Antibodies Practice Questions

Correct answer: Fc region

The Fc (fragment crystallizable) region is composed of constant domains of the heavy chains. It interacts with cell surface receptors called Fc receptors and complement proteins to mediate immune responses.

Correct answer: IgG

IgG is the most abundant antibody in the blood and can be transported across the placenta via the neonatal Fc receptor (FcRn). This provides the newborn with a temporary defense against pathogens encountered by the mother.

Correct answer: Two identical heavy chains and two identical light chains

A standard antibody monomer is a Y-shaped molecule consisting of four polypeptide chains. These are two identical heavy (H) chains and two identical light (L) chains, held together by disulfide bridges.

Correct answer: IgM

IgM is the first isotype synthesized by B cells upon their initial encounter with an antigen. Because it often exists as a pentamer, it has high avidity and is very efficient at agglutinating pathogens.

Correct answer: Secretory component

The secretory component is a fragment of the polymeric immunoglobulin receptor (pIgR) that remains attached to dimeric IgA. It masks cleavage sites, protecting the antibody from digestive enzymes in mucosal secretions.

Correct answer: IgE

IgE binds with high affinity to FcεRI receptors on mast cells and basophils. Upon cross-linking by an allergen or parasite antigen, it triggers the degranulation and release of histamine.

Correct answer: Epitope

An epitope, or antigenic determinant, is the specific chemical group or molecular surface feature on an antigen. A single antigen can possess multiple different epitopes recognized by different antibodies.

Correct answer: J chain

The J (joining) chain is a small glycoprotein that facilitates the polymerization of IgM and IgA. In IgM, it links the Fc regions of the monomers to form a stable pentameric structure.

Correct answer: Somatic hypermutation

Somatic hypermutation occurs in the germinal centers of secondary lymphoid organs. B cells with mutations that result in higher-affinity receptors are preferentially selected to survive, leading to affinity maturation.

Correct answer: IgD

IgD is co-expressed with IgM on the surface of mature, naive B cells. While it serves as a B cell receptor, its concentration in the serum is extremely low, and its effector role is less defined than other isotypes.

Correct answer: Two Fab fragments and one Fc fragment

Papain cleaves the antibody above the hinge region, resulting in three distinct pieces. The two Fab fragments retain antigen-binding capacity, while the Fc fragment remains intact but cannot bind antigen.

Correct answer: 10

Valency refers to the number of antigen-binding sites. Since each monomer in the pentamer has two binding sites, a pentameric IgM molecule has 10 potential binding sites.

Correct answer: A change in the heavy chain constant region from mu to gamma, alpha, or epsilon

Isotype (or class) switching allows a B cell to produce antibodies with the same antigen specificity but different effector functions. This is achieved by DNA recombination that replaces the Cμ constant region with another (e.g., Cγ).

Correct answer: Plasma cells and myeloma cells

Hybridomas are created by fusing a specific antibody-producing B cell (plasma cell) with an immortalized cancerous B cell (myeloma). This results in a cell line that grows indefinitely while secreting a single type of antibody.

Correct answer: Disulfide linkages

The interaction between the paratope of an antibody and the epitope of an antigen is strictly non-covalent. Disulfide linkages are covalent bonds used to hold the internal polypeptide chains of the antibody itself together.

Correct answer: IgA

IgA is the primary mediator of mucosal immunity. It is transported across epithelial cells into external secretions, where it prevents the attachment of viruses and bacteria to mucosal surfaces.

Correct answer: The total binding strength of a multivalent antibody with an antigen

While affinity measures the strength of a single interaction, avidity accounts for the accumulated strength of multiple binding sites. This is why IgM, with 10 binding sites, has very high avidity despite potentially lower individual affinity.

Correct answer: The variable domains of both heavy and light chains

Three hypervariable loops, also called Complementarity Determining Regions (CDRs), are located within the variable domains of both chains. Together, these six loops (3 from H, 3 from L) form the surface that contacts the antigen.

Correct answer: Kappa and Lambda

In humans, antibodies contain either two kappa (κ) or two lambda (λ) light chains. A single antibody molecule will never have one of each; they are always identical.

Correct answer: It provides flexibility so the Fab arms can bind variably spaced epitopes

The hinge region is a proline-rich sequence in IgG, IgA, and IgD that provides segmental flexibility. This allows the Fab arms to open and close to accommodate epitopes on the surfaces of pathogens.