Immunology Practice Questions

Q: The most potent antigen-presenting cell is?

Dendritic cells. ***Dendritic cells*** - Dendritic cells are the **most potent antigen-presenting cells** [1], crucial for T-cell activation and initiating immune responses. - They capture, process, and present antigens to T cells, leading to a **robust adaptive immune response** [1]. *T cells* - T cells are primarily **effector cells** in the immune system, not main antigen presenters. - While they can present antigens through MHC molecules, they are less efficient compared to dendritic cells. *NK cells* - Natural Killer (NK) cells primarily target **virus-infected** and **tumor** cells, functioning in innate immunity. - They do not serve as antigen-presenting cells and do not activate T cells as effectively as dendritic cells. *B cells* - B cells can present antigens and produce **antibodies**, but they are not as potent as dendritic cells in initiating T-cell responses. - Their main role is in **humoral immunity**, while dendritic cells excel in activating naïve T cells. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 199-200, 204-208.

Q: A possible source of the "second signal" to a B-cell bound by specific antigen includes

Antigen-specific T-cells. ***Antigen-specific T-cells*** - T-cells, specifically **helper T-cells (Th2 cells)**, provide co-stimulatory signals (the "second signal") to antigen-bound B-cells via interactions such as **CD40L on the T-cell binding to CD40 on the B-cell**. - This interaction, along with cytokine release from the T-cell, is crucial for B-cell **activation, proliferation, differentiation**, and class switching. *Epstein-Barr virus (EBV) infection* - EBV can activate B-cells **polyclonally** and promote their proliferation and differentiation, bypassing the need for T-cell help or an initial antigen-specific signal. - However, this is an antigen-independent activation mechanism, not a typical "second signal" for a naturally antigen-bound B-cell. *Non-specific endotoxin (LPS)* - **Lipopolysaccharide (LPS)** is a potent **mitogen** for B-cells, meaning it can cause their proliferation and activation without specific antigen recognition. - While it can initiate B-cell responses, it functions as a **T-cell-independent activator**, not providing the co-stimulatory "second signal" required for T-cell dependent antigens. *Differentiated plasma cells* - **Plasma cells** are the terminally differentiated effector cells of the B-cell lineage that primarily produce and secrete **antibodies**. - They are not involved in providing the "second signal" for the initial activation of naive B-cells; rather, they are the end product of B-cell activation.

Q: IgE is secreted by which of the following cells?

Plasma cells. ***Plasma cells*** - Plasma cells are **terminally differentiated B lymphocytes** that are specialized in producing and secreting large quantities of antibodies, including **IgE**. - While other cells like mast cells and basophils have receptors for IgE and play roles in IgE-mediated reactions, they do not synthesize **IgE** themselves. *Mast cell* - Mast cells are key players in allergic reactions and express **FcεRI receptors** that bind to IgE antibodies. - Upon binding the antigen, they degranulate, releasing **histamine** and other mediators, but they do not produce IgE. *Basophils* - Basophils also express **FcεRI receptors** for IgE and are involved in allergic responses, releasing inflammatory mediators. - Similar to mast cells, they do not synthesize IgE, but rather bind pre-formed **IgE** antibodies. *Eosinophils* - Eosinophils are involved in allergic reactions and defense against **parasitic infections**, and their granules contain toxic proteins. - They can be activated by IgE-mediated mechanisms but are not producers of **IgE** antibodies.

Q: CD marker for cytotoxic T-cells:

CD 8. ***CD 8*** - CD 8 is a specific **marker for cytotoxic T-cells**, which play a crucial role in directly killing infected or cancerous cells [1]. - Cells expressing CD 8 are essential for **cell-mediated immunity**, particularly in recognizing and eliminating virus-infected cells. *CD 21* - CD 21 is a marker primarily associated with **B-lymphocytes** and acts as a receptor for **C3d**, involved in the activation of B-cells. - It does not play a role in the identification or function of **cytotoxic T-cells**. *CD 4* - CD 4 is a marker for **helper T-cells**, which assist other cells in the immune response but do not directly kill targets [1]. - It is involved in enhancing the immune response rather than cytotoxic functions. *CD 45* - CD 45 is a pan-leukocyte marker found on all **leukocytes** and is not specific to cytotoxic T-cells. - While it indicates the presence of immune cells, it does not differentiate between various T-cell subtypes. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 198-199.

Q: Which complement component is involved in both the classical and alternative pathways?

C3. ***C3*** - **C3** is a central component of the complement system, acting as a point of convergence for both the **classical** and **alternative pathways**. - Its cleavage product, **C3b**, is crucial for opsonization and the formation of the **C5 convertase**, thereby initiating the **membrane attack complex (MAC)**. *C1* - **C1** is unique to the **classical complement pathway** and is responsible for its activation, typically initiated by antigen-antibody complexes. - It consists of C1q, C1r, and C1s, and its primary role is to cleave **C4** and **C2**. *C2* - **C2** is exclusively involved in the **classical** and **lectin pathways**, where it is cleaved by C1 or **MASP** respectively. - The resulting fragment, **C2a**, combines with C4b to form the **C3 convertase** (C4b2a). *C4* - Similar to C2, **C4** is a component of the **classical** and **lectin pathways**, not the alternative pathway. - It is cleaved by C1 or **MASP** to form **C4a** and **C4b**, with C4b binding to the pathogen surface.

Q: Which among the following is NOT transmitted transplacentally?

IgE. ***IgE*** - **IgE antibodies do NOT cross the placenta** due to their molecular structure and lack of specific Fc receptors on placental syncytiotrophoblasts. - Only **IgG** is actively transported across the placenta via FcRn receptors to provide passive immunity to the fetus. - Maternal IgE does not provide passive immunity or transmit allergic predispositions transplacentally. *Herpes* - **Herpes simplex virus (HSV)** transmission is predominantly **intrapartum** (85% of cases) during vaginal delivery through contact with infected genital secretions. - **Congenital (transplacental) HSV** is rare, representing only **5% of neonatal HSV cases**, but can cause severe outcomes including skin lesions, chorioretinitis, and neurological defects. - While transplacental transmission can occur, it is **uncommon** compared to intrapartum transmission. *CMV* - **Cytomegalovirus (CMV)** is the **most common congenital infection** and is readily transmitted transplacentally. - Occurs in **0.5-2% of all live births** and can result from primary or recurrent maternal infection. - **Congenital CMV** can lead to sensorineural hearing loss, intellectual disability, microcephaly, chorioretinitis, and other neurological sequelae. *EB virus* - **Epstein-Barr virus (EBV)** transplacental transmission is **extremely rare** and not well-documented. - Unlike CMV, EBV is **not considered a typical congenital infection**, and most EBV infections occur postnatally through saliva contact. - While there are rare case reports, congenital EBV is not a recognized cause of routine prenatal screening or concern.

Q: Which complement proteins representing both classical and alternative pathways are primarily synthesized in the liver?

C1, C4, C2, factor B. ***C1, C4, C2, factor B*** - This option correctly identifies key complement proteins from **both the classical pathway (C1, C4, C2) and the alternative pathway (factor B)** that are synthesized in the liver. - These proteins represent the liver's critical role in producing components for **multiple complement activation pathways**. - This is the most specific answer highlighting the liver's role in synthesizing proteins from different complement pathways. *C3, C6, C9* - While **C3, C6, and C9** are indeed synthesized in the liver, this option is incomplete. - It omits other major liver-derived complement components from the classical and alternative pathways. - C3 is the most abundant complement protein, but this option doesn't represent both pathways comprehensively. *Factor H, factor I, DAF* - **Factor H** and **Factor I** are regulatory proteins synthesized in the liver, but **DAF (Decay-accelerating factor, CD55)** is a membrane-bound complement regulatory protein. - DAF is expressed on cell surfaces and is not a secreted liver-derived complement protein. - This option focuses on regulatory proteins rather than pathway components. *C1-C9, factor B, factor D, properdin* - While this option is broadly correct (most of these are liver-synthesized), it lists an extensive range of proteins without specifically highlighting the pathway representation. - The question asks for proteins representing both pathways, and this comprehensive list is less focused than the specific classical and alternative pathway components in the correct answer. - This option is too exhaustive for the focused intent of the question.

Q: Which immunoglobulin provides natural passive immunity to a newborn through the placenta?

IgG. ***IgG*** - **IgG** is the only immunoglobulin that can cross the **placenta**, providing the fetus with **passive immunity** from the mother's antibodies. - This maternal IgG protects the newborn from various infections during the first few months of life until its own immune system matures. *IgA* - **IgA** is primarily found in **mucosal secretions**, such as breast milk, saliva, tears, and gastrointestinal fluids. - While important for newborn immunity via **breastfeeding**, it does not cross the placenta. *IgE* - **IgE** is mainly involved in **allergic reactions** and defense against **parasitic infections**. - It does not cross the placenta to provide passive immunity to the fetus. *IgD* - **IgD** is primarily found on the surface of **B lymphocytes** and is involved in B-cell activation. - Its function is not related to passive fetal immunity through the placenta.

Q: Which immunoglobulin is most efficient at fixing C1q in the classical complement pathway?

IgM. ***IgM*** - **IgM** is the most efficient immunoglobulin at activating the **classical complement pathway** due to its pentameric structure, which provides multiple binding sites for C1q. - The **Fc portion** of IgM, when bound to an antigen, undergoes a conformational change that exposes binding sites for the **C1q component** of complement. *IgA* - **IgA** primarily functions in **mucosal immunity** and does not efficiently activate the classical complement pathway. - It can weakly activate the **alternative complement pathway** but is not known for fixing C1. *IgG* - **IgG** can activate the classical complement pathway, but it requires two or more IgG molecules to be in close proximity on the cell surface to effectively bind C1q, making it less efficient than IgM. - Its **Fc region** binds C1q only when **antigen-antibody complexes** are formed, but it's not the primary immunoglobulin for initiating C1 binding in solitary form. *IgE* - **IgE** is primarily involved in **allergic reactions** and defense against parasites, binding to Fc receptors on mast cells and basophils. - It does **not activate the complement system** via the classical pathway and therefore does not fix C1.

Question 1

The most potent antigen-presenting cell is?

Accepted Answer

Dendritic cells

Answer

B cells

Answer

T cells

Answer

NK cells

Question 2

A possible source of the "second signal" to a B-cell bound by specific antigen includes

Accepted Answer

Antigen-specific T-cells

Answer

Non-specific endotoxin (LPS)

Answer

Differentiated plasma cells

Answer

Epstein-Barr virus (EBV) infection

Question 3

IgE is secreted by which of the following cells?

Accepted Answer

Plasma cells

Answer

Mast cell

Answer

Basophils

Answer

Eosinophils

Question 4

Which of the following statements about the secondary immune response is false?

Accepted Answer

Immune response against a subsequent antigenic challenge is absent.

Answer

The lag period is absent or significantly shorter.

Answer

There is a negative phase in the response.

Answer

Only T-dependent antigens are recognized.

Question 5

CD marker for cytotoxic T-cells:

Accepted Answer

CD 8

Answer

CD 45

Answer

CD 4

Answer

CD 21

Question 6

Which complement component is involved in both the classical and alternative pathways?

Accepted Answer

C3

Answer

C1

Answer

C2

Answer

C4

Question 7

Which among the following is NOT transmitted transplacentally?

Accepted Answer

IgE

Answer

Herpes

Answer

CMV

Answer

EB virus

Question 8

Which complement proteins representing both classical and alternative pathways are primarily synthesized in the liver?

Accepted Answer

C1, C4, C2, factor B

Answer

C3, C6, C9

Answer

Factor H, factor I, DAF

Answer

C1-C9, factor B, factor D, properdin

Question 9

Which immunoglobulin provides natural passive immunity to a newborn through the placenta?

Accepted Answer

IgG

Answer

IgA

Answer

IgE

Answer

IgD

Question 10

Which immunoglobulin is most efficient at fixing C1q in the classical complement pathway?

Accepted Answer

IgM

Answer

IgA

Answer

IgG

Answer

IgE

Immunology — MCQs

Immunology — MCQs

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