Immunology: Immunoglobulin Structure, Function: MCQ

Immunoglobulins, also known as antibodies, are crucial components of the immune system that play a vital role in defending the body against pathogens. Understanding the structure and function of immunoglobulins is essential for comprehending their importance in immune responses.
Structure of Immunoglobulins:
Immunoglobulins belong to a larger family of proteins called globulins and are characterized by their Y-shaped structure. Each immunoglobulin molecule consists of four protein chains: two identical heavy chains (H chains) and two identical light chains (L chains).
The H chains are further divided into constant (C) and variable (V) regions. The V regions, located at the N-terminal ends of both H and L chains, are highly diverse and form the antigen-binding site of the immunoglobulin. The C regions, found in the middle and C-terminal portions of the H and L chains, are relatively more conserved.
Immunoglobulin Function:
The primary function of immunoglobulins is to recognize and bind to specific foreign substances, known as antigens. This binding specificity is attributed to the antigen-binding site formed by the variable regions of the H and L chains.
Upon encountering an antigen, the immunoglobulin undergoes a conformational change that enhances its ability to neutralize or eliminate the antigen. This can occur through various mechanisms, including:
Neutralization: Immunoglobulins can bind to toxins or viruses, preventing them from interacting with target cells and neutralizing their harmful effects.
Opsonization: Immunoglobulins can coat bacteria or other pathogens, marking them for recognition and phagocytosis by immune cells such as macrophages and neutrophils.
Complement Activation: Immunoglobulins can initiate the classical pathway of the complement system, leading to the recruitment of complement proteins and subsequent destruction of the target.
Antibody-Dependent Cell-Mediated Cytotoxicity (ADCC): Immunoglobulins can bind to target cells, such as infected cells or cancer cells, and recruit immune cells, such as natural killer (NK) cells, to eliminate the targeted cells.
The diverse functions of immunoglobulins are mediated by the different classes and subclasses of antibodies, including IgG, IgM, IgA, IgE, and IgD. Each class has unique properties and effector functions that contribute to the overall immune response.

Multiple Choice Question on Immunoglobulin Structure, Function

1) Immunoglobulin is the plasma protein that specifically binds to antigens. Identify the region of electrophoresis that consists of these major immunoglobulins.

a) Alpha region
b) Beta region
c) Gamma region
d) None of the above

2) The five classes of immunoglobulin include the following, except?
a) IgA
b) IgD
c) IgE
d) IgH

3) Which of the following class of immunoglobulin is pentameric structure?
a) IgA
b) IgD
c) IgH
d) IgM

4) Which of the following class of immunoglobulin is dimeric structure?
a) IgA
b) IgD
c) IgH
d) IgM

5) The IgA and IgMs consist of which of the following chain can allow its polymerization?
a) H chain
b) L chain
c) J chain
d) V chain

6) The monomeric immunoglobulin consists of heterodimers of heavy (H) and light (L) chains bound together by non-covalent interaction and disulfide bonds. 
Which of the following is the antigen-binding site?
a) Fab
b) Fc
c) Hinge region
d) None of the above

7) The hinge region of the immunoglobulin consists of the disulfide bond that held the heterotetramer together. Also, it contributes to the flexibility of the antibody chain. 
Which of the following antibody class do not have a hinge region?
a) IgA
b) IgD
c) IgE
d) IgG

8) The hypervariable complementarity determining region (CDR) is responsible for which of the following function?
a) binding to antigen
b) binding to FcR
c) binding to complement
d) None of the above

9) Identify the protease that results in two different fragments of antibodies namely Fab and Fc fragments as shown in the figure below:

a) Pepsin
b) Trypsin
c) Papain
d) Fucin

10) Identify the protease that results in two different fragments of antibodies namely Fab and Fc fragments as shown in the figure below:

a) Pepsin
b) Trypsin
c) Papain
d) Fucin

11) The variable heavy and light chain makes up the antigen recognition region which consists of six complementarity determining regions (CDRs) (three from each heavy and light chain). 
In addition, a stretch of amino acid sequence also known as the framework region...................................
a) assist in the recognition of antigen
b) act as a scaffold to support CDR
c) are highly variable
d) None of the above

12) The variable heavy and light chains make up the antigen recognition region.
Which of the following is mostly involved in antigen binding?
a) Variable light chain
b) Variable heavy chain
c) Both of the above
d) None of the above

13) The contact area of the antigen binding area may consist of a protrusion or depression that complementarity matches the antigen. 
This contact area span approximately ............................. based on well-studied Lysozyme/anti-Lysozyme interaction.
a) 15-22 amino acids
b) 5-12 amino acids
c) 25-60 amino acids
d) None of the above

14) The antigen-antibody interactions are considered inducible which means
a) The antigen binding site has performed a site that exactly fits the antigen.
d) The antigen binding site is rigid
c) The antigen binding site undergoes conformation changes after contact with the antigen
d) None of the above

15) Which of the following antibody have four constant regions (CH1, CH2, CH3, CH4)?
a) IgA
b) IgD
c) IgG
d) IgM

16) The effector function of the antibody requires its Fc region. The Fc region binds to cells or proteins to mediate its function. 
Which of the following is the effector function of the antibody?
a) Antigen binding to antibodies promotes opsonization
b) Antigen binding to antibody activates complement
c) Antigen binding to antibody activates cell cytotoxicity
d) All of the above

17) Which of the following immunoglobulins are secretory and present in the milk?
a) IgG
b) IgM
c) IgA
d) IgE

18) The receptor that is responsible for the transport of IgAs across the epithelial barriers:
a) Poly Fc receptor
b) Poly Ig receptor
c) Poly Fab receptor
d) All of the above

19) Which of the following antibody is produced as a primary immune response and have higher valency to remove clear antigens?
a) IgM
b) IgG
c) IgA
d) IgE

20) Which of the following is the passive immunity transferred from the mother to its offspring?
a) Transplacental transfer of IgGs
b) Transfer of IgAs in the milk
c) Both a & b
d) None of the above

21) Which of the following antigen-bound antibodies bind to the Fc receptor present on the basophils and tissue mast cells, and release various pharmacoactive mediators involved in anaphylaxis?
a) IgA
b) IgD
c) IgM
d) IgE

22) The Fc receptor is a plasma membrane glycoprotein that binds to different immunoglobulin and triggers effective functions. 
Which of the following Fc receptor is involved in the transfer of IgG from mother to fetus?
a) Fc€R
b) FcRN
c) FcµR
d) FcγR

23) The immunoglobulin superfamily is the group of membrane proteins that possess one or more homologous immunoglobulin domains. 
Which of the following is NOT an immunoglobulin superfamily?
a) T cell receptor
b) beta2 microglobulin
c) Insulin receptor
d) Platelet-derived growth factor

24) B-cell receptors consist of membrane-bound immunoglobulin and a small heterodimer protein required for signaling. 
Which of the following is the heterodimer protein?
a) Igα & Igβ
b) Igµ & Igγ
c) Igα & Igγ
d) None of the Above

25) Multiple Myeloma is characterized by excessive production of immunoglobulin and the presence of a light chain in urine. 
Which of the following cells are responsible for the production of immunoglobulin?
a) T cells
b) Plasma cells
c) B-cells
d) Dendritic cells

26) Which of the following complement protein binding to IgG facilitates phagocytosis?
a) C2a
b) C2b
c) C3a
d) C3b

27) Which of the following subclass of IgG molecule is the most potent activator of the complement pathway?
a) IgG1
b) IgG2
c) IgG3
d) IgG4

28) Which of the following subclass of IgG does not readily cross the placental barriers?
a) IgG1
b) IgG2
c) IgG3
d) IgG4

29) Which of the immunoglobulin isotype have the shortest half-life?
a) IgG
b) IgM
c) IgA
d) IgE

30) Which of the immunoglobulin isotype have the longest half-life?
a) IgG
b) IgM
c) IgA
d) IgE

Multiple Choice Answers:
1-c)Gamma region 
Immunoglobulins are often referred to as gamma globulins because, during plasma protein electrophoresis, they migrate and concentrate in the gamma region of the albumin/globulin separation pattern. This term, "gamma globulin," reflects the localization of immunoglobulins in the specific region of the electrophoretic pattern where they are observed.

Five classes of immunoglobulins are IgG, IgG, IgE, IgD, IgA
IgG (Immunoglobulin G): IgG is the most abundant class of immunoglobulins in the blood and tissue fluids. It plays a crucial role in immune defense against bacteria, viruses, and toxins. IgG can cross the placenta, providing passive immunity to the fetus.
IgA (Immunoglobulin A): IgA is primarily found in secretions such as saliva, tears, breast milk, and mucous membranes. It provides protection at mucosal surfaces, preventing pathogens from entering the body. IgA exists in both monomeric form (found in blood) and dimeric form (found in secretions).
IgM (Immunoglobulin M): IgM is the first immunoglobulin produced during an initial immune response. It is a pentamer (consisting of five monomers) and is efficient at clumping together pathogens. IgM is involved in the early stages of neutralizing and eliminating microbes.
IgD (Immunoglobulin D): IgD is found in low concentrations in the blood and is primarily expressed on the surface of B cells. Its exact function is not entirely clear, but it is thought to play a role in B cell activation and differentiation.
IgE (Immunoglobulin E): IgE is involved in allergic reactions and immune responses against parasites. It is present in low concentrations in the blood but has a high affinity for receptors on mast cells and basophils. When IgE binds to an allergen, it triggers the release of inflammatory mediators, leading to allergic symptoms.

 IgM exists as a pentamer, which means it is made up of five individual antibody subunits joined together. The subunits are connected by a J chain and held together by disulfide bonds. This pentameric structure gives IgM its characteristic shape.

Two IgA Monomers: sIgA is composed of two IgA monomers, each consisting of two heavy (H) chains and two light (L) chains. The heavy chains in IgA are denoted as α (alpha) chains, and the light chains can be either kappa (κ) or lambda (λ) chains.

5-c)J chain
J Chain: The two IgA monomers in sIgA are connected by a polypeptide called the J chain. The J chain helps stabilize the dimeric structure and is responsible for the binding of the secretory component (SC) to IgA.
The J chain is a polypeptide that is primarily associated with IgM antibodies. It plays a role in the polymerization and structural stabilization of IgM molecules. The J chain helps connect the individual IgM monomers, promoting the formation of pentameric IgM structures.

Each Fab region is composed of one complete light chain and the variable (V) and constant (C) regions of one heavy chain of the antibody molecule. The V regions of both the heavy and light chains contribute to the formation of the antigen-binding site. Fab region consists of the antigen-binding side that is variable and confers diversity specificity and affinity of the antibody.

7-c) IgE
The hinge region of an antibody is a flexible segment that connects the Fab (antigen-binding fragment) and Fc (crystallizable fragment) regions. 
Location: The hinge region is located between the two Fab regions of an antibody molecule. It is found in the "middle" of the antibody structure, connecting the arms of the Y-shaped antibody molecule.
Amino Acid Composition: The hinge region is composed of a relatively long stretch of amino acids that vary in length and composition among different antibody classes. For example, IgG antibodies have a longer hinge region compared to IgM antibodies.
Flexibility: The hinge region is highly flexible due to the presence of amino acids that lack secondary structure, such as proline and glycine. This flexibility allows the Fab arms to move independently of each other, increasing the range of motion and enabling the antibody to bind to antigens at different angles.
IgE does not have a flexible region between the Fab and Fc regions. Instead, IgE has a relatively short and rigid linker region that connects the Fab and Fc portions

8-a) binding to antigen
CDRs are specific segments within the variable (V) domains of both the heavy and light chains of an antibody molecule. There are three CDRs in each chain, designated as CDR1, CDR2, and CDR3. CDRs play a crucial role in the binding of antibodies to antigens. They form the antigen-binding site within the variable regions of the antibody, directly interacting with the target antigen.

9-c) Papain
Papain cleaves the antibody molecule at the hinge region, which is a flexible segment that connects the Fab (antigen-binding fragment) and Fc (crystallizable fragment) regions. The cleavage occurs specifically at the disulfide bonds present in the hinge region.

Pepsin digestion of an antibody produces F(ab')2 fragment.

11-b)act as a scaffold to support CDR

12-b) Variable heavy chain
The heavy chain and light chain of the antibody molecule work together to form the antigen-binding site. The heavy chain contributes a larger portion to the antigen-binding site, while the light chain complements it.

13-a) 15-22 amino acids
The antigen contact area on an antibody involves the complementarity-determining regions (CDRs) within the variable regions of both the heavy and light chains. These CDRs, particularly CDRs 1, 2, and 3, form loops that protrude from the antibody structure and directly interact with the antigen. The CDRs contribute to the specificity and affinity of the antibody-antigen interaction.

14-c) The antigen binding site undergoes conformation changes after contact with the antigen 

15-d) IgM
Not all antibody isotypes have a distinct CH4 domain. For example, IgG and IgD antibodies have CH1, CH2, and CH3 domains, while IgM antibodies have CH1, CH2, and CH3 domains followed by additional constant domains known as CH4-like domains.

16-d) All of the above
Opsonization: The Fc region can bind to specific receptors on immune cells, such as phagocytes (e.g., macrophages and neutrophils), through a process called opsonization. This opsonization enhances the phagocytosis of pathogens, promoting their clearance from the body.
Antibody-Dependent Cellular Cytotoxicity (ADCC): The Fc region can also interact with Fc receptors on certain immune cells, such as natural killer (NK) cells. This interaction triggers ADCC, where the NK cells recognize the antibody-coated target cells (e.g., infected or tumor cells) and induce their destruction.
Complement Activation: Some antibody isotypes, such as IgG and IgM, can activate the complement system through their Fc regions. A complement system is a group of proteins that, when activated, leads to the destruction of pathogens via various mechanisms, including direct lysis, opsonization, and inflammation.
Neonatal Fc Receptor (FcRn) Binding: The Fc region interacts with the neonatal Fc receptor (FcRn) in specific tissues and cells. This interaction plays a critical role in the transport of antibodies across epithelial barriers, such as the placenta (providing passive immunity to the fetus) and the intestinal mucosa (facilitating the transfer of antibodies from breast milk to the infant).
Modulation of Immune Responses: The Fc region can influence the magnitude and quality of immune responses. It can regulate the activation and function of immune cells, modulate inflammation, and contribute to immune regulation and tolerance.

17-c) IgA
Secretory IgA (sIgA) is a specialized form of immunoglobulin A (IgA) antibody that is primarily found in mucosal secretions, such as saliva, tears, colostrum (first milk produced by mammary glands after giving birth), and mucosal surfaces of the respiratory, gastrointestinal, and genitourinary tracts.

18-b) Poly Ig receptor
The receptor responsible for the secretion of IgA antibodies across mucosal epithelial cells is called the polymeric immunoglobulin receptor (pIgR). The pIgR is a transmembrane protein expressed on the basolateral surface of epithelial cells lining mucosal tissues, such as the respiratory, gastrointestinal, and genitourinary tracts.
The polymeric immunoglobulin receptor binds to the dimeric form of IgA antibodies, known as secretory IgA (sIgA). This binding occurs in the lamina propria, which is the connective tissue underlying the mucosal epithelium. The pIgR interacts with the J chain of sIgA, which stabilizes the dimeric structure, allowing it to be transported across the epithelial layer.
After binding to sIgA, the pIgR undergoes internalization and is transported through the epithelial cell via transcytosis. The pIgR-bound sIgA is transported within vesicles across the cell and is subsequently released on the apical surface of the epithelial cell. During transcytosis, a portion of the pIgR, known as the secretory component (SC), remains associated with the transported sIgA, forming the secretory IgA complex.
IgM is a pentameric structure with 10 antigen-binding sites. 

 20-b)Transfer of IgAs in the milk
IgE-mediated anaphylaxis is a severe and potentially life-threatening allergic reaction that occurs when the immune system overreacts to an allergen in individuals sensitized to it. This type of anaphylaxis is primarily mediated by the immunoglobulin E (IgE) antibody class.
Here's how IgE-mediated anaphylaxis occurs:
Sensitization: Initially, an individual is sensitized to a specific allergen through exposure. The immune system recognizes the allergen as foreign and produces specific IgE antibodies against it.
Allergen re-exposure: Upon re-exposure to the same allergen, the allergen binds to the IgE antibodies already present on the surface of mast cells and basophils, which are types of immune cells involved in allergic responses.
Cross-linking and activation: When the allergen binds to multiple IgE antibodies on the mast cells or basophils, it causes cross-linking of the IgE-receptor complexes. This cross-linking triggers the release of inflammatory mediators, including histamine, leukotrienes, prostaglandins, and cytokines, from the mast cells and basophils.
Systemic effects: The released inflammatory mediators cause a rapid and widespread allergic reaction throughout the body. This leads to a range of symptoms that can include hives, swelling (angioedema), difficulty breathing, wheezing, low blood pressure, gastrointestinal distress, and in severe cases, loss of consciousness and cardiovascular collapse.

22)-a) FcRn
 FcRn is responsible for the transfer of maternal IgG antibodies across the placenta to provide passive immunity to the developing fetus. The FcRn receptor in the placenta binds to IgG antibodies and transports them from the maternal circulation into fetal circulation.

23-c) Insulin receptor
The immunoglobulin superfamily (IgSF) includes several protein families and subgroups, some of which are:
Immunoglobulins (antibodies): The IgSF includes the immunoglobulin proteins, which are involved in recognizing and binding to specific antigens in immune responses. Antibodies are composed of Ig domains and are produced by B cells as part of the adaptive immune system.
Cell Adhesion Molecules (CAMs): CAMs are proteins involved in cell-cell and cell-extracellular matrix interactions. They play critical roles in processes such as cell migration, tissue development, and immune cell activation. Examples of CAMs in the IgSF include neural cell adhesion molecules (NCAMs) and intercellular adhesion molecules (ICAMs).
Co-stimulatory Molecules: Certain proteins in the IgSF act as co-stimulatory molecules and modulate immune cell activation. For example, CD28 and CTLA-4 are IgSF proteins expressed on T cells that regulate T cell activation and immune responses.
Signaling Receptors: Many receptors involved in cell signaling belong to the IgSF. Examples include receptors of the tumor necrosis factor (TNF) receptor superfamily, such as CD40 and CD95 (Fas), which play roles in immune regulation and cell death signaling.
Neural Cell Recognition Molecules: The IgSF also includes molecules involved in neural development and synaptic function. Examples include neural cell adhesion molecules (NCAM) and contactins, which mediate interactions between neurons and contribute to neural circuit formation.

24-a)Igα & Igβ 
Igα (CD79a) and Igβ (CD79b) are transmembrane proteins that help in signal transduction.

25-b)Plasma cells
Plasma cells are specialized cells derived from activated B lymphocytes (B cells) that play a crucial role in the immune response. They are primarily responsible for the production and secretion of antibodies, also known as immunoglobulins (Igs).

C3b can covalently bind to the surface of pathogens or immune complexes that are bound by IgG antibodies. This process is called opsonization, where C3b acts as an opsonin, marking the target for recognition and uptake by phagocytic cells, such as macrophages and neutrophils. The binding of C3b to the Fc region of IgG-coated antigens enhances the efficiency of phagocytosis by promoting the interaction between the target and the phagocyte's complement receptors.

IgG3 antibodies have a longer hinge region, making them more flexible and capable of binding to multiple antigenic sites simultaneously. They are potent activators of the complement system and are effective against viral infections. IgG3 antibodies are also associated with autoimmune diseases.

IgG2 antibodies have limited ability to cross the placental barrier compared to other IgG subclasses, such as IgG1. The placental barrier serves as a protective barrier between the maternal and fetal bloodstreams, allowing essential nutrients and oxygen to pass through while preventing the transfer of potentially harmful substances.
The transport of IgG antibodies across the placenta occurs through a specialized transport mechanism called neonatal Fc receptor (FcRn)-mediated transcytosis. FcRn is expressed on the syncytiotrophoblast cells of the placenta, which are responsible for facilitating the transfer of maternal IgG antibodies to the developing fetus.

The half-life of IgE antibodies, also known as immunoglobulin E, is relatively short compared to other immunoglobulin classes, such as IgG. The exact half-life of IgE can vary among individuals and depends on various factors, including physiological conditions and the presence of specific allergens.

IgG: IgG antibodies have the longest half-life among the immunoglobulin classes, ranging from about 21 to 28 days. This extended half-life allows IgG antibodies to provide sustained protection and long-term immunity.
IgA: The half-life of IgA antibodies is relatively shorter compared to IgG. The estimated half-life of IgA in the bloodstream is around 5 to 6 days. However, it's important to note that the half-life of secretory IgA, which is found in mucosal secretions, can vary and may be shorter due to degradation in the mucosal environment.
IgM: IgM antibodies have a relatively short half-life compared to IgG. The half-life of IgM is estimated to be around 5 to 7 days. IgM antibodies are usually the first antibodies produced during an immune response and serve as the initial defense against pathogens.
IgE: IgE antibodies have the shortest half-life among the immunoglobulin classes. The half-life of IgE is relatively rapid, ranging from just a few hours to a couple of days. This short half-life helps regulate the allergic response and prevents prolonged activation of mast cells and basophils.