adaptive immunity

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Adaptive ImmunityChapter 7

Mosby items and derived items © 2006 by Mosby, Inc.

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Adaptive Immunity Antigens Specificity Memory Antibodies Lymphocytes


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Adaptive Immunity Clonal diversity

Production of T and B lymphocytes Antigen recognition Lymphocyte specificity

Clonal selection Antigen processing Cellular interaction


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Humoral and Cell-Mediated Immunity The cellular and humoral responses are not

independent Humoral immunity

“Fluid” immunity Circulating antibody

Cell-mediated immunity T cell differentiation


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Active vs. Passive Immunity Active immunity

Antibodies or T cells produced after either a natural exposure to an antigen or after immunization

Passive immunity Preformed antibodies or T lymphocytes are

transferred from a donor to a recipient


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Recognition and Response Required for a successful immune response Clusters of differentiation (CD)

Originally used to describe proteins found on the surface of lymphocytes

Now it is a labeling system used to identify a family of proteins on many cells


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Antigens Immunogens vs. antigens Antigenic determinant (epitope) Self-antigen Tolerance

Central and peripheral tolerance Molecular size

Haptens Allergens


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Antigen Presentation Antigen-presenting cells (APCs) Major histocompatibility complex (MHC)

Glycoproteins on the surface of all human cells (except RBCs)

Also referred to as human leukocyte antigens (HLAs) MHC class I molecules

A, B, and C MHC class II molecules

DR, DP, and DQ MHC class III molecules


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Antigen Presentation


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Transplantation Cells in transplanted tissue from one

individual will have a different set of MHC surface antigens than those of the recipient

Therefore, a recipient can mount an immune response against the foreign MHC molecules

Haplotype Combination of A, B, C, DR, DQ, and DP alleles


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Transplantation


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CD1 Antigen-presenting molecules Found on antigen-presenting and thymus cells Present lipid antigens


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Antigen Recognition Antigen is directly recognized by circulating

antibody, antigen receptors on B cells (BCR), and T lymphocytes (TCR)


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Antibodies Also called immunoglobulins Produced by plasma cells Classes of antibody

IgG, IgA, IgM, IgE, and IgD Characterized by antigenic, structural, and functional

differences


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Antibodies


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Antibodies


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Immunoglobulin G (IgG) Most abundant class (80%-85%) Transported across the placenta Four classes

IgG1, IgG2, IgG3, and IgG4


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Immunoglobulin A (IgA) Two classes

IgA1 molecules are found predominantly in the blood

IgA2 molecules are found predominantly in normal body secretions

IgAs found in body secretions are dimers anchored by a J chain and a “secretory” piece Secretory piece may function to protect IgAs

against enzyme degradation


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Immunoglobulin M (IgM) Largest of the immunoglobulins Pentamer stabilized by a J chain First antibody produced during the primary

response to an antigen Synthesized during fetal life


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Immunoglobulin D (IgD) Limited information on IgD function Low concentration in the blood Located primarily on the surface of

developing B lymphocytes Function as one type of B cell antigen

receptor


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Immunoglobulin E (IgE) Least concentrated of the immunoglobulin

classes in the circulation Mediator of many common allergic responses Defender against parasites


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Antibody Structure Antigen-binding fragment (Fab)

Recognition sites (receptors) for antigenic determinants

Crystalline fragment (Fc) Responsible for biological function

Polypeptide chains (4) Light chains (2) and heavy chains (2)


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Antibody Structure Hinge region Constant and variable regions

Complementary determining regions (CDRs) Framework regions (FRs)


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Antigen Binding Amino acid sequences of the variable regions of the

heavy and light chains Framework regions control antibody folding Lock and key

Noncovalent chemical interactions Antibody valence

IgG, IgD, and IgE—2 IgA—4 IgM—theoretically 10, likely 5


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B Cell Receptor Complex Located on surface of B cells Consists of:

Antigen-recognition molecules Monomer IgM and IgD

Accessory intracellular-signaling molecules Ig-alpha and Ig-beta heterodimers


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T Cell Receptor Complex Antibody-like transmembrane protein (TCR) Accessory proteins for intracellular signaling

Referred to as CD3


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Generation of Clonal Diversity All necessary receptor specificities are

produced Takes place in the primary (central) lymphoid

organs Results in immature but immunocompetent T

and B cells Primarily occurs in the fetus


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Clonal Selection Immunocompetent T and B cells migrate

from the primary lymphoid organs to the secondary lymphoid organs to await antigen

Primarily after birth Clonal selection is initiated by antigen Final products

Plasma cells that produce antibody, effector cells that help Th, Tc, or Treg, and memory B and T cells


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T Cell Maturation The thymus is the central lymphoid organ of T cell

development T cells move from the thymic cortex to the medulla Changes

Development of the T cell receptors and expression of surface molecules

T cells are released into the blood and take up residence in the secondary lymph organs


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Antigen Processing and Presentation Antigens require processing and presentation

by antigen-presenting cells (APCs) Dendritic cells, macrophages, and B lymphocytes

For processing and presentation to occur, the antigen must be of the appropriate type, the lymphocytes must be prepared to recognize the presented antigen, and the antigen must be presented appropriately


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Antigen Processing and Presentation


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Helper T Lymphocytes “Help” the antigen-driven maturation of B and T

cells Facilitate and magnify the interaction between APCs

and immunocompetent lymphocytes Steps

Th interacts through antigen-specific and antigen-independent mechanisms

Undergoes differentiation Mature Th interacts with plasma or T-effector cells


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Helper T Lymphocytes Subsets

Th1 cells provide help in developing cell-mediated immunity

Th2 cells provide help in developing humoral immunity

Differences based on cytokine production


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B Cell Activation When an immunocompetent B cell encounters

an antigen for the first time, B cells with specific BCRs are stimulated to differentiate and proliferate

A differentiated B cell becomes a plasma cell A plasma cell is a factory for antibody

production Single class or subclass of antibody


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Primary and Secondary Responses Primary response

Initial exposure Latent period or lag phase

B cell differentiation is occurring After 5 to 7 days, an IgM antibody for a specific

antigen is detected An IgG response equal or slightly less follows the

IgM response


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Primary and Secondary Responses Secondary response

More rapid Larger amounts of antibody are produced Rapidity is due to the presence of memory cells

that do not have to differentiate IgM is produced in similar quantities to the

primary response, but IgG is produced in considerably greater numbers


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Class Switch Immunocompetent B cells use IgM and IgD

as receptors During clonal selection, B cells have the

option of changing the class of the antibody One of four IgGs, one of two IgAs, IgE, or an

IgM in a pentamer form DNA rearrangement


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B Cell Clonal Selection


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T Cell Activation Binding antigen to specific T cell receptors Allows:

Direct killing of foreign or abnormal cells Assistance or activation of other cells

T regulatory cells (Treg) Regulate the immune response to avoid attacking

“self” Memory T cells


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T Cell Activation


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Superantigens (SAGs) Bind the variable portion of the TCR and the

MHC class II molecules outside of their antigen-presentation sites

Activates a large population of T lymphocytes regardless of antigen specificity

SAGs induce an excessive production of cytokines Causes fever, low blood pressure, fever, and

potentially shock


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Antibody Function Direct

Neutralization Agglutination Precipitation

Indirect Opsonization

Degree of antibody protection is assessed by an antibody titer


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Secretory (Mucosal) Immune System Lymphoid tissues that protect the external

surfaces of the body Antibodies present in tears, sweat, saliva,

mucus, and breast milk IgA is the dominant immunoglobulin

Small numbers of IgG and IgM are present


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Secretory (Mucosal) Immune System


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IgE Function Provides protection from large parasites

Initiates an inflammatory reaction to attract eosinophils

When produced against innocuous environmental antigens, they are a common cause of allergies Fc portions of IgEs are bound to mast cells


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IgE Function


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Cell-Killing Mechanisms Cytotoxic T lymphocytes

Destroy cancer cells or cells infected with virus Perforin, granzymes, or direct receptor

interactions


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Cell-Killing Mechanisms


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Other Cells Natural killer (NK) cells

Complement Tc cell mechanisms Regulatory T cells (Treg)

Provide peripheral tolerance Affect recognition of antigen and suppress

proliferative steps of antigen recognition


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Fetal and Neonatal Immunity Antibody function is deficient

Capable of primary IgM response; unable to produce an IgG challenge

Immunity provided by maternal antibody Trophoblastic cells transport maternal IgG across

the placenta Newborn IgG levels are near adult levels


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Fetal and Neonatal Immunity


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Aging and Immune Function Decreased T cell activity

Thymic size is 15% of its maximum size Decreased production of specific antibodies Increase in circulating antigen-antibody

complexes Increase in circulating autoantibodies Decrease in circulating memory B cells

adaptive immunity

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