Antigens and Antibodies — MCQs
All of the following forces are involved in antigen-antibody reactions, except:
In ABO blood grouping, which is False?
Which immunoglobulin is known to be heat-labile?
CD40 deficiency in a person signifies?
A researcher is studying the interactions between foreign antigens and human immune cells. She has isolated a line of lymphocytes that is known to bind antigen-presenting cells. From this cell line, she has isolated a cell surface protein that binds to class I major histocompatibility complex molecules. The continued activation, proliferation and survival of this specific cell line requires which of the following signaling molecules?
Adjuvants given along with antigens are going to
CD 40 marker is absent in the person. Which of the following would be seen?
In chronic allergy, which Ig is more persistent in the body?
Which of the following is the most potent stimulator of Naive T-cells?
Which of the following statements about the secondary immune response is false?
Antigens and Antibodies Indian Medical PG Practice Questions and MCQs
Question 1: All of the following forces are involved in antigen-antibody reactions, except:
- A. Electrostatic bond
- B. Hydrogen bond
- C. Covalent bond (Correct Answer)
- D. Van der Waals forces
Explanation: ***Covalent bond*** - **Covalent bonds** are strong, irreversible bonds that involve the sharing of electrons between atoms. - Antigen-antibody interactions are predominantly **non-covalent** and reversible, allowing for dynamic binding and release. *Vander Waal's forces* - **Van der Waals forces** are weak attractive forces that arise from temporary fluctuations in electron distribution, creating transient dipoles. - They are crucial in antigen-antibody binding, especially when the molecules are in **close proximity**, contributing to overall affinity. *Electrostatic bond* - **Electrostatic (ionic) bonds** occur between oppositely charged groups on the antigen and antibody surfaces. - These interactions are significant for **initial recognition** and overall binding stability, particularly at appropriate pH levels. *Hydrogen bond* - **Hydrogen bonds** form between a hydrogen atom covalently linked to an electronegative atom (like oxygen or nitrogen) and another electronegative atom. - They play a vital role in the **specificity and strength** of antigen-antibody interactions by providing numerous weak, directional contacts.
Question 2: In ABO blood grouping, which is False?
- A. IgM is most common antibody in ABO
- B. ABO are carbohydrate Ag
- C. ABO antibodies are natural and present since birth (Correct Answer)
- D. Ab are present only if Ag is absent
Explanation: ***ABO antibodies are natural and present since birth*** - **ABO antibodies** are naturally occurring, but they are typically **not present at birth** [1]. - They develop within the **first 3 to 6 months of life** as a response to exposure to similar antigens in the environment (e.g., bacteria). *IgM is most common antibody in ABO* - The primary **ABO antibodies** (anti-A, anti-B) are indeed predominantly **IgM antibodies** [1]. - IgM antibodies are large pentameric structures, and their size prevents them from crossing the placenta [2]. *ABO are carbohydrate Ag* - The **ABO blood group antigens** (A, B, H) are **carbohydrate structures** (glycans) found on the surface of red blood cells and other tissues. - These carbohydrate chains are attached to proteins or lipids. *Ab are present only if Ag is absent* - This statement is a fundamental principle of ABO blood grouping: individuals naturally produce antibodies against the **ABO antigens** they **lack** [1]. - For example, a person with **Type A blood** (A antigen present) will have **anti-B antibodies** (B antigen absent). **References:** [1] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Blood And Bone Marrow Disease, pp. 627-628. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of Infancy and Childhood, pp. 469-470.
Question 3: Which immunoglobulin is known to be heat-labile?
- A. IgA
- B. IgG
- C. IgM (Correct Answer)
- D. IgE
Explanation: ***IgM*** - **IgM** is known for its **heat lability** and is readily denatured at 56°C within a few minutes. - This characteristic is due to its **pentameric structure** held together by disulfide bonds and J chains, which are sensitive to thermal denaturation. - Heat lability of IgM is clinically important in complement fixation tests and other laboratory assays where heat inactivation is performed. - IgM is the first antibody produced in primary immune response and its heat sensitivity distinguishes it from other immunoglobulins. *IgA* - **IgA** exists in monomeric (serum) and dimeric (secretory) forms and shows moderate stability to heat. - Secretory IgA is relatively stable as it needs to function in harsh mucosal environments, though not as heat-resistant as IgG. - Does not exhibit the pronounced heat lability characteristic of IgM. *IgG* - **IgG** is the most stable immunoglobulin and is highly resistant to heat denaturation. - Can withstand temperatures up to 60-70°C without significant loss of activity. - Its monomeric structure with strong intramolecular bonds provides exceptional thermal stability. - Most abundant antibody in serum and has the longest half-life. *IgE* - **IgE** is actually quite stable to heat and can withstand 56°C for extended periods. - While it has a short half-life in serum (2-3 days), this is due to receptor binding rather than heat instability. - Important in type I hypersensitivity reactions and parasitic infections. - Does not show the characteristic heat lability that defines IgM.
Question 4: CD40 deficiency in a person signifies?
- A. IgG increase
- B. T cell absent
- C. IgM increase (Correct Answer)
- D. B cell absent
Explanation: ***IgM increase*** - A deficiency in **CD40**, or its ligand **CD40L** (found on T helper cells), disrupts **T-cell-dependent B cell activation** and **class switching**. - Without proper signaling through CD40/CD40L, B cells cannot undergo **isotype switching** from **IgM** to IgG, IgA, or IgE, leading to elevated IgM levels and deficiencies in other antibody classes. *IgG increase* - **IgG levels** would likely be **decreased** in CD40 deficiency due to the impaired ability of B cells to undergo **class switching** from IgM to other antibody isotypes. - The primary role of CD40/CD40L interaction is to facilitate this class switching process. *T cell absent* - **CD40 deficiency** does not directly cause the absence of **T cells**; rather, it affects the ability of T cells to adequately activate B cells. - T-cell absence or severe dysfunction would be indicative of a different primary immunodeficiency, such as **SCID (Severe Combined Immunodeficiency)**. *B cell absent* - **CD40 deficiency** does not result in the absence of **B cells**; B cells are present but are dysfunctional in terms of antibody class switching. - Conditions like **X-linked agammaglobulinemia (XLA)** are characterized by the absence or severe deficiency of B cells.
Question 5: A researcher is studying the interactions between foreign antigens and human immune cells. She has isolated a line of lymphocytes that is known to bind antigen-presenting cells. From this cell line, she has isolated a cell surface protein that binds to class I major histocompatibility complex molecules. The continued activation, proliferation and survival of this specific cell line requires which of the following signaling molecules?
- A. Interleukin 1
- B. Interleukin 4
- C. Interleukin 2 (Correct Answer)
- D. Interleukin 8
- E. Interleukin 6
Explanation: ***Interleukin 2*** - The description of the lymphocyte binding the **constant portion of MHC class I** and requiring a signaling molecule for activation, proliferation, and survival points to a **T cell**. - **Interleukin-2 (IL-2)** is a crucial cytokine for the proliferation, differentiation, and survival of T lymphocytes, acting in an autocrine or paracrine fashion after T cell activation. *Interleukin 1* - **Interleukin-1 (IL-1)** is primarily involved in inflammation and fever, produced by macrophages and other innate immune cells. - While it can act as a costimulator for T cells, it is not the primary cytokine required for their sustained proliferation and survival after initial activation. *Interleukin 4* - **Interleukin-4 (IL-4)** is a key cytokine in humoral immunity, promoting B cell proliferation and differentiation, and inducing IgE class switching. - It also plays a role in the differentiation of naive T cells into **Th2 cells**, but it is not the main cytokine for general T cell proliferation and survival. *Interleukin 8* - **Interleukin-8 (IL-8)**, also known as CXCL8, is a chemokine primarily responsible for attracting and activating neutrophils to sites of infection or inflammation. - It does not have a direct role in the sustained proliferation and survival of activated lymphocytes. *Interleukin 6* - **Interleukin-6 (IL-6)** is a pleiotropic cytokine involved in acute phase reactions, hematopoiesis, and the immune response, particularly B cell differentiation and antibody production. - Although it can influence T cell responses, it is not the primary growth factor for activated T lymphocytes as IL-2 is.
Question 6: Adjuvants given along with antigens are going to
- A. reduce the antigenicity
- B. increase antigenicity (Correct Answer)
- C. reduce the toxigenicity
- D. increase toxigenicity
Explanation: ***increase antigenicity*** - Adjuvants are substances added to vaccines to **enhance the immune response** to the co-administered antigen. - They work by various mechanisms, such as forming an antigen depot, activating antigen-presenting cells, and stimulating cytokine production, all leading to a **stronger and more prolonged immune response**. *reduce the antigenicity* - This statement is incorrect as adjuvants are specifically designed to **potentiate, not diminish**, the immune response to an antigen. - Reducing antigenicity would counteract the primary purpose of vaccination, which is to induce protective immunity. *reduce the toxigenicity* - Adjuvants generally do not directly impact the **toxigenicity** of an antigen, which refers to its ability to produce toxins. - While some vaccines detoxify bacterial toxins (e.g., toxoids), this is a separate process from the action of adjuvants that boost immune recognition. *increase toxigenicity* - This is incorrect; adjuvants are not intended to make an antigen more toxic. - Their role is to enhance immunogenicity safely, and increasing toxicity would be counterproductive and harmful.
Question 7: CD 40 marker is absent in the person. Which of the following would be seen?
- A. Impaired Macrophage function
- B. Impaired NK cell function
- C. Impaired B cell function (Correct Answer)
- D. Impaired T cell function
Explanation: ***Impaired B cell function*** - The **CD40 receptor** on B cells is crucial for receiving co-stimulatory signals from **CD40 ligand (CD40L)**, primarily expressed on activated T cells. - Absence of CD40 on B cells prevents proper **T-cell dependent antibody class switching** and germinal center formation, leading to impaired B cell activation, immunoglobulin production, and immune responses. - This condition is seen in **Hyper-IgM Syndrome Type 3** (very rare autosomal recessive disorder). *Impaired Macrophage function* - While macrophages express CD40, its absence would primarily affect their ability to be fully activated by T cells and present antigens, but the most direct and profound impact of absent CD40 is on B cells themselves. - Macrophages have other activation pathways not directly dependent on CD40. *Impaired NK cell function* - **Natural killer (NK) cells** primarily recognize and kill target cells lacking MHC class I molecules or those expressing activating ligands, independent of CD40 signaling. - NK cell function is not directly regulated by the CD40-CD40L interaction. *Impaired T cell function* - While **T cells express CD40L** (the ligand for CD40), the question specifies the absence of the **CD40 marker** itself, which is expressed on B cells, not T cells. - T cell function involves antigen recognition, activation, and cytokine production, which are not directly mediated by CD40 expression on T cells. - T-cell function would be indirectly affected due to the lack of proper B cell help and antigen presentation, but the direct impact of absent CD40 is on the cell expressing it (B cells).
Question 8: In chronic allergy, which Ig is more persistent in the body?
- A. Ig A
- B. Ig E (Correct Answer)
- C. Ig G
- D. Ig M
Explanation: ***Ig E*** - **IgE** is the primary antibody involved in **allergic reactions**, binding to receptors on **mast cells** and **basophils** to trigger histamine release. - In chronic allergy, sustained exposure to allergens leads to continuous production of IgE, making it a **persistent** and dominant immunoglobulin in the allergic response. *Ig A* - **IgA** is mainly found in **mucosal secretions**, such as tears, saliva, and gut, protecting against pathogens at these sites. - While important for immunity, IgA does not play a direct role in the **immediate hypersensitivity reactions** characteristic of chronic allergies. *Ig G* - **IgG** is the most abundant antibody in serum, providing **long-term immunity** against pathogens through neutralization, opsonization, and complement activation. - Though present, IgG is not the **primary mediator** of the **allergic response** in chronic allergy, instead often associated with protective immunity or certain non-IgE mediated hypersensitivities. *Ig M* - **IgM** is the first antibody produced during a **primary immune response** and is effective at activating the complement system. - It is predominantly found in the bloodstream and functions as a **short-term defender**, but it is not directly involved in the pathogenesis or persistence of chronic allergies.
Question 9: Which of the following is the most potent stimulator of Naive T-cells?
- A. Macrophages
- B. B-cell
- C. Mature dendritic cells (Correct Answer)
- D. Follicular dendritic cells
Explanation: ***Mature dendritic cells*** - **Mature dendritic cells** are the most potent professional antigen-presenting cells (APCs) for activating **naive T cells** due to their efficient antigen processing, presentation abilities, and high expression of costimulatory molecules (e.g., CD80, CD86) and MHC-peptide complexes. - Activated by pathogens or inflammatory signals, they migrate to secondary lymphoid organs where they initiate primary immune responses by presenting antigens to and activating naive T cells. *Follicular dendritic cells* - **Follicular dendritic cells** primarily present intact antigens to **B cells** in germinal centers of secondary lymphoid organs, playing a crucial role in B cell maturation, selection, and antibody production. - They lack MHC class II molecules and thus cannot directly present antigens to naive T cells. *Macrophages* - While **macrophages** are professional APCs, they are generally less efficient than mature dendritic cells at activating **naive T cells**, especially in the initiation of primary immune responses. - They are more involved in presenting antigens to already activated T cells and clearing pathogens, often acting as secondary APCs. *B-cell* - **B cells** can act as APCs, but they are generally less efficient than **dendritic cells** in activating **naive T cells**, especially for the primary immune response. - Their primary role in antigen presentation is to present processed antigens to **helper T cells** to receive costimulation for their own activation and differentiation into plasma cells, often after being activated themselves.
Question 10: Which of the following statements about the secondary immune response is false?
- A. The lag period is absent or significantly shorter.
- B. There is a negative phase in the response.
- C. Only T-dependent antigens are recognized.
- D. Immune response against a subsequent antigenic challenge is absent. (Correct Answer)
Explanation: ***Immune response against a subsequent antigenic challenge is absent.*** - This statement is **false** because the secondary immune response is characterized by a **much stronger and faster** immune response upon subsequent exposure to the same antigen. - The presence of **memory cells** ensures that the immune system is highly prepared to combat the antigen more efficiently than during the primary response. *The lag period is absent or significantly shorter.* - This statement is **true** for the secondary immune response. The **memory B and T cells** can be rapidly activated, reducing the time needed to mount an effective response. - Unlike primary responses that can take 5-10 days to produce antibodies, secondary responses typically produce antibodies within **1-3 days**. *There is a negative phase in the response.* - This statement is **false** for the secondary immune response. The **negative phase** is characteristic of the **primary immune response**, not the secondary response. - The negative phase in primary response refers to a transient drop in antibody concentration after initial antigen exposure due to antigen-antibody complex formation. However, the **secondary response shows immediate and robust antibody production** without this negative phase due to pre-existing memory cells. - While this statement is technically false, the question asks for THE false statement, and Option D is more obviously and fundamentally false. *Only T-dependent antigens are recognized.* - This statement is **partially false** but has some truth in context. While **T-dependent antigens** generate the most robust secondary responses with strong memory cell formation, the immune system doesn't ONLY recognize T-dependent antigens. - **T-independent antigens** can elicit responses but typically generate weaker, shorter-lived immunity without strong memory formation. The classical, robust secondary immune response with anamnestic features is predominantly associated with T-dependent antigens.
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