days
- Log phase antibody titer rises logarithmically
- Plateau phase antibody titer stabilizes
- Decline phase antibody titer decreases because
the antibodies are metabolized or cleared from
the circulation - Mostly IgM low-affinity antibodies
83Fig. 32.19
84- After a period of time, give animal a series of
booster injections with same antigen to stimulate
secondary immune response, or anamnestic response - Shorter lag phase, higher antibody titer
- Mostly IgG high-affinity antibodies (affinity
maturation) - Withdraw blood from animal allow it to clot and
remove fluid (serum), which is referred to as
antiserum since it is from a specifically
immunized host
85Fig. 32.19
86- Limitations
- This method results in polyclonal antibodies
which have different epitope specificities thus
sensitivity is lower, and the antibodies often
cross-react with closely related antigens - Repeated injections with antiserum from one
species into another can cause serious allergic
reactions - Antiserum contains a mixture of antibodies, not
all of which are of interest
87- Hybridomas - overcome some of the limitations of
antisera - Inject animals with antigen
- Separate spleen cells (which contain plasma
cells) - Fuse spleen cells with myeloma cells (tumor cells
of the immune system that produce large
quantities of antibodies and that are easy to
culture)
88- Culture fused cells (hybridomas) so that each
grows into a separate colony - Some plasma cells that fused with a myeloma cell
will produce the desired antibody - Screen colonies for those producing desired
antibody
89- Can grow many desired colonies to obtain large
amounts of antibody - Antibodies produced by this method are monoclonal
(react with only one epitope) since they come
from the fusion of a single plasma cell with the
tumor cell
90Figure 32.20
91- Have a variety of uses in which high specificity
is required - Tissue typing for transplants
- Identification and epidemiological study of
infectious microorganisms - Identification of tumor and other surface antigens
92- Classification of leukemias and T-cell
populations - Sensitive diagnostic procedures
- Immunotoxins - used in the targeted delivery of a
toxic substance to a particular cell type
chemotherapeutic agents
93Box 32.1
94 95- They are monoclonal antibodies made by subjecting
an animal to an enzyme-substrate transition-state
analogue and then producing a hybridoma - The binding pocket on the antibody lowers the
energy of activation of a reaction by ensuring
the proper orientation of the reactant(s)
96- Currently, catalytic antibodies have been
produced that can transform relatively simple
compounds, but the potential is great if
antibodies that act on proteins and nucleic acids
can be produced