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ADAPTIVE IMMUNITY
• IMMUNITY CAN BE ACQUIRED IN TWO WAYS
1. ACTIVE IMMUNITY RESULTS FROM ACTIVATION OF AN
INDIVIDUAL’S OWN LYMPHOCYTES
• PATHOGEN INFECTION OR VACCINATION
2. PASSIVE IMMUNITY RESULTS FROM OBTAINING ANOTHER
INDIVIDUAL’S ANTIBODIES
• TRANSFER OF MATERNAL ANTIBODIES ACROSS PLACENTA
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ADAPTIVE IMMUNITY
• ORGANS OF THE IMMUNE SYSTEM
• PRIMARY LYMPHOID ORGANS
• BONE MARROW AND THYMUS
• SECONDARY LYMPHOID ORGANS
• LYMPH NODES, SPLEEN, AND MUCOSAL-ASSOCIATED
LYMPHOID TISSUE (MALT)
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• Organs of the immune system
• Primary lymphoid organs
• Bone marrow
• Thymus
• Secondary lymphoid organs
• Lymph nodes
• Spleen
• Mucosa-associated
lymphoid tissue or MALT
Thymus
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Spleen
Tonsils
Cervical lymph nodes
Axillary
lymph nodes
Mucosa-
associated
lymphatic
tissue
(MALT) (in
small intestine)
Inguinal
lymph
nodes
Lymphatic vessels
Red bone marrow
CELL-MEDIATED IMMUNITY
• T LYMPHOCYTES ARE OF TWO TYPES
• CYTOTOXIC T CELLS (TC)
• CD8+ CELLS
• HELPER T CELLS (TH)
• CD4+ CELLS
• DISTINGUISHED BY TYPE OF MHC MARKERS RECOGNIZED AND
ROLES AFTER ACTIVATION
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CELL-MEDIATED IMMUNITY
• CYTOTOXIC T CELLS
• RECOGNIZE “ALTERED-SELF”CELLS, PARTICULARLY THOSE THAT ARE
VIRALLY INFECTED OR TUMOR CELLS
• RECOGNIZE FOREIGN PEPTIDES BOUND TO SELF-MHC CLASS I PROTEINS
• CLONAL EXPANSION AND DIFFERENTIATION INTO ACTIVATED CELLS AND
MEMORY CELLS
• ACTIVATED CELLS INDUCE APOPTOSIS IN CELLS WITH SAME SPECIFICITY
AS FIRST CELL
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Cytotoxic T cells induce apoptosis
of “altered-self” cells. And destroy
tumors
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Dendritic cell
CD8CytokinesCytokines
Virus
MHC class I
Viral peptide
TCR specific for this
MHC–peptide complex
Naive cytotoxic T cell
Memory cytotoxic T cells Activated cytotoxic T cells
Clonal
expansion
Destroys
altered cell
Persists after
alter ed cells
are destroyed
Infected cell
Apoptosis of infected cell
a. 2500× b. 2500×
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a-b: © Dr.Andrejs Liepins/Photo Researchers, Inc.
CELL-MEDIATED IMMUNITY
• HELPER T CELLS
• SECRETE CYTOKINES THAT PROMOTE ACTIVATION OR DIFFERENTIATION
OF IMMUNE SYSTEM CELLS
• TH CELLS RESPOND TO EXOGENOUS ANTIGEN THAT IS TAKEN UP BY AN
ANTIGEN PRESENTING CELL
• ANTIGEN IS PARTIALLY DIGESTED, THEN COMPLEXED WITH MHC CLASS II
PROTEINS
• COMPLEX IS TRANSPORTED TO AND DISPLAYED ON THE CELL SURFACE
• ACTIVATED TH CELLS GIVE RISE TO EFFECTOR CELLS AND MEMORY CELLS
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Dendritic cell
CD4
B7
CD28
Bacterial peptide
Cytokines (IL-2)
Cytokines (IL-4)
Antigen
(such as a
bacterium) MHC class II
TCR specific for this MHC–peptide complex
Clonal
expansion
Memory helper T cell
Activated
helper T cell
Memory B cell
Naive B cell
Clonal
expansion
Plasma cells secreting antibody specific to this antigen
Humoral Response Cellular Response
Macrophage that can
better destroy invading
antigens
Cytokines (IFN-)
Activated
helper T cell
Memory helper T cell
Naive
helper T cell
Cytokines (IL-12)
Macrophage
HUMORAL IMMUNITY• BEGINS WHEN NAIVE B CELLS IN SECONDARY LYMPH ORGANS
MEET ANTIGENS
• B CELLS ARE ACTIVATED WHEN THEIR SURFACE IGS BIND TO A
SPECIFIC EPITOPE ON AN ANTIGEN
• TH CYTOKINES MAY ALSO BE REQUIRED
• ACTIVATION RESULTS IN CLONAL EXPANSION AND
DIFFERENTIATION INTO PLASMA AND MEMORY CELLS
• PLASMA CELLS PRODUCE SOLUBLE ANTIBODIES AGAINST THE
SAME EPITOPE
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IMMUNE RESPONSES
• THE FIRST ENCOUNTER WITH A FOREIGN ANTIGEN IS CALLED THE
PRIMARY IMMUNE RESPONSE
• ONLY FEW B OR T CELLS CAN RECOGNIZE ANTIGEN AND MOUNT RESPONSE
• CLONAL EXPANSION OCCURS – MEMORY CELLS
• SECOND ENCOUNTER IS CALLED THE SECONDARY IMMUNE RESPONSE
• THIS TIME THERE IS A LARGE CLONE OF MEMORY CELLS THAT CAN
RECOGNIZE THE ANTIGEN
• IMMUNE RESPONSE IS FASTER AND MORE EFFECTIVE
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11Development of active immunity to smallpox
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Am
ou
nt
of
an
tib
od
y
Exposure
to cowpoxTime
Exposure
to cowpox
Primary response
Secondary response
This interval
may be years
IgM
IgG
AUTOIMMUNITY
• IMMUNOLOGICAL TOLERANCE
• ACCEPTANCE OF SELF CELLS
• AUTOIMMUNE DISEASES ARE CAUSED BY THE FAILURE OF IMMUNE
TOLERANCE
• RESULT IN ACTIVATION OF AUTOREACTIVE T CELLS, AND PRODUCTION
OF AUTOANTIBODIES BY B CELLS
• CAUSE INFLAMMATION AND ORGAN DAMAGE
• ALLEVIATED BY CORTICOSTEROIDS AND NSAIDS, INCLUDING ASPIRIN
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ALLERGY
• REFERS TO A GREATLY HEIGHTENED RESPONSE TO A FOREIGN ANTIGEN, OR
ALLERGEN
• MOST COMMON TYPE IS KNOWN AS IMMEDIATE HYPERSENSITIVITY
• RESULTS FROM EXCESSIVE IGE PRODUCTION
• SEASONAL HAY FEVER
• SYSTEMIC ANAPHYLAXIS – SEVERE AND LIFE-THREATENING
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Allergen
Dendritic cell
Helper T cell
Allergen
Helper T cell
B cell
Cytokines (IL-4)
Plasma cell
Mast cell
Allergen
Initial Exposure
Subsequent Exposure
Memory B cell
Receptor for IgEAllergen-
specific IgE
Histamine and
other mediators
of inflammation
are released
Sequence of an
allergic response
ALLERGY
• DELAYED-TYPE HYPERSENSITIVITY PRODUCES SYMPTOMS WITHIN ABOUT 48
HOURS OF A SECOND EXPOSURE TO AN ALLERGEN
• MEDIATED BY TH CELLS AND MACROPHAGES
• CONTACT DERMATITIS
•CAUSED BY VARIED MATERIALS, SUCH AS
POISON IVY, NICKEL IN JEWELRY, AND
COSMETICS
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ANTIBODIES IN MEDICINE
• BLOOD TYPE
• DETERMINED BY ANTIGENS FOUND ON SURFACE OF RED BLOOD CELLS
• ABO BLOOD TYPES – TYPES A, B, AB, AND O
• RH FACTOR – RH POSITIVE AND RH NEGATIVE
• IMMUNE SYSTEM IS TOLERANT OF ITS OWN RBC ANTIGENS, BUT MAKES
ANTIBODIES THAT BIND TO THOSE THAT DIFFER (AGGLUTINATION)
• FOR EXAMPLE, PEOPLE WITH TYPE A BLOOD MAKE ANTIBODIES AGAINST THE B
ANTIGEN
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ANTIBODIES IN MEDICINE
• IN BLOOD TRANSFUSIONS, THE ANTIGENS OF THE DONOR HAVE TO BE
MATCHED TO THE ANTIBODIES OF THE RECIPIENT
• FOR INSTANCE, A TYPE A PERSON CANNOT DONATE TO A TYPE B OR TYPE O
• THESE WOULD HAVE ANTI-A ANTIBODIES
• BLOOD IS TYPED BY AGGLUTINATION REACTIONS, USING CIRCULATING IGM
ANTIBODIES
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EVADING THE IMMUNE SYSTEM
•SOME PATHOGENS CAN ALTER THEIR SURFACE ANTIGENS TO
AVOID IMMUNE SYSTEM DETECTION
• INFLUENZA VIRUS EXPRESSES 2 SURFACE PROTEINS:
HEMAGLUTININ (HA) AND NEURAMINIDASE (NA)
• ANTIGENIC DRIFT – POINT MUTATIONS TO THE HA AND NA GENES
• ANTIGENIC SHIFT – SUDDEN APPEARANCE OF A NEW VIRUS
SUBTYPE WHERE HA AND/OR NA PROTEINS ARE COMPLETELY
DIFFERENT
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EVADING THE IMMUNE SYSTEM
•HIV – HUMAN IMMUNODEFICIENCY VIRUS
•MOUNTS A DIRECT ATTACK ON TH CELLS
• BINDS TO CD4 PROTEINS AND IS ENDOCYTOSED
• AN INDIVIDUAL IS CONSIDERED TO HAVE AIDS WHEN THEIR
TH CELL LEVEL HAS DROPPED SIGNIFICANTLY
• IMMUNOSUPPRESSION RESULTS IN AN INCREASE IN
OPPORTUNISTIC INFECTIONS AND CANCERS
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7600×
© CDC/Science Source/Photo Researchers, Inc.