Immune System 3rd LineSpecific Immunity(Line3)-characterized by specificity, inducibility, clonality, memory, and
tolerance.
The Lymphocyte response in depth:
#1-4
Overview:
I. Development of Dual lymphocyte systemStem cells diverge into 2 populations. B lymphocytes-bone marrow (born in and educated the
same place-home school) which are involved in humoral immunity (antibody production) and Tlymphocytes (born in bone marrow and then move out to thymus to become educated). Tinvolved in CMI (cell mediated immunity). Education involves becoming committed to
responding to one antigen. Gene shuffling-build the T and B receptors on these cells during the
education process. Also, lymphocytes that respond to self antigens are eliminated(tolerance).
Resulting cells are now immunocompetent (primed to react should they encounter their
partner antigen). Once educated, lymphocytes migrate to regional lymphoid tissues. Here
they provide a lifelong source of immune cells. B lymphocytes secrete protective antibodies. T
lymphocytes help B-cells and also kill host cells that harbor pathogens or have become
cancerous.
II. Entrance and Processing of Antigens and Clonal selectionWhen foreign cells enter fluids of host they encounter blood, lymph and RES. Here they
encounter a battery of cells that screen, trap, and eliminate. Macrophages/Dendritic cells
usually first to react. Ingest, process at the site of infection. Then they migrate (it takes about a
day) to the lymph tissue (especially nodes) and present the antigens to the B and T
lymphocytes. This presentation (usually along with additional signals) activates a specific
lymphocyte. Usually activation of a B lymphocyte requires assistance of T-helper cells.
III. Activation of Lymphocytes and Clonal expansion
Challenge by ag stimulates B and T cells to differentiate and proliferate. The response is so
powerful that it could kill normal body cells if the process were not properly regulated.
Activation requires multiple signals in the right sequence (binding ag and lymphokines that bind
to receptors on cell's surfaces for example). Nuclear launch analogy. If activated, lymphocytes
forms clone, some of which are memory cells, and others effector cells (B-plasma cells for
example)
IV. Activated B lymphocytes and their products-Humoral Immunity
B cells found in spleen, MALT, lymph nodes, and a small % in the blood. Active progeny of B
cells are effector plasma cells (no surface receptors, they secrete proteins). Synthesize and
secrete antibodies into fluids (tissue fluids, lymph, and blood-humoral). Antibodies mark
antigens for destruction or neutralization. Memory cells also produced.
V. Activated T lymphocytes and CMIT cell types and responses are extremely varied. T cells- found in the blood (70-80% of the
circulating lymphocytes are T cells) and lymph nodes. T-cells only respond to AGs that are part
of cell surfaces. Sensitization by antigen causes T cell to differentiate into either: 1) T helper-2)
suppressor cells 3) cytotoxic cells. T cells produce lymphokines but no secreted antibodies.
Essential preliminary concepts to understand specific immunity (3rd
line of defense)
1. cell surface molecules (MHC,CD, B and T cell receptors)
2. Cytokines
3. antigens
4. antibodies.
1. Cell surfaces- molecules (MHC antigens, CD receptors, and B and T cell receptors).
Several different types of receptors on each cell. Major function of receptors are 1) perceive
and attach to non-self (antigens) 2) help in recognition of self 3) receive and transmit chemical
messages among other cells of the system 4) aid in cellular development. Major cells with
receptors are lymphocytes and macrophages/dendritic cells
#5
A. MHC molecules
MHC surface molecules (antigens) are glycoproteins on all cells except rbc. Also known as HLA
antigens (first discovered on leukocytes). MHC molecules play a key role in recognition of
foreign vs. self and are key players in the process of antigen presentation (dendritic, macro,
b-cell). The MHC holds and positions the antigen fragments for presentation to other immune
cells.
#6
2 routes of antigen processing . Intracellular pathogens class I, extracellular class II.
Class I-On all cells except rbc. First recognized because they regulate acceptance or rejection of
tissue grafts. Each person inherits a unique set of class I genes. Closer people related, more
likely it is that their MHC profile is similar. Individual differences exist however. You have
enough unique Class I antigens that your tissues are likely to be antigenic to others. Each
nucleated cell has a fragment (epitope) of every cell polypeptide it produces (including viral and
cancer peptides) displayed on its surface as part of an MHC I so it can be shown to other
immune cells. For our purposes- important in elimination of virus infected/cancerous cells.
#7
Class II-code for immune regulatory receptors. Found only cell surface of APC (macrophages,
dendritic cells, B, and T-helper cells). If an AG can’t be displayed with a MHC II, it can’t trigger
an immune response. Thus, MHC’s determine which Ag fragments might trigger an immune
response. When a pathogen is phagocytized, pieces of it are loaded onto an MHC II receptor
and shipped to the cell surface. This cell can then present ag to other immune cells.
T cells are MHC restricted-Class I, T-killer, Class II, t-helper.
#8
B. CD molecules-Lymphocytes have additional receptors that define cell type. Various
lymphocytes often just referred to as CD-8 (for example)
C. B and T Cell Receptors:
#9
B cell receptor structure- Ig-G model. 2 heavy and 2 light chains. Light chain is kappa or
lambda. Heavy is one of five types (A-E see below). Hinge region between the light and heavy
chains (allows swiveling of Fab region to fit angle of antigen better). Hypervariable region
(model of ab-this is the hands). Igg has a valence of 2, allows cross linking.
B cell receptors are unique (compared to T cells) in that they have the same basic structure
(minus the membrane anchoring piece in the B-cell) as the antibody molecule that B cells
secrete. This is not the case for T-cells because they secrete no antibody. Each receptor
recognizes on one specific antigen. 50,000 copies of same bcr/cell
#10
T-cell receptor structure (similar to B except one heavy and one light chain)- Binding of ag to
tcr/mhc complex activates a cascade that triggers cell development and cytokine production.
TCR complex has two components, one that recognizes self (MHC) and one that recognizes
foreign (ag bind site). The receptor only recognizes an epitope if it associated with an MHC
receptor (restricted). T cells can’t bind free ag, only antigens that are prat of cell surfaces.
How are the B and T cell receptors (and antibodies) made?
How many foreign antigens will a person encounter in their life? Potentially, billions. Each
antigen will need a cell with receptors for that specific antigen (1 ag/receptor). How many
different types of immune cells are there?
Possibilities?
A. One (e very immune cell has receptors on its surface for every different antigen)? Nonot enough room on the cell surface for all of those different receptors (even one copy
of each).
B. There billions of different types of cells each with specificity for a different antigen?
Yes-it has been shown that every B or T lymphocyte carries a unique receptor for a
unique antigen.
Generation of Receptor Diversity:
So where do all the diverse cells with unique receptors come from?
The progenitors of all B and T cells are stem cells. How do stem cells have enough genes to pass
on to their offspring to code for all of this diversity of receptors? It is estimated that there are
1023 different types of B and t cells each with a unique receptor. Does that mean that stem
cells have that many genes that code for receptor structure and that they pass these onto their
offspring? No-there are only about 20-25,000 genes in the human genome.
In reality, there are slightly more than 200 genes in stem cells that code for lymphocyte
receptors. How do 200 genes give rise to 10 23 receptors?
As part of their education process in bone marrow (B cells) and thymus (t-cells) the
lymphocytes randomly recombine segments from 3 immunoglobulin regions in DNA (like
shuffling a deck of cards)
#11
Lymphocyte development involves shuffling blocks of genes (while B cells are developing in
bone marrow or T-cells developing in Thymus). RSS sites (recombination signal sequence) are a
key to this process. An enzyme called RAG (recombination activating gene protein) randomly
combines one of each kind of various segments to form a BCR gene. Segregation,
rearrangement and assembly results in 2 complete transcriptional units-one that codes for an H
chain and one that codes for a light. This phase of development is antigen independent.
#12
The result is 5 X 10 6 receptor combinations. Where does the remaining diversity come from to
get to 10 23? (that one hundred billion trillion -10 X the stars in the universe)
A. RAG randomly removes portions of D and J segments before joining them together
B.
Another enzyme randomly adds nucloetides to the heavy chain VDJ combination.
C.
B cell genes in the lymph node populations udergo rapid mutation in the V-regions
D. As memory cells divide, they become increasingly adapted to better fit antigen during
clonal selection
#13
Isotype class switch –
Once a cell has undergone gene shuffling, it always produces an antibody/receptor with the
same antigen specificity. However, it can produce several classes of antibody with that same
specificity (class switching) depending on circumstances. This involves associating a new Fc
region (5) with the other gene products.
The most common class switch is from IGM to IGG. Some lymphocytes then switch IGG to IGA.
Making memory B- cellsMemory are B-cells that have already undergone switching. Long lived. B-cells that haven’t
class switched produce suicide protein (apoptosis). After switching, cells produce an antisuicide protein. During their life time memory cells hyper mutate.
#14,15
Once synthesized the IG is transported to cell surface and acts as a receptor. Now the cell is
antigenically committed but it is still immature (naïve cell). It needs additional signals to give
rise to effector and memory cells. Upon stimulation by appropriate antigen/cytokine, B cells
form plasma and memory cells and plasma cells secrete antibody with same antigenic
specificity as receptor. In order for this to happen, a minimum of one signal is required (ag
binding). Usually a second signal is also required (lymphokines).
# 16,17
Clonal DeletionSince T ( and B) cell receptors are generated randomly (in the thymus and bone marrow
respectively), it’s inevitable that some cells will be formed that will react autoantigens. Body
eliminates these through clonal deletion (in thymus and bone marrow). Exposure of these cells
to autoantigens stimulates them to undergo apoptosis. If this doesn’t happen properly, you
can have autoimmune disease.
# 18
Clonal Selection- When then surviving non-self antigen binds to its assigned lymphocyte, it
stimulates that cell to divide many times to form a clone of identical cells (these then form
effector cells and memory cells). This process is called clonal selection.
2. Cytokines-soluble regulatory proteins that act as intracellular messengers
Most immune cells won’t react to antigens without receiving the appropriate chemical signals
(along with stimulation by an antigen). Many cytokines are redundant and form a complex
cytokine network.
#19
a. Interlukins signal among leukocytes. Designated in order of discovery (35 identified to date).
b. Interferons-(gamma secreted by T-helpers activate macrophages).
c. growth factors-stimulate leukocyte stem cells to divide. Body can control progression of
immune response by limiting the production of growth factors.
d. Tumor necrosis factor-Macrophages and T cells secrete to kill tumor cells and regulate the
immune response and inflammation.
e. chemokines-stimulate leukocytes to moves to site of infection (chemotaxis)
#20-Selected immune cytokines
3. Antigens:(Antibody generating molecules).
Antigenicity has special requirements. Size, shape, and complexity key elements. Molecules
with complex composition (proteins, glycoproteins) are more immunogenic than large
repeating polymers. MW must be > 1000 to stimulate abs.
#21
Lymphocyte responds to a small piece of Ag molecule (ag determinant).
Many complex molecules have many ag determinants (mosaic antigens). These stimulate many
different types of lymphocytes (polyclonal response). Examples include components of bacterial
cell walls, capsules, pili, flagella, toxins, and capsids of virus.
#22
Haptens-small foreign molecules (drugs, metals, env. chems.) that are too small to elicit
immune response (an example of an individual ag determinant). Immunogenic if conjugated to
a larger carrier (serum proteins)-allergies.
#23
Types of Ags-(many are from human sources)1) autoantigens (self)- may be hidden when immune system is developing. (eye). If later
exposed, may stimulate abs (autoimmune). Basis of rheumatoid arthritis?
2) alloantigens-found on cells of some members of species but not others. Basis of blood
grouping and MHC profiles. Tissue rejection.
3) Heterophile ags-same ag found in different species (cross reactions-false serological tests).
Grp A strep and heart valve tissue (rheumatic fever). Homologous ag-that which induces the ab
(strep), heterologous ag, other which react with the induced ab (heart valve)(cross reactions)
4) Allergens
5) Superantigens-Potent stimulators of T-cells. (enterotoxin of Staph). Cross link T cell
receptors and trick T-cells to release massive amounts of cytokines. Damages blood vessels and
cause multi organ failure. Basis of toxic shock syndrome.
6) T-dependent and T-independent Ags:
A. T independent ag's are in the minority.
They can directly stimulate B cells to secrete ab. Only 1 signal required for this . These are
usually simple molecules with repeating units (CHO's). They activate B cells to to make IG-m
only (no memory response). Why? No T-helper involved so no interlukins involved (second
signal). Thus no class switching (IGM to IGG)because that is mediated by by IL-4.
B. T-dependent-Majority of ags.
These antigens can only activate B cells if a T-helper is involved. Remember, T-helper are MHC
II restricted. They can only react with an antigen presented to them by an APC. This process
involves internalization and presentation of ag by APC .
Antigens stimulate antibodies.
4. Antibodies
#24
AB structure- (just like B-cell receptor except secreted) Ig-G model. 2 heavy and 2 light chains.
Light chain is kappa or lambda. heavy is one of five types (see below). Hinge region between
the light and heavy chains (allows swiveling of Fab region to fit angle of antigen better).
Hypervariable region (model of ab-this is the hands). Igg has a valence of 2, allows cross linking.
Fc region-effector region. Fc binds to receptors on body's cells (granulocytes and
agranulocytes). Effect antibody binding has on cell depends on type of cell (if it is a mast cell it
may degranluate, if it is a macrophage it may stimulate phagocytosis). Fc region influences
permeability of ab (crossing placenta, distribution of ab (is it secreted). It also may or may not
fix complement (attach).
Fc region determines class which antibody belongs to.
Threats confronting the immune system are variable so its not surprising that there are several
different classes of antibodies to respond to these varied threats. Class of Ab produced
depends on the type of invading antigen, portal of entry, and the antibody function required
(determined by the Fc region of the antibody).
#25
Roles of ABs- (why make Abs handout)-react with ags and immobilize them, tags antigens for
destruction (opsonins-kick me), neutralizes (fills surface receptors on virus and alters structures
of toxins), stimulates cell destruction (turns on complement system), agglutinates (basis of
many serological tests), killing by oxidation (antibodies catalyze reactions that produce
substances that kill bacteria (H2o2, ozone), and antibody cellular toxicity (Fc regions bind to NK
cells, these secrete perforin (forms channels in cell membrane) by which granzymes enter,
these cause apoptosis). This is similar to opsonins in that ab’s coat the surface of pathogens.
However, opsonins result in phagocytosis of the antibody coated pathogen rather than
apoptosis.
Some abs can cause disease (autoantibodies).
#26
5 Classes of Abs (isotypes)-Type of AB produced depends on type of Ag, portal of entry, and
antibody function required
1. M-(macro)-1st class secreted by B cells. Found mainly in blood. Most effective in fixing
complement also an opsonin. Once it produces IG-M, a plasma cell (if it gets the right signal)
may recombine its variable region with a new Fc region (class switching) to produce a new class
of Ab. The most common class switch is to produce IG-G. This only happens in response to Tdependent antigens.
2. G-produce by activated memory cells. Makes up majority of class of proteins in blood serum
called gamma globulins (80%). Longest lasting. Half-life is 23 days versus 4-5 for other classes.
Neutralizes toxins, opsonin, fixes complement, antibody dependent cell toxicity, and only one
that can cross the placenta. It also can easily leave the blood and enter extracellular space so it
can bind to the pathogen before it enters the circulatory system.
3. A-2 forms-serum (12% of blood antibodies) and secretory. Secretory form prevents binding
of ags to mucosal surfaces. Found in mucous secretions of salivary glands, intestine, nasal
membranes, breast, lung, and urogenital tract. Secretory piece added by gland itself. Protects
ab against digestive enzymes. Most important in local immunity to enteric and urogenital
pathogens. Also secreted into milk. Total amount of secretory A produced is greater than of
serum IG-G.
4. D-basically a BCR. Triggers B cell activation and involved in immune suppression. Not all
mammals produce this. Role?
5. E- Fc region reacts with eosinophils, trigger release of cell damaging molecules onto the
surface of cells (especially parasitic worms). Also binds to basophils and mast cells receptors to
cause degranulation. Initiates inflammatory response, mediates allergies.
In developed countries, IG-E more often associated with allergies than parasitic worms.
Primary vs Secondary Immune response.
#27
Primary response-ag is being concentrated in lymphoid tissues, processed by APC’s, and b
synthesis finally begins. Anamenestic response yields 1000X amount of ab as primary. Faster
because memory B cells don’t have to go through the early steps of activation and require
fewer signals to form plasma cells.
#28
Types of Immunity
1. Active-Immunological stimulus activates T and B lymphocytes-results in antibody release.
Characterized by memory and is long lasting. Takes a long time to develop. Naturalbiological experience (subclinical can stimulate protection). Artificial-induced medically
(vaccines and immune serum). Vaccines for 24 diseases.
2. Passive-receive antibodies produced by a donor-no memory, immediate protection, short
lived. Natural-maternal antibodies-up to one year. Breast milk-intestinal protection not
available otherwise. Artificial-horse serum, injected with toxins. Pooled gamma globulins
(from recovering patients)-rabies, and hepatitis.
CombinationsNotes-natural active-Many viral diseases cause life-long immunity. Other pathogens immunity
that last a few months to years. Even a subclinical infection can stimulate active immunity.
Part B-Line 3-Specific Immunity-Interplay of B/T lymphocytes and macrophages
#29
2 Branches to Immune System-Humoral and Cell Mediated. Lots of overlap between two
branches. T-helper cells are involved in both Humoral and CMI.
#30
I. Cell Mediated Immunity
Cell mediated- The body uses cell-mediated immune responses to fight intracellular pathogens
such as viruses that have invaded body cells, as well as abnormal body cells such as cancer cells.
CMI respond to intracellular pathogens by activating T-killers . T cells have a unique T-cell
receptor and are MHC restricted (they can only see foreign antigen in the spatial context of the
molecular architecture of cell surfaces).
#31
Additionally T cells express CD proteins that define what type of cell they are (T-killers have CD8, T-helpers CD-4). CD molecules serve multiple roles including receptors, cell adhesion, and cell
communication. CD 8 recognizes MHC I and CD4 MHC II
The players in CMI:
1) Antigen presenting cells: present antigen to both TH-1 (Ag + Class II MHC) and Tc cells (Ag+
Class I MHC)
1) T-helpers (1)-CD4: T-helper most prevalent of all T cells- 65% of total. Hiv depress them
(binds to CD-4 receptor). Help regulate activity Tc (and B cells) by providing necessary signals
and growth factors. After stimulation by APC, TH-1 stimulates Tc cell via cytokine signaling
2) T-cytotoxic (CD 8). After activation by TH-1 and infected cell (Ag + Class I MHC on infected
cell), Tc kills host cell by perforin/granzyme pathway. This process accounts for much of our
immunity to foreign cells (fungi and protozoans), viral infections, and cancers (T deficient
people more prone to cancer). It is also responsible for rejection of transplanted tissue.
#32
Ist event in CMI is usually Tc activation. Steps involved in activation of cytotoxic
T cells:
Tc activation require 2 signals (activation in lymph nodes):TCR/mhc I/ag APC complex binding
ad IL-2 produced by activated TH-1.
1) Antigen presentation by APC to Th-1 and Tc cells. Dendritic cells phago. pathogens at
site of infection (skin for example) and then migrate to lymph nodes (it takes about a
day to do this) T-helper cell in lymph nodes browses all APC’s in every lymph node every
day.
2) TH-1 and Tc activation. Activated Tc cell moves to infected tissue and binds to cells that
have ag on surface. Tc also produce lymphokines including MAF (macrophage activation
factor) , MIF (migration inhibition factor) and, transfer factor (mobilizes other
nonsensitized T cells to take on characteristics of Tc)
3) Tc Clonal expansion
4) Tc Self-stimulation
#33
Activated Tc cells kill infected/cancerous cells via the The PerforinGranzyme Cytotoxic Pathway# 3435
In the perforin-granzyme pathway, cytotoxic T cells destroy their targeted cells by secreting
perforins and granzymes, toxic protein molecules. Perforins perforate cytoplasmic membranes,
and granzymes enter the target cell and activate capase enzymes which cause apoptosis.
CMI must be regulated carefully regulated or else it could kill normal host cells (as opposed to
virus infected/cancerous cells). Tc cells must receive signals in a precise order (combination
lock) to be activated. T reg cells (CD-4/25)also control immune response by shutting down
innate immunity (when the threat has been eliminated) and also repressing autoimmunity. To
prevent autoimmunity, T cells require additional signals from an APC. If they do not receive
these signals, they will not respond.
# 36
II.
Humoral ImmunityGoal is to produce AB that will help clear ag. B cell differentiation-bone marrow . Certain bone
marrow sites harbor stromal cells. Huge cells that nurture lymphocytes and provide hormonal
signals that influence the development of B cells. These signals stimulate cell division and gene
shuffling that results in billions of different distinct B cells each with a unique receptor. These
are released to regional lymphoid sites (lymph nodes, spleen, and GALT). 90% of B cells die
within a few days. Reseeded by bone marrow. These cells have class I and II MHC receptors
plus B cell receptor. However, they are not MHC restricted.
Humoral response-antibodies produced by effector B cells (plasma cells) are directed against
extracellular pathogens. Plasma cells are basically antibody secreting factories. An activated
plasma cells has no BCR and can produce thousands of antibody molecules per second. It dies in
a few days.
# 37
Inducement of T-Independent Antibody Immunity
T-independent antigens can directly stimulate B cell activation without needing the assistance of
macrophages and T-helpers
Because some large antigens have repeating epitopes that can be processed by B cells without
the help of T cells, they are called T-independent antigens. The repeating subunits of Tindependent antigens allow extensive cross-linking between numerous BCRs on a B cell which
activates transcriptions which stimulates the B cell to proliferate. Stimulated B-cells produce
IGM but do not class switch to produce OGG. T-independent antibody immunity is relatively
weak, disappears quickly, and induces little immunological memory. Children have stunted Tindep ag response. Pathogens that have T-ind ags are not eliminated. Example-Hemophilus
influenza (capsule ags are t-indep) can cause meningitis if the child has not been vaccinated.
# 38,39
Inducement of T-Dependent Antibody Immunity with Clonal Selection
# 40,41
T-dependent antigens lack the numerous, repetitive epitopes and large size of T-independent
antigens. Immunity against them requires the assistance of type 2 helper T (Th2) cells. These ags
don’t cross link BCR (which is a normal B-cell activation signal) so these cells need help (signals)
from other cells (T-helpers). If the Ag is T-depend, B cell activation requires 2 signals (Ag biding
and cytokines)
# 42,43
A T-dependent antibody immune response involves the following series of interactions among
antigen-presenting cells, helper T cells, and B cells, which are mediated by cytokines:
1. Antigen presentation for Th activation and cloning. Activated T-helper produces
membrane protein CD-40L which binds to the CD-40 on the B lymphocyte. This provides
the second signal for AB production.
2. Differentiation of helper T cells into Th2 cells
3. Activation of B cell by the Th2 cell such that it proliferates rapidly and differentiates into
either a memory B cell (retains its BCR and can last decades) or plasma cell (does not
have a BCR and dies in a few days)
Once B-lymphocyte activated, it produces clones of itself. These clones produce ab with a
slightly higher specificity for ag because of hypermutation. Plasma cells die in a few days but
ab’s can last a few weeks. Once plasma cells start secreting AB, they can class switch if they
receive the proper signal. IGMIGG-GIGA (possibly). As the antibody immune response
progresses, plasma cells that secrete antibody with a higher affinity for the epitope survive
at a higher rate such that the antibody specificity becomes progressively better over the
course of the immune response.
Summary of T/B cell activation requirements:
THo activated by APC+ag/MHC IITH1 and Th2
B lymph acitiv by TH-2 and ag/MHC II
Tc-activated by TH1 and APC/ag/MHCI
Memory B Cells and the Establishment of Immunological Memory
A small percentage of the cells produced by B cell proliferation do not secrete antibodies
but survive as memory B cells, long-lived cells (some last a lifetime) with BCRs
complementary to the specific antigen that triggered their production. In a primary
immune response, it can take days for relatively small amounts of antibodies to be
produced. When an antigen is encountered a second time, the activation of memory cells
in the secondary immune response ensures that the immune response is rapid and strong.
Enhanced immune responses to subsequent exposures are called memory responses.
Summaries-
# 44-46
Additional Considerations:
IR also affected by numerous factors1. Virus-AIDS
2. Sterols-tends to depress wbc. Stress produces cortisols. Cortisols used to suppress immune
system in tissue transplants. Sterols also used to treat chronic inflammation.
3. Exercise-positive benefits on immune system. Produces endorphins and enkephalins (natural
opiates). Also enhances immune activity. Also stimulates production of IL-I and If.
4. Mind IS connection. IL-2 also affects brain and nervous system. Nervous system produces ILHeavy enervation of lymphoid tissue and bone marrow (connection between mind and
immunity).
Modified 11-14