Immunoglobulins and Antigens Practice Questions
34 free Immunoglobulins and Antigens practice questions for the Zoology. Tap an option to answer — you get instant feedback, the correct answer, and a detailed explanation for every question.
Immunoglobulins are best described as:
- A Antigens secreted directly by pathogens to evade host immunity
- B Proteins produced by B cells that bind and neutralize specific foreign antigens
- C Antibody molecules involved in immune defense
- D Hormones that regulate the intensity of the immune response
Correct answer: Antibody molecules involved in immune defense
Immunoglobulins are antibodies produced by plasma cells (differentiated B cells). They specifically recognize and bind antigens, facilitating neutralization, opsonization, and complement activation.
The basic structure of an immunoglobulin molecule consists of:
- A Two heavy chains covalently linked, with no light chains
- B Two light chains only, without any heavy chain component
- C Two heavy and two light chains
- D Four identical chains of the same molecular weight and sequence
Correct answer: Two heavy and two light chains
An immunoglobulin molecule has a Y-shaped structure formed by two identical heavy chains and two identical light chains. These chains are held together by disulfide bonds forming the classic bivalent antibody.
The antigen-binding site of an antibody is formed by:
- A Constant regions of heavy chains, which remain identical across antibody classes
- B Variable regions of both heavy and light chains
- C The Fc region, which mediates effector functions only
- D Disulfide bonds linking the two heavy chains
Correct answer: Variable regions of both heavy and light chains
The antigen-binding site (paratope) is created by the hypervariable (complementarity-determining) regions within the variable domains of both the heavy and light chains. These regions confer exquisite antibody specificity.
Which class of immunoglobulin is most abundant in human serum?
- A IgA
- B IgE
- C IgG
- D IgM
Correct answer: IgG
IgG is the most abundant immunoglobulin in blood and extracellular fluid. It plays a major role in opsonization and neutralization.
Which immunoglobulin is the first to be produced during a primary immune response?
- A IgA
- B IgG
- C IgE
- D IgM
Correct answer: IgM
IgM is the first antibody produced when an antigen is encountered for the first time. It is effective in activating the complement system.
Which immunoglobulin is mainly found in mucosal secretions such as saliva and milk?
- A IgA
- B IgD
- C IgE
- D IgG
Correct answer: IgA
IgA is the principal antibody in mucosal secretions. It provides local immunity by preventing pathogen attachment to epithelial surfaces.
The Fc region of an antibody is primarily involved in:
- A Antigen binding at the paratope site
- B Complement activation and effector cell interaction
- C Determining antibody specificity through hypervariable loop conformation
- D Formation of the variable domains via VDJ recombination
Correct answer: Complement activation and effector cell interaction
The Fc region interacts with Fc receptors on immune cells (macrophages, NK cells) and with complement protein C1q. It mediates effector functions such as opsonization, antibody-dependent cellular cytotoxicity (ADCC), and complement activation.
Which immunoglobulin class is associated with allergic reactions?
- A IgA
- B IgG
- C IgE
- D IgM
Correct answer: IgE
IgE binds to mast cells and basophils. Upon antigen binding, it triggers the release of histamine and other mediators.
An antigen is defined as a substance that:
- A Destroys pathogens directly
- B Is produced by antibodies
- C Elicits an immune response
- D Functions only inside cells
Correct answer: Elicits an immune response
An antigen is any substance that can be recognized by the immune system and induce an immune response. It often originates from pathogens or foreign particles.
The specific region of an antigen recognized by an antibody is called:
- A Paratope
- B Epitope
- C Hapten
- D Isotype
Correct answer: Epitope
An epitope is the specific part of an antigen that binds to the antibody. Each antigen may have multiple epitopes.
Haptens differ from complete antigens because they:
- A Cannot bind to antibodies because they lack the molecular weight needed for recognition
- B Are always proteins with a full complement of T-cell and B-cell epitopes
- C Require a carrier protein to become immunogenic
- D Are rapidly destroyed by circulating antibodies upon first exposure
Correct answer: Require a carrier protein to become immunogenic
Haptens can bind to pre-existing antibodies but are too small to stimulate an immune response on their own. They must be coupled to a larger carrier protein (e.g., albumin) to elicit T-cell help and induce antibody production.
Which immunoglobulin exists mainly as a pentamer?
- A IgA
- B IgD
- C IgG
- D IgM
Correct answer: IgM
IgM molecules are pentameric in nature. This structure allows efficient activation of the complement system.
Disulfide bonds in immunoglobulins mainly provide:
- A Antigen-binding specificity
- B Structural stability
- C Catalytic enzymatic activity
- D Direct membrane attachment to B cells
Correct answer: Structural stability
Disulfide bonds hold the heavy and light chains together and link the two heavy-chain subunits. They stabilize the overall quaternary structure of the antibody molecule without contributing to antigen recognition.
Which immunoglobulin can cross the placental barrier?
- A IgA
- B IgD
- C IgE
- D IgG
Correct answer: IgG
IgG is the only antibody class that crosses the placenta. It provides passive immunity to the developing fetus.
The variability in antibodies is mainly due to differences in:
- A Constant regions
- B Fc region
- C Variable regions
- D Number of heavy chains
Correct answer: Variable regions
The variable regions differ among antibodies and determine antigen specificity. This diversity allows recognition of numerous antigens.
Which immunoglobulin is present on the surface of immature B cells?
- A IgA
- B IgD
- C IgE
- D IgM
Correct answer: IgM
IgM is the first immunoglobulin expressed on the surface of immature B cells, functioning as the B-cell antigen receptor. Upon maturation, most naive B cells co-express both IgM and IgD on their surface.
Antigen-antibody interaction is primarily based on:
- A Covalent bonding between antigen and antibody
- B Ionic and non-covalent interactions
- C Peptide bond formation at the antigen surface
- D Enzymatic reactions catalyzed by antibodies
Correct answer: Ionic and non-covalent interactions
Antigen-antibody binding is mediated by non-covalent interactions including hydrogen bonds, ionic forces, van der Waals forces, and hydrophobic interactions. These interactions ensure specificity and reversibility of the complex.
Which immunoglobulin is present in very low concentration in serum but acts as a B-cell receptor?
- A IgA
- B IgD
- C IgG
- D IgM
Correct answer: IgD
IgD is present in very low concentrations in serum but is prominently co-expressed with IgM on the surface of mature naive B cells as part of the B-cell receptor complex. It plays a role in B-cell activation and immune regulation.
The term 'isotype' refers to:
- A Different antigenic determinants on an antigen
- B Different classes of immunoglobulins
- C Different epitopes recognized by B cells
- D Different binding affinities of antibodies
Correct answer: Different classes of immunoglobulins
Isotypes are the different classes of immunoglobulins — IgG, IgA, IgM, IgE, and IgD — determined by the constant region of the heavy chain. All individuals of the same species share the same isotypes.
Which function of antibodies helps in marking pathogens for phagocytosis?
- A Neutralization of toxins
- B Agglutination of bacteria
- C Opsonization of pathogens
- D Precipitation of antigens
Correct answer: Opsonization of pathogens
Opsonization involves coating pathogens with antibodies (or complement proteins) to enhance their recognition and ingestion by phagocytes such as macrophages and neutrophils. This marks pathogens for destruction.
Which immunoglobulin is most abundant in human serum?
- A IgA
- B IgM
- C IgG
- D IgE
Correct answer: IgG
IgG is the most abundant immunoglobulin in serum. It plays a major role in secondary immune response and long-term immunity.
Which immunoglobulin can cross the placenta to provide passive immunity?
- A IgA
- B IgM
- C IgG
- D IgD
Correct answer: IgG
IgG can cross the placental barrier and provide immunity to the fetus. This is an important mechanism of passive immunity.
Which immunoglobulin is the first to be produced during primary immune response?
- A IgA
- B IgM
- C IgG
- D IgE
Correct answer: IgM
IgM is the first antibody produced during initial exposure to an antigen. It provides early defense before IgG production.
Which immunoglobulin is mainly found in mucosal secretions like saliva and milk?
- A IgG
- B IgM
- C IgA
- D IgE
Correct answer: IgA
IgA is present in mucosal secretions and protects body surfaces. It prevents pathogen entry at mucosal sites.
Which immunoglobulin is associated with allergic reactions?
- A IgG
- B IgA
- C IgM
- D IgE
Correct answer: IgE
IgE binds to allergens and triggers histamine release from mast cells. It is responsible for allergic responses.
Which immunoglobulin is found on the surface of immature B cells?
- A IgG
- B IgM
- C IgA
- D IgE
Correct answer: IgM
IgM is present on immature B cells as a receptor. It helps in recognizing antigens.
Which immunoglobulin acts mainly as a B-cell receptor along with IgM?
- A IgG
- B IgA
- C IgD
- D IgE
Correct answer: IgD
IgD is found on the surface of mature B cells. It functions as a receptor for antigen binding.
Which part of the antibody binds to antigens?
- A Fc region
- B Fab region
- C Constant region
- D Hinge region
Correct answer: Fab region
The Fab region contains the antigen-binding site. It interacts directly with the antigen.
What is the main function of the Fc region of an antibody?
- A Bind antigen
- B Provide structural support
- C Interact with immune cells
- D Synthesize proteins
Correct answer: Interact with immune cells
The Fc region binds to receptors on immune cells. It mediates immune responses such as phagocytosis.
Which function of antibodies enhances phagocytosis?
- A Neutralization
- B Lysis
- C Opsonization
- D Precipitation
Correct answer: Opsonization
Opsonization marks pathogens for destruction by phagocytes. Antibodies coat the pathogen to enhance recognition.
What provides structural stability to immunoglobulins?
- A Hydrophobic interactions
- B Hydrogen bonds
- C Ionic bonds
- D Disulfide bonds
Correct answer: Disulfide bonds
Disulfide bonds link the heavy and light chains. They maintain the structure of the antibody.
The basic structure of an immunoglobulin consists of:
- A Two heavy chains only
- B Two light chains only
- C Four identical polypeptide chains
- D Two heavy and two light chains
Correct answer: Two heavy and two light chains
Immunoglobulins consist of two heavy chains and two light chains. These chains form a Y-shaped structure.
What determines the specificity of an antibody?
- A Constant region
- B Fc region
- C Hinge region
- D Variable region
Correct answer: Variable region
The variable region of an antibody determines its specificity. It binds to a specific antigen epitope.
What does the term 'isotype' refer to in immunoglobulins?
- A Different antigen types
- B Different classes of antibodies
- C Different binding sites
- D Different allotypic classifications
Correct answer: Different classes of antibodies
Isotype refers to different classes of immunoglobulins such as IgG, IgA, IgM, IgD, and IgE. These classes differ in structure and function.