Hasan Immunoglobulins fixed
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86 Terms
Why are antibodies considered bifunctional molecules?
They bind antigen through the Fab/variable region and perform effector functions through the Fc/constant region.
What is the primary activity of an antibody?
The primary activity is specific antigen binding.
What are secondary antibody activities?
Secondary activities include complement activation, opsonization, placental transfer, Fc receptor binding, and secretion into body fluids.
Why does the variable region determine specificity?
The variable region forms the antigen-binding site that directly contacts the epitope.
Why does the constant region determine secondary activity?
The constant region forms the Fc region, which controls receptor binding, complement activation, transport, and isotype function.
What is the basic structure of an immunoglobulin?
An immunoglobulin has two identical heavy chains and two identical light chains.
Why are antibodies glycoproteins?
They contain mostly polypeptide plus carbohydrate that contributes to structure and biologic behavior.
Where are antibodies found in the body?
Antibodies are found in serum, extravascular fluids, secretions, and on B-cell surfaces.
Why is surface immunoglobulin important on B cells?
It functions as the B-cell receptor for antigen recognition.
What are hypervariable regions?
Hypervariable regions are highly variable parts of the antibody that fold together to form the antigen-binding pocket.
What are CDRs?
Complementarity determining regions are the closest contact sites between antibody and antigen.
Why are CDRs important?
They determine the fine specificity of antigen binding.
What forms the antigen-binding site?
The folded VH and VL regions together form the antigen-binding cleft.
Why does an antibody bind only part of a large antigen?
The binding site recognizes an epitope, usually only a small portion of the antigen.
What forces mediate antigen-antibody binding?
Hydrogen bonds, ionic bonds, hydrophobic interactions, and Van der Waals forces.
Why is antigen-antibody binding reversible?
Binding is noncovalent, so antigen and antibody can dissociate.
Why are antibodies monospecific but bivalent?
A typical antibody has two identical binding sites for the same antigen specificity.
Why does bivalency matter?
Bivalency improves avidity and allows cross-linking of antigens.
What is the hinge region?
The hinge is the flexible region between CH1 and CH2 that allows Fab arms to move.
Why is the hinge region clinically important?
It is exposed and can be cleaved by enzymes to generate antibody fragments.
What does papain digestion produce?
Papain produces two Fab fragments and one Fc fragment.
Why are Fab fragments monovalent?
Each Fab fragment contains one antigen-binding site.
Why do Fab fragments lack Fc function?
They do not contain the Fc region.
Why can Fab fragments be therapeutically useful?
They bind antigen without activating Fc-mediated effects and may penetrate tissue or clear rapidly.
What is DIGIBIND?
DIGIBIND is digoxin immune Fab used to treat life-threatening digoxin toxicity.
How does DIGIBIND reverse digoxin toxicity?
Fab fragments bind digoxin in blood, shifting digoxin away from tissue receptors and allowing elimination.
Why was digoxin conjugated to albumin to make DIGIBIND antibodies?
Digoxin is a small hapten and needed a carrier protein to become immunogenic.
What does pepsin digestion produce?
Pepsin produces F(ab')2 and small Fc-derived peptides.
Why is F(ab')2 bivalent?
The two Fab arms remain connected by hinge disulfide bonds.
Why does F(ab')2 lack Fc function?
Pepsin removes the Fc portion.
Why might F(ab')2 be useful?
It preserves bivalent binding while avoiding Fc receptor binding and Fc-mediated effects.
Why do antibody fragments penetrate tissues better than intact antibodies?
Fragments are smaller than whole antibodies.
Why does removing Fc reduce nonspecific binding?
Cells with Fc receptors cannot bind fragments through Fc if the Fc portion is absent.
What determines antibody class?
The heavy-chain constant region determines antibody class.
What are the five major human antibody classes?
IgG, IgA, IgM, IgD, and IgE.
Why do antibody classes differ in function?
Different heavy-chain constant regions produce different Fc-mediated activities.
What are IgG subclasses?
IgG1, IgG2, IgG3, and IgG4.
Why do IgG subclasses matter?
They differ in complement activation, Fc receptor binding, half-life, and placental transfer.
Which IgG subclass activates complement best?
IgG3 is the strongest complement activator among IgG subclasses.
Which IgG subclass activates complement poorly?
IgG4 activates complement very poorly.
Why is IgG important in serum immunity?
IgG is the major serum immunoglobulin and supports opsonization, complement activation, memory responses, and placental transfer.
Why does IgG have a long half-life?
FcRn protects IgG from degradation and recycles it.
Why can IgG cross the placenta?
FcRn transports IgG across the placental barrier.
Why is IgG important for fetal immunity?
Maternal IgG provides passive protection to the fetus.
Why is IgG an opsonizing antibody?
IgG binds Fcγ receptors on phagocytes, promoting uptake of antibody-coated targets.
Why is IgM the first antibody made?
Naïve B cells initially express and secrete IgM before class switching.
Why is IgM important in B-cell development?
Surface IgM is the first complete B-cell receptor expressed by immature B cells.
Why is IgM efficient at agglutination?
Pentameric IgM has many binding sites and can cross-link antigens effectively.
Why is IgM efficient at complement activation?
One antigen-bound IgM pentamer can efficiently activate the classical complement pathway.
Why is IgM mainly intravascular?
Its large pentameric size limits movement out of blood vessels.
Why is IgM called a macroglobulin?
Pentameric IgM is very large, around 900,000 molecular weight.
What is the J chain?
The J chain helps join polymeric IgM and IgA.
Why is IgA important at mucosal surfaces?
IgA is the major antibody in external secretions.
Where is IgA found?
IgA is found in saliva, tears, milk, nasal, bronchial, intestinal, vaginal, and other secretions.
How does serum IgA differ from secretory IgA?
Serum IgA is mostly monomeric, while secretory IgA is usually dimeric.
Why is secretory IgA stable in secretions?
The secretory piece protects IgA from degradation.
What produces the secretory piece?
Epithelial cells produce the secretory piece.
Why is secretory IgA tetravalent?
Dimeric IgA has four antigen-binding sites.
Why is IgE important in allergy?
IgE binds mast cells and basophils through FcεRI and triggers mediator release when cross-linked by allergen.
Why is IgE low in serum but biologically powerful?
Most IgE is cell-bound, and small amounts can trigger strong mast-cell responses.
What is FcεRI?
FcεRI is the high-affinity IgE receptor on mast cells and basophils.
Why is IgE useful in parasite defense?
It supports mast-cell, basophil, and eosinophil responses involved in anti-parasite immunity.
Why is IgD important?
IgD functions mainly as a surface receptor on naïve B cells along with IgM.
Why does IgD have little serum role?
It is present only in trace amounts in serum.
What are Fc receptors?
Fc receptors are cell-surface receptors that bind antibody Fc regions.
Why are Fc receptors important?
They link antigen recognition to effector functions such as phagocytosis, degranulation, feedback inhibition, and transport.
What does FcγRI bind?
FcγRI binds IgG.
Why is FcγRI important on phagocytes?
It helps macrophages, monocytes, dendritic cells, neutrophils, and eosinophils respond to IgG-coated targets.
Why does antigen-linked IgG trigger Fc receptor signaling better than free IgG?
Cross-linking of receptors by antigen-bound antibody is needed for activation.
What does FcγRIIB1 do?
FcγRIIB1 provides negative feedback to limit antibody synthesis.
Why is FcγRIIB1 important?
It prevents excessive antibody production and immune activation.
What does FcRn do?
FcRn transports IgG across epithelia and protects IgG from degradation.
Why is FcRn important clinically?
It explains IgG placental transfer and IgG's long serum half-life.
How do NK cells use Fc receptors?
NK cells bind antibody-coated targets through Fc receptors during ADCC.
What is ADCC?
ADCC is antibody-dependent cell-mediated cytotoxicity.
What are allotypes?
Allotypes are genetically determined antibody differences between individuals of the same species.
Why do allotypes usually not affect antibody function?
They are usually small constant-region differences that do not disrupt antibody activity.
Why can allotypes become immunogenic?
Another person's allotypic variant may be recognized as foreign after pregnancy, transfusion, or autoimmune disease.
Why is the heavy chain more important than the light chain for effector function?
The heavy-chain constant region forms most of the Fc region and determines class-specific activity.
Why is antibody class important clinically?
Class predicts location, function, receptor binding, and immune mechanism.
Why can two antibodies have the same specificity but different function?
They can share the same variable region but have different constant regions/isotypes.
Why can two antibodies have the same class but different specificity?
They can share the same constant region but have different variable regions.
Why is understanding antibody fragments useful?
Fragments separate antigen-binding functions from Fc-mediated functions.
Why is whole IgG useful for long-lasting therapy?
The Fc region allows FcRn recycling and longer serum persistence.
Why are antibody fragments cleared faster than whole IgG?
They lack FcRn recycling and are smaller.
Why is antibody structure directly tied to biological activity?
Fab determines what is bound, while Fc determines what immune response follows.