Microbiology - MCQ Practice Questions
Microbiology sits behind a lot of applied biology, so the fundamentals here carry into medicine, food technology and biotechnology alike. Practice covers bacterial structure and growth, viruses, fungi and parasites, sterilisation and culture techniques, immunology, and microbial genetics. Technique based questions explain the purpose of each step, because that is usually what separates a memorised protocol from a usable one.
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Which type of hypersensitivity reaction is responsible for anaphylaxis, and which immunoglobulin class is primarily involved?
Understanding:
We need to identify the type of hypersensitivity reaction underlying anaphylaxis and the immunoglobulin class involved.
Step 1: Classify hypersensitivity reactions
The Gell and Coombs classification divides hypersensitivity into four types:
Step 2: Identify the mechanism of anaphylaxis
Anaphylaxis is the prototypical Type I (immediate) hypersensitivity reaction. On first antigen exposure, IgE antibodies are produced and bind to high-affinity FcεRI receptors on mast cells and basophils. On re-exposure, the antigen cross-links IgE-bound receptors, triggering degranulation and release of histamine, leukotrienes, and prostaglandins, leading to systemic anaphylaxis.
Step 3: Rule out other options
Type II involves IgG/IgM against cell-surface antigens (e.g., haemolytic anaemia). Type III involves immune complex deposition (e.g., serum sickness). Type IV is T-cell mediated with no immunoglobulin involvement (e.g., contact dermatitis, tuberculin reaction).
Answer:
Anaphylaxis is a Type I hypersensitivity reaction primarily mediated by IgE antibodies bound to mast cells and basophils.
Quick Tip:
Remember the mnemonic: Type I = Immediate = IgE = mast cells. Anaphylaxis, allergic rhinitis, and asthma are classic Type I disorders.
A patient with recurrent bacterial infections is found to have normal B and T cell counts but deficiency in a serum protein that is the most abundant immunoglobulin and provides the majority of secondary immune response. Which immunoglobulin is deficient?
Understanding:
We need to identify the most abundant serum immunoglobulin responsible for the secondary (anamnestic) immune response.
Step 1: Review immunoglobulin classes and their serum levels
The approximate serum concentrations of immunoglobulins are:
Step 2: Identify the role in secondary immune response
IgG is the predominant immunoglobulin produced during secondary (anamnestic) immune responses. It is the only immunoglobulin capable of crossing the placenta (providing passive immunity to neonates), can fix complement (via classical pathway), and opsonises pathogens. It has the longest half-life (~23 days) among immunoglobulins.
Step 3: Correlate with the clinical scenario
Recurrent bacterial infections with normal lymphocyte counts but low serum immunoglobulin of the most abundant class points specifically to IgG deficiency, the hallmark of common variable immunodeficiency (CVID) or selective IgG subclass deficiency.
Answer:
The deficient immunoglobulin is IgG, the most abundant serum immunoglobulin and the principal mediator of secondary humoral immune responses.
Quick Tip:
IgG is the only Ig that crosses the placenta. IgM is the first Ig produced in a primary response and is the largest immunoglobulin (pentamer). Do not confuse them.
Which of the following cells are the primary antigen-presenting cells (APCs) in the skin, responsible for initiating adaptive immune responses against cutaneous antigens?
Understanding:
We need to identify the professional antigen-presenting cells located specifically in the skin.
Step 1: Identify tissue-specific macrophage/dendritic cell types
Dendritic cells and macrophages are distributed throughout body tissues with tissue-specific names:
Step 2: Role of Langerhans cells
Langerhans cells are immature dendritic cells residing in the epidermis. They capture antigens at the skin surface, process them, and migrate to regional lymph nodes where they mature into fully functional dendritic cells. They express MHC class II molecules and co-stimulatory molecules (CD80, CD86), enabling them to present antigens to naive T helper cells and initiate adaptive immunity. They are characterised by Birbeck granules (tennis racket-shaped) on electron microscopy.
Step 3: Confirm the answer
Kupffer cells serve hepatic immune surveillance; microglial cells are CNS-resident macrophages; mesangial cells provide structural and phagocytic support in the glomerulus but are not classical APCs.
Answer:
Langerhans cells are the primary antigen-presenting cells of the skin, identified by their characteristic Birbeck granules on electron microscopy.
Quick Tip:
Birbeck granules are pathognomonic for Langerhans cells and are also seen in Langerhans cell histiocytosis (formerly called Histiocytosis X).
The classical pathway of complement activation is initiated by which of the following?
Understanding:
We need to identify the initiating event specific to the classical pathway of complement activation.
Step 1: Overview of complement pathways
There are three pathways of complement activation:
Step 2: Mechanism of classical pathway initiation
C1 is the first component of the classical pathway. It is a macromolecular complex consisting of C1q, C1r, and C1s (in the ratio C1q:C1r2:C1s2). C1q has six globular heads that bind to the Fc regions of IgG or IgM within an antigen-antibody complex. Binding of C1q triggers auto-activation of C1r, which then cleaves and activates C1s. Activated C1s cleaves C4 and C2, forming the C3 convertase C4b2a.
Step 3: Eliminate distractors
MBL binding to mannose residues initiates the lectin pathway. Spontaneous C3 hydrolysis initiates the alternative pathway (tick-over mechanism). Factor B activation by microbial surfaces is also part of the alternative pathway.
Answer:
The classical complement pathway is initiated by binding of C1q to antigen-antibody complexes, specifically via the Fc regions of IgG or IgM.
Quick Tip:
Only IgM and IgG (except IgG4) can activate the classical complement pathway. IgM is far more efficient per molecule because a single IgM pentamer provides multiple Fc binding sites for C1q.
A child presents with recurrent severe bacterial and fungal infections. Laboratory findings reveal an absence of CD18 surface protein on neutrophils, leading to inability of neutrophils to adhere to vascular endothelium. Which immunodeficiency disorder does this child most likely have?
Understanding:
We need to identify an immunodeficiency characterised by absence of CD18 (β2-integrin subunit) causing defective neutrophil adhesion.
Step 1: Role of CD18 in neutrophil function
CD18 is the β2-integrin subunit that pairs with different α subunits to form functional integrins:
These integrins are essential for neutrophil rolling, firm adhesion to endothelium, and transmigration to infection sites.
Step 2: Mechanism of LAD type I
In Leukocyte Adhesion Deficiency (LAD) type I, there is an autosomal recessive mutation in the gene encoding CD18 (ITGB2 gene). This prevents surface expression of all β2-integrins, so neutrophils cannot firmly adhere to inflamed endothelium or migrate to sites of infection. Clinically: recurrent severe bacterial and fungal infections, delayed umbilical cord separation, markedly elevated neutrophil count in blood (neutrophilia) despite absence of pus formation.
Step 3: Distinguish from other options
Answer:
This child has Leukocyte Adhesion Deficiency (LAD) type I, caused by deficiency of CD18, the β2-integrin subunit required for neutrophil adhesion to vascular endothelium.
Quick Tip:
A hallmark clue for LAD is persistently elevated blood neutrophil count (neutrophilia) with no pus formation at infection sites, because neutrophils cannot exit the bloodstream.
Which of the following cytokines is produced by activated macrophages and is the principal mediator responsible for fever, acute-phase protein synthesis, and cachexia during systemic inflammatory responses?
Understanding:
We need to identify the cytokine primarily responsible for fever, acute-phase protein synthesis, and cachexia (wasting) in systemic inflammation.
Step 1: Identify the key cytokines of innate immunity
Macrophages activated by pattern recognition (e.g., LPS binding to TLR4) produce pro-inflammatory cytokines including:
Step 2: Functions of TNF-α
TNF-α is the master regulator of acute inflammatory responses:
Step 3: Eliminate distractors
Answer:
TNF-α is the principal macrophage-derived cytokine responsible for fever, acute-phase protein synthesis, and cachexia during systemic inflammation.
Quick Tip:
TNF-α was originally named "cachectin" because of its role in cancer-related wasting (cachexia). Anti-TNF biologics (infliximab, etanercept) are now used therapeutically in rheumatoid arthritis and other inflammatory diseases.
MHC class I molecules present antigens to which subset of T lymphocytes, and which co-receptor on those T cells binds MHC class I?
Understanding:
We need to identify the T cell subset that interacts with MHC class I molecules and the co-receptor involved in this interaction.
Step 1: MHC class I structure and distribution
MHC class I molecules (HLA-A, HLA-B, HLA-C in humans) are expressed on virtually all nucleated cells. They present endogenous peptides (e.g., viral proteins, tumour antigens) generated by proteasomal degradation via the TAP transporter pathway to the endoplasmic reticulum.
Step 2: T cell recognition of MHC class I
CD8+ cytotoxic T lymphocytes (CTLs) recognise antigen presented in the groove of MHC class I molecules via their T cell receptor (TCR). The CD8 co-receptor binds to the α3 domain of MHC class I, stabilising the TCR-MHC-peptide interaction and transducing activating signals via Lck (a tyrosine kinase).
Step 3: Contrast with MHC class II
MHC class II molecules (HLA-DP, HLA-DQ, HLA-DR) present exogenous antigens (from phagocytosed pathogens) to CD4+ T helper cells. CD4 co-receptor binds to the β2 domain of MHC class II.
The rule: CD8 × MHC I = 8 × 1 = 8; CD4 × MHC II = 4 × 2 = 8. This mathematical trick helps remember both interactions.
Answer:
MHC class I molecules present antigens to CD8+ cytotoxic T cells, and the CD8 co-receptor binds the α3 domain of MHC class I to stabilise the interaction.
Quick Tip:
The multiplication trick: CD4 × 2 = 8 and CD8 × 1 = 8. Both equal 8, confirming CD4 pairs with MHC II and CD8 pairs with MHC I.
Opsonisation is a critical process that enhances phagocytosis of pathogens. Which of the following combinations of opsonins is MOST effective in promoting phagocytosis by neutrophils and macrophages?
Understanding:
We need to identify the most effective opsonin combination that promotes phagocytosis by neutrophils and macrophages.
Step 1: Define opsonisation
Opsonisation is the coating of a pathogen with host molecules (opsonins) that are recognised by specific receptors on phagocytes, thereby enhancing phagocytic uptake.
Step 2: Key opsonins and their receptors on phagocytes
The two most potent opsonins are:
Together, IgG and C3b act synergistically as the most effective opsonin pair, because C3b is deposited on antibody-coated particles via the classical complement pathway, and both receptors on phagocytes engage simultaneously.
Step 3: Analyse distractors
Answer:
The most effective opsonin combination for phagocytosis is IgG (acting via Fcγ receptors) and C3b (acting via complement receptor CR1), which together synergistically promote uptake by neutrophils and macrophages.
Quick Tip:
Remember: C3a and C5a are chemotactic anaphylatoxins (they call phagocytes to the scene); C3b is the opsonin (it coats the pathogen for recognition). These functions are distinct and commonly tested.
Regulatory T cells (Tregs) are essential for maintaining peripheral tolerance and preventing autoimmunity. Which of the following transcription factors is considered the master regulator of Treg development and function?
Understanding:
We need to identify the master transcription factor that drives regulatory T cell (Treg) differentiation and function.
Step 1: T cell subset-specific transcription factors
Each CD4+ T helper subset is governed by a lineage-defining transcription factor:
Step 2: Role of FoxP3 in Tregs
FoxP3 is the master transcription factor that:
Step 3: Confirm by elimination
T-bet, GATA-3, and RORγt are master regulators of Th1, Th2, and Th17 cells respectively — none of them define Treg lineage.
Answer:
FoxP3 is the master transcription factor for regulatory T cells (Tregs), and its mutation causes IPEX syndrome, a life-threatening autoimmune disorder.
Quick Tip:
IPEX syndrome (FoxP3 mutation) is the classic exam association. It presents in infant males with diabetes mellitus, enteropathy, eczema, and recurrent infections — an X-linked condition caused by absence of functional Tregs.
In the context of B cell maturation and immunoglobulin gene rearrangement, which of the following processes is responsible for generating the vast diversity of antigen-binding specificities of antibodies?
Understanding:
We need to identify the process specifically responsible for generating the primary diversity of antigen-binding specificities of antibodies during B cell maturation.
Step 1: Sources of antibody diversity
Multiple mechanisms contribute to antibody diversity:
1. VDJ recombination (combinatorial diversity): random joining of Variable (V), Diversity (D), and Joining (J) gene segments for heavy chains; VJ joining for light chains — generates the primary repertoire BEFORE antigen exposure
2. Junctional diversity: imprecise joining, addition of P-nucleotides and N-nucleotides (by TdT) at V-D and D-J junctions
3. Combinatorial pairing of heavy and light chains
4. Somatic hypermutation: point mutations in variable region genes AFTER antigen exposure, in germinal centres, enabling affinity maturation
Step 2: Distinguish VDJ recombination from other processes
VDJ recombination generates the PRIMARY and VAST initial diversity of antigen-binding sites (estimated to produce >10^11 different specificities). It occurs in the bone marrow during B cell development before any antigen encounter and is mediated by RAG-1 and RAG-2 enzymes.
Step 3: Eliminate distractors
Answer:
VDJ recombination is the primary mechanism responsible for generating the vast diversity of antigen-binding specificities of antibodies, occurring in the bone marrow before antigen encounter under the action of RAG-1 and RAG-2 recombinases.
Quick Tip:
Class switch recombination changes the antibody class (effector function) but keeps the antigen-binding site identical. This distinction between diversity generation (VDJ) and isotype switching is a favourite exam concept.