CD3 is a marker for which type of cells?
In the context of diagnosing syphilis, which of the following is an example of a precipitation test?
Haptens are immunogenic when they covalently bind to which type of carrier?
Which human IgG subclass has the highest serum concentration?
Which of the following does not stimulate active immunity?
Which of the following is a specific feature of acquired immunity?
To which part of an antigen do monoclonal antibodies specifically bind?
What type of immunity is primarily associated with the administration of transfer factor?
Which of the following tests is not classified as an in vivo test?
Which microorganism is the most common cause of pyogenic osteomyelitis?
NEET-PG 2015 - Microbiology NEET-PG Practice Questions and MCQs
Question 71: CD3 is a marker for which type of cells?
- A. B - cells
- B. T - cells (Correct Answer)
- C. NK - cells
- D. Monocytes
Explanation: ***T - cells*** - **CD3** is a complex of proteins that is universally expressed on the surface of all **T lymphocytes** (T cells). - It plays a crucial role in **T cell activation** by transducing signals from the T cell receptor (TCR) to the cell's interior. *B - cells* - **B cells** are characterized by the expression of unique surface markers like **CD19**, **CD20**, and surface **immunoglobulins**, not CD3. - Their primary function is to produce **antibodies** and present antigens. *NK - cells* - **Natural Killer (NK) cells** are a type of lymphocyte that lacks both CD3 and a T cell receptor (TCR), differentiating them from T cells. - They express markers such as **CD16** and **CD56** and are involved in innate immunity, particularly against viral infections and tumor cells. *Monocytes* - **Monocytes** are myeloid cells, not lymphocytes, and are characterized by markers such as **CD14** and **CD68**. - They differentiate into macrophages and dendritic cells, playing a significant role in phagocytosis and antigen presentation.
Question 72: In the context of diagnosing syphilis, which of the following is an example of a precipitation test?
- A. Rose waaler test
- B. Widal test
- C. Latex agglutination
- D. Kahn test (Correct Answer)
Explanation: ***Kahn test*** - The Kahn test is a **flocculation** or **precipitation** test used for diagnosing syphilis. - It detects **reagin antibodies** in the patient's serum that react with a non-treponemal antigen (cardiolipin antigen). *Rose waaler test* - The Rose Waaler test is an **agglutination test** used to detect **rheumatoid factor** in patients with rheumatoid arthritis, not syphilis. - It involves sheep red blood cells sensitized with rabbit anti-sheep erythrocyte antibody. *Widal test* - The Widal test is an **agglutination test** used for the diagnosis of **typhoid fever**, detecting antibodies against *Salmonella* O and H antigens. - It is not used for the diagnosis of syphilis. *Latex agglutination* - Latex agglutination is a general type of **agglutination test** where antigen or antibody is coated onto latex particles. - While used in various diagnoses, it is not a specific precipitation test for syphilis in the context of classic methods like the Kahn test.
Question 73: Haptens are immunogenic when they covalently bind to which type of carrier?
- A. Lipid carrier
- B. Polysaccharide carrier
- C. Protein carrier (Correct Answer)
- D. None of the above carrier
Explanation: ***Protein carrier*** - Haptens are small molecules that are **antigenic** but not **immunogenic** on their own; they acquire immunogenicity when covalently bound to a larger carrier molecule. - **Proteins** are highly effective carriers because their complex structures and multiple epitopes can induce strong T-cell help, which is crucial for a robust antibody response against the hapten. *Lipid carrier* - While some lipids can be antigenic (e.g., glycolipids), they generally do not serve as effective carriers for haptens to induce a strong adaptive immune response, especially T-cell-dependent responses. - **Lipids** are less likely to be processed and presented by MHC molecules in a way that generates potent helper T-cell activation. *Polysaccharide carrier* - Some polysaccharides can be immunogenic themselves (e.g., bacterial capsular polysaccharides) and can induce T-cell-independent antibody responses. - However, for haptens to become immunogenic and induce a **T-cell-dependent antibody response**, a protein carrier is typically required. *None of the above carrier* - This option is incorrect because haptens do require a carrier to become immunogenic, and specific types of carriers are more effective than others. - The type of carrier chosen significantly impacts the **strength and nature of the immune response** to the hapten.
Question 74: Which human IgG subclass has the highest serum concentration?
- A. IgG1 (Correct Answer)
- B. IgG2
- C. IgG3
- D. IgG4
Explanation: ***IgG1*** - **IgG1** constitutes approximately **60-70%** of the total IgG in human serum, making it the most abundant subclass. - Its high concentration reflects its crucial role in **neutralizing toxins**, agglutinating viruses, and opsonizing bacteria for phagocytosis. *IgG2* - **IgG2** makes up about **20-30%** of total IgG and is primarily important in protecting against **polysaccharide-encapsulated bacteria**. - While significant, its serum concentration is notably lower than that of IgG1. *IgG3* - **IgG3** is the least abundant IgG subclass, accounting for only **5-8%** of total IgG, and is characterized by a shorter half-life. - Despite its low concentration, it is highly efficient in activating the **complement system** due to its flexible hinge region. *IgG4* - **IgG4** comprises approximately **3-6%** of total IgG and is unique for its ability to undergo **Fab arm exchange**, leading to bispecific antibodies. - It does not activate complement and is often associated with allergy and protection against parasitic infections, but its concentration is significantly less than IgG1.
Question 75: Which of the following does not stimulate active immunity?
- A. Clinical infection
- B. Vaccination
- C. Transplacental antibody transfer in newborn (Correct Answer)
- D. Subclinical infection
Explanation: ***Transplacental antibody transfer in newborn*** - This is a form of **passive immunity**, where pre-formed **antibodies from the mother** are transferred to the newborn, providing immediate but temporary protection. - It does not involve the newborn's own immune system generating an immune response or creating **memory cells**. *Subclinical infection* - Even without overt symptoms, a **natural infection** exposes the immune system to pathogens, triggering an active immune response and generating **memory cells**. - This leads to **long-term immunity** against future exposures to the same pathogen. *Clinical infection* - A **symptomatic natural infection** involves the immune system actively responding to the pathogen, producing antibodies and **memory cells**. - This process is the basis of **naturally acquired active immunity** and provides durable protection. *Vaccination* - Vaccines contain weakened or inactive forms of pathogens, or their components, which stimulate the immune system to produce **antibodies** and **memory cells** without causing disease. - This is an example of **artificially acquired active immunity**, providing long-lasting protection.
Question 76: Which of the following is a specific feature of acquired immunity?
- A. Immunological memory (Correct Answer)
- B. Affected by genetic makeup
- C. No antigen exposure
- D. Immediate response
Explanation: ***Immunological memory*** - A key characteristic of **acquired immunity** is the ability to "remember" previous encounters with specific pathogens. - This memory leads to a more rapid and robust immune response upon subsequent exposure to the same pathogen. - This is the **defining feature** that distinguishes acquired immunity from innate immunity. *Affected by genetic makeup* - While genetic makeup can influence the *efficiency* of the acquired immune system, it is not a **specific feature** that distinguishes it from innate immunity. - **Both innate and acquired immunity** are affected by genetic factors, determining baseline resistance and immune response capability. *No antigen exposure* - **Acquired immunity** is specifically characterized by its *dependence* on antigen exposure to develop specific responses. - The phrase "no antigen exposure" describes how the **innate immune system** functions, providing immediate, non-specific protection without prior contact with a pathogen. *Immediate response* - **Innate immunity** provides an immediate, non-specific response to pathogens. - **Acquired immunity** takes time to develop (days to weeks) after initial antigen exposure, but provides a faster response upon re-exposure due to immunological memory.
Question 77: To which part of an antigen do monoclonal antibodies specifically bind?
- A. None of the options
- B. Specific epitope on the antigen (Correct Answer)
- C. Both the epitope and the paratope
- D. Part of the antibody that binds to the epitope
Explanation: ***Specific epitope on the antigen*** - Monoclonal antibodies are designed to recognize and bind to a **unique, specific region** on an antigen, known as an **epitope**. - This high specificity is crucial for their clinical applications, such as targeted therapies and diagnostic tests. *Both the epitope and the paratope* - The **epitope** is the part of the antigen, while the **paratope** is the part of the antibody that binds to the epitope. - An antibody binds to an epitope, not to both itself and its own binding site. *None of the options* - This option is incorrect because there is a correct answer among the choices provided, which accurately describes the binding site of monoclonal antibodies. *Part of the antibody that binds to the epitope* - This describes the **paratope**, which is the antigen-binding site on the antibody, not the part of the antigen to which the antibody binds. - The question specifically asks about the part of the antigen.
Question 78: What type of immunity is primarily associated with the administration of transfer factor?
- A. Natural active immunity
- B. Artificial active immunity
- C. Artificial passive immunity
- D. Adoptive immunity (Correct Answer)
Explanation: ***Adoptive immunity (Correct)*** - **Transfer factor** consists of small, dialyzable molecules extracted from immune T lymphocytes of an immune donor - Its administration transfers **cell-mediated immunity** from donor to recipient, which defines adoptive immunity - This represents transfer of **immune cells or their products** (not antibodies), providing antigen-specific cellular immunity - Also called **adoptive immunotherapy** or adoptive transfer *Artificial passive immunity (Incorrect)* - Involves the **transfer of pre-formed antibodies** (e.g., antitoxins, immunoglobulins, antiserum) from an immune individual or animal - Provides **immediate but temporary humoral protection** - Does NOT transfer cellular immunity - this is the key distinction from adoptive immunity - Examples: Anti-rabies immunoglobulin, anti-tetanus serum *Natural active immunity (Incorrect)* - Occurs when an individual is **naturally exposed to an antigen** (infection) and produces their own antibodies and immune cells - The host's own immune system **actively responds** to develop long-lasting immunity - Example: Immunity after recovering from measles or chickenpox *Artificial active immunity (Incorrect)* - Achieved through **vaccination** with attenuated, inactivated, or subunit antigens - The recipient's body is **actively stimulated** to produce protective immunity - Provides long-lasting protection through immunological memory
Question 79: Which of the following tests is not classified as an in vivo test?
- A. Schick test
- B. Elek's gel precipitation test (Correct Answer)
- C. Tuberculin test
- D. Lepromin test
Explanation: ***Elek's gel precipitation test*** - This test is an **in vitro** method used to detect the production of **diphtheria toxin** by *Corynebacterium diphtheriae* strains in a lab setting, not within a living organism. - It involves the precipitation of toxin and antitoxin in an agar gel, forming visible lines. *Schick test* - The Schick test is an **in vivo** test used to determine susceptibility to **diphtheria** by injecting diphtheria toxin subcutaneously and observing for a localized inflammatory reaction. - A positive reaction (inflammation) indicates a lack of protective antitoxin antibodies. *Lepromin test* - The Lepromin test is an **in vivo** test used to assess the type of **leprosy** a patient has and to evaluate their cell-mediated immune response to *Mycobacterium leprae*. - It involves the intradermal injection of lepromin antigen and observing for a delayed hypersensitivity reaction. *Tuberculin test* - Also known as the **Mantoux test**, this is an **in vivo** test used to screen for exposure to **tuberculosis** by injecting purified protein derivative (PPD) intradermally. - A positive reaction indicates a **cell-mediated immune response** to *Mycobacterium tuberculosis*.
Question 80: Which microorganism is the most common cause of pyogenic osteomyelitis?
- A. S. aureus (Correct Answer)
- B. Streptococcus spp.
- C. Corynebacterium spp.
- D. Neisseria gonorrhoeae (gonococcus)
Explanation: ***Staph aureus*** - **_Staphylococcus aureus_** is the most frequent cause of **pyogenic osteomyelitis** across all age groups and routes of infection. - Its ability to adhere to bone, form biofilms, and produce toxins contributes to its prevalence in bone infections. *Streptococcus spp.* - While various **_Streptococcus_** species can cause infections, they are less common causes of pyogenic osteomyelitis compared to **_Staphylococcus aureus_**. - **Group A _Streptococcus_** can cause severe invasive infections but rarely involves primary bone infection. *Corynebacterium spp.* - **_Corynebacterium_** species, particularly **_Corynebacterium striatum_**, are increasingly recognized as opportunistic pathogens, especially in immunocompromised individuals or those with foreign bodies. - However, they are not the most common cause of osteomyelitis in the general population. *Neisseria gonorrhoeae (gonococcus)* - **_Neisseria gonorrhoeae_** can cause **disseminated gonococcal infection (DGI)**, which may include joint involvement (**septic arthritis**). - While it can lead to bone pain and swelling, it primarily affects joints and is a less common cause of direct **pyogenic osteomyelitis** than **_S. aureus_**.