Ch. 04 Immunology

Antibody

Secreted form of B cell antigen receptor

recognizes protein/carb antigens by disabling or rendering it susceptible to destruction

where are antibodies located

blood, lymph, on mucosal surfaces

Antibody specificity

each antibody binds to one antigen or a few structurally related antigens

Antibody diversity

recognition of enormous range of antigens

Immunoglobulin

general name for antibodies and B-cell antigen receptors

Naive mature B cells

resting B cells that express membrane-bound immunoglobulin before encountering antigens

Plasma cells

effector B-lympocytes that produce/secrete antibodies during an immune response

Clonal selection

Antigen stimulates B cells to proliferate/differentiate into plasma cells

Agammaglobulinemia

Unable to make antibodies

renders high susceptibility to certain types of infection

How is Agammaglobulinemia caused?

a critical kinase, BTK, on the X chromosome is defective

B-cell to plasma cell pathway

Mature naive resting B cell encounters an antigen → stimulates it → gives rise to antibody-secreting plasma cells

Antibody composition

Polypeptides with variable and constant regions

Carbs attached to heavy chain portion

Heavy chains

2 identical 50 kDa polypepyides forming stem and parts of the Y arms

Light chains

two identical 25 kDa polypeptides paired with heavy chains in each arm

Variable regions

domains with sequence diversity that determine antigen specificity

V_H

Variable region of the heavy chain

V_L

Variable region of the light chain

Constant regions

Domains with limited variation providing structural stability and effector function

C_H

Constant region of the heavy chain

C_L

Light chain constant region

Antigen-binding site

Region in variable area that binds to antigen and determines specificity

Isotypes of immunoglobulins

differences in heavy-chain constant regions, each with distinct functions

List of isotypes in heavy chain

IgG (γ)

IgM (μ)

IgD (δ)

IgA (α)

IgE (ε)

Light-chain isotypes list

Kappa and lambda

What are the differences in heavy chain isotypes?

length of heavy chains, locations of disulfide bonds, hinge regions in IgG, IgA, and IgD (not IgM and IgE)

carbohydrate group distribution

Differences in light-chain isotypes?

No functional differences

Every antibody has either kappa or gamma light chains, never both

varies with animal species

Membrane bound immunoglobulins occur as what -mers?

all isotypes occur as monomers

Soluble immunoglobulin monomers

IgD, IgE, IgA, and IgG

Soluble immunoglobulin dimers

IgA

Soluble immunoglobulin pentamers

IgM

IgG molecule can be dissected into

2 Fab fragments and 1 Fc fragment

How is a IgG molecule dissected

• protease cleaves hinge of each heavy chain

• reducing agent breaks disulfide bonds connecting the hinges

Fab fragment

Fragment antigen binding

Arm portion of antibody containing antigen-binding site

Fc fragment

fragment crystallizable

stem portion mediating effector functions through receptor and protein binding

Flexible hinge region

bendable part of antibody that connects two antigen-binding arms to stem

What does flexible hinge region allow

allows antibody to bind with both arms to many different antigen arrangements

Immunoglobulin-like domains

Structural motifs found in many immune proteins beyond antibodies

Immunoglobulin superfamily

group of cell-surface and secreted proteins sharing immunoglobulin-like domain structures

Each Fab and Fc fragment is made up of

4 immunoglobulin domains

Each Fab/Fc fragment makes

similarly shaped globular structure

Light chain C domain structure

Light chain V domain structure

Hypervariable regions

short segments in V_H and V_L domains where amino acid sequence diversity is greatest

Framework regions

conserved sequences that flank hypervariable regions and maintain variable domain structure

Complementarity-determining regions

Hypervariable loops that form surfaces complementary to antigen structure

Three hypervariable regions

HV1, HV2, HV3 aka CDR1-3

different amino acid sequences

surrounded by four framework regions on both sides (FR1-4)

Antigen

triggers an immune response

Antigenic determinant (epitope)

part of antigen where an antibody binds

can be linear or discontinuous

Multivalent antigen

antigen with multiple epitopes

can have either different or repeated epitopes

Affinity

Binding strength of an antibody determined by shape and chemical complementarity of the antigen-binding site to the epitope

Antiserum

Contains antibodies against immunizing antigen and any contaminating macromolecules

Polyclonal antiserum

blood serum containing differently specified antibodies

Immunizing antigen/Immunogen

Antigen used to induce an immune response and antibody production during immunization

hybridoma

Hybrid cell formed by fusion of an antibody-producing B cell from an immunized mouse with an immortal myeloma cell

Monoclonal antibodies

• Identical antibodies from single hybridoma clone, specific to one epitope

• Made in 1980

• Eliminate many diseases

Flow cytometry

analytical technique that detects and classifies cells based on labeled antibody binding to their surface molecules

How is a mouse monoclonal antibody made

1. Myeloma cells fuse with antigen-immunized mouse B cells

2. Fused cells are grown in drug-infused medium which kill myeloma and unfused lymphocytes

3. Hybridoma which makes the right antibody is selected for and cloned to make a large culture

Immunoglobulin is first made in

membrane-bound form on B-cell surface

Secreted antibodies are made by

alternative pattern of heavy-chain RNA processing

Antibodies with different constant regions

have different effector functions

Naive B cells express

IgM and IgD on surfaces

Naive B cells have

• the same light chains and same heavy chain V region

• 1 Antigen specificity

Ig-alpha and Ig-beta

Transmembrane proteins that pair with immunoglobulin to form a complete B-cell receptor complex

disulfide-linked complex

What do the cytoplasmic tails of the Ig-alpha and Ig-beta proteins do

Interact with intracellular signaling proteins that signal B cells to divide/differentiate

How does the disulfide-linked complex interact with the immunoglobulin

Can help it transport it to the B cell surface from the Endoplasmic reticulum lumen

Functional B cell receptor composition

• Immunoglobulin

• Hydrophobic transmembrane sequence

• Ig-alpha and Ig-beta disulfide complex next to the receptor outside the cell

• Cytoplasmic portion (short for the actual receptor, long tails for Ig-alpha and Ig-beta proteins)

After antigen encounter, what do B cells do

Antibodies are produced by an alternative pattern of heavy-chain RNA processing

Heavy chain IgM

μ

IgD heavy chain

ẟ

IgM

Most common class of antibody made as a response to an antigen

neutralizes and agglutinates antigen, activates complement

IgD

Important as an antigen receptor on B cells

neutralizes and agglutinates antigen

IgG

Most common class of antibody in the blood

major class produced in secondary responses

crosses the placenta for fetal/newborn immunity

neutralizes/agglutinates/opsonizes antigen

activates complement

enhance NK cell activity

IgE

Involved in allergies such as hay fever

neutralizes/agglutinates antigen

binds to mast cells and basophils, which causes histamine release

IgA

Crosses epithelial cells

present on mucosal surfaces and breast milk

imprtant for newborn immunity

neutralizes/agglutinates antigen

Neutralization

antibodies block pathogen activtity

Agglutination

Multiple pathogens are aggregated by antibody molecules

Opsonization

Pathogens bound by antibodies to be more efficiently phagocytized

Complement activation

Antibodies bound to pathogens activate complement cascade to lyse the cell

Enhanced NK cell activity

Abnormal body cells are bound to be recognized by NK cells and lysed.

Pentameric IgM

5 IgM monomers are cross-linked to each other and a J chain polypeptide via disulfide bonds

allows for strong binding to repetitive epitopes

Monomeric IgA

made by plasma cells in lymph nodes, spleen, and bone marrow

secreted into blood

Dimeric IgA

Two monomers joined by a J chain

made in lymphoid tissues under mucosa

secreted into gut lumen

Also in milk, saliva, sweat, tears

FcεR receptor

high-affinity that binds IgE in the absence of antigen, present on mast cells in epithelium, eosinophils at mucosal surfaces, and basophils in blood

What happens when antigen binds to cell-surface IgE on Mast cells, eosinophils, and basophils

strong physical and inflammatory reactions that expel and kill infecting parasites

What are IgE major effects in its response in countries with a low amount of parasite infections

allergies and asthma

IgG flexibility

small size and flexibility allow it to access antigens in extracellular spaces of damaged and infected tissues

How do the subclasses of IgG differ?

Relative length of hinge and number of disulfide bonds in that hinge

IgG1

IgG2

IgG3

IgG4

In circulation IgG4 molecules can

exchange one heavy chain + light chain combination, allowing it to have two different antigen binding sites, and interact with a pathogen/antigen through one binding site