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.
180 questions | 100% Free
Which of the following fungi is associated with 'Tinea versicolor' (Pityriasis versicolor), causing hypopigmented or hyperpigmented skin patches?
Understanding:
We need to identify the fungus responsible for Tinea versicolor, a superficial skin infection presenting with hypo- or hyperpigmented patches.
Step 1: Identify the causative fungus
Malassezia furfur (also known as Pityrosporum orbiculare/ovale) is the causative organism of Tinea versicolor. It is a lipophilic, dimorphic yeast that is part of normal skin flora but causes disease under warm, humid conditions or in immunocompromised states.
Step 2: Pathogenesis of pigmentation changes
M. furfur produces azelaic acid and other metabolites that inhibit tyrosinase in melanocytes, causing hypopigmentation (in dark skin). On light skin, inflammation causes hyperpigmentation. The organism produces 'spaghetti and meatballs' appearance on KOH preparation — short hyphae (spaghetti) and yeast cells (meatballs).
Step 3: Eliminate distractors
Answer:
Malassezia furfur is the causative organism of Tinea versicolor.
Quick Tip:
KOH preparation showing 'spaghetti and meatballs' = Malassezia furfur. Culture requires lipid-supplemented media (olive oil overlay) due to its lipophilic nature.
Which of the following is the most common opportunistic fungal infection in HIV/AIDS patients with a CD4 count below 100 cells/μL?
Understanding:
We need to identify the most common opportunistic fungal infection specifically when CD4 count drops below 100 cells/μL in HIV/AIDS.
Step 1: CD4 thresholds for opportunistic infections
Step 2: Cryptococcus neoformans in AIDS
Cryptococcus neoformans causes cryptococcal meningitis, the most common cause of meningitis in AIDS patients and a leading cause of AIDS-related mortality globally. India ink preparation shows the characteristic wide polysaccharide capsule. Latex agglutination test detects cryptococcal antigen in CSF.
Step 3: Clarify the distinction
While Pneumocystis jirovecii pneumonia is among the most common AIDS-defining infections overall, Cryptococcus neoformans is the most common at CD4 < 100 and is specifically the most common fungal cause of meningitis in this population.
Answer:
Cryptococcus neoformans is the most common opportunistic fungal infection at CD4 < 100 cells/μL.
Quick Tip:
India ink preparation showing encapsulated yeast in CSF of an AIDS patient = Cryptococcus neoformans until proven otherwise. Amphotericin B + flucytosine is the induction treatment.
Which of the following is the reservoir of Rabies virus in India, making it responsible for the majority of human rabies cases?
Understanding:
We need to identify the primary reservoir responsible for the majority of human rabies cases specifically in the Indian context.
Step 1: Global vs. Indian epidemiology
Globally, rabies reservoirs vary by region:
Step 2: Indian epidemiology
India accounts for approximately 36% of the world's rabies deaths. Domestic dogs are responsible for >95% of human rabies cases in India. The rabies virus (Lyssavirus genus, Rhabdoviridae family) is transmitted through saliva via bites.
Step 3: Pathological landmark
Negri bodies — eosinophilic intracytoplasmic inclusion bodies found in hippocampal neurons (Ammon's horn) and Purkinje cells of the cerebellum — are pathognomonic of rabies infection.
Answer:
Dogs are the primary reservoir responsible for most human rabies cases in India.
Quick Tip:
Post-exposure prophylaxis (PEP) for rabies includes: wound washing + rabies immunoglobulin (passive) + rabies vaccine (active). The vaccine schedule is Days 0, 3, 7, 14 (Essen regimen).
Which serotyping method is used to identify Salmonella typhi, based on its somatic, flagellar, and surface antigens?
Understanding:
We need to identify the serotyping scheme used to classify Salmonella species, including Salmonella typhi, based on their O, H, and Vi antigens.
Step 1: Antigenic structure of Salmonella
Salmonella species possess three key antigens:
Step 2: The Kauffmann-White scheme
The Kauffmann-White scheme is the internationally recognized classification system that serotypes all Salmonella strains based on combinations of O, H, and Vi antigens. S. typhi is classified as O:9,12; H:d; Vi.
Step 3: Eliminate distractors
Answer:
The Kauffmann-White scheme is used to serotype Salmonella species.
Quick Tip:
The Widal test detects antibodies against O and H antigens of S. typhi — it uses the same antigen system classified by the Kauffmann-White scheme. A rising titer (4-fold) is diagnostically significant.
Which of the following viruses is the causative agent of infectious mononucleosis and is associated with Burkitt's lymphoma and nasopharyngeal carcinoma?
Understanding:
We need to identify the virus that causes infectious mononucleosis AND is associated with specific malignancies.
Step 1: Epstein-Barr Virus (EBV)
EBV (Human Herpesvirus 4) is a double-stranded DNA virus of the Herpesviridae family. It infects B lymphocytes via the CD21 (CR2) receptor using its gp350 glycoprotein.
Step 2: Clinical associations of EBV
Step 3: Eliminate distractors
Answer:
Epstein-Barr Virus (EBV) causes infectious mononucleosis and is linked to Burkitt's lymphoma and nasopharyngeal carcinoma.
Quick Tip:
The Monospot (Paul-Bunnell heterophile antibody) test is positive in EBV mononucleosis but NEGATIVE in CMV mononucleosis — a key diagnostic differentiator.
Which antibiotic targets the 30S ribosomal subunit by binding to the 16S rRNA, preventing aminoacyl-tRNA binding?
Understanding:
We need to identify an antibiotic that targets the 30S ribosomal subunit by binding to 16S rRNA and blocking aminoacyl-tRNA from entering the A site.
Step 1: Mechanism of streptomycin
Streptomycin is an aminoglycoside antibiotic that binds irreversibly to the 16S rRNA of the 30S ribosomal subunit. This binding:
Step 2: Mechanisms of other antibiotics (eliminate distractors)
Step 3: 30S vs. 50S inhibitors
Answer:
Streptomycin targets the 30S subunit by binding to 16S rRNA, preventing aminoacyl-tRNA binding.
Quick Tip:
Mnemonic: '30S: A-TET-aminoglycoside' — aminoglycosides and tetracyclines both target the 30S subunit but by different mechanisms (aminoglycosides cause misreading; tetracyclines block tRNA entry reversibly).
A patient returning from sub-Saharan Africa presents with intermittent fever every 48 hours, hemolytic anemia, and splenomegaly. Blood smear shows ring-form trophozoites and 'banana-shaped' gametocytes. Which Plasmodium species is responsible?
Understanding:
We have a patient with malaria from sub-Saharan Africa presenting with tertian fever (every 48 hours), and the blood smear shows ring-form trophozoites with characteristic crescent/banana-shaped gametocytes.
Step 1: Identify the pathognomonic feature
Banana-shaped (crescent-shaped) gametocytes are PATHOGNOMONIC of Plasmodium falciparum. No other Plasmodium species has this gametocyte morphology.
Step 2: Other smear features of P. falciparum
Step 3: Fever periodicity
P. falciparum is the deadliest species, causing cerebral malaria, severe anemia, and blackwater fever.
Answer:
Plasmodium falciparum is responsible, identified by its banana-shaped gametocytes.
Quick Tip:
Banana/crescent-shaped gametocytes = P. falciparum exclusively. In competitive exams, this single morphological clue is sufficient to identify the species.
Which of the following bacteria produces a heat-labile exotoxin (LT) and a heat-stable exotoxin (ST) that cause watery diarrhea by activating adenylate cyclase and guanylate cyclase respectively?
Understanding:
This question asks about the bacterium that produces both a heat-labile toxin (LT) acting via adenylate cyclase and a heat-stable toxin (ST) acting via guanylate cyclase to cause watery diarrhea.
Step 1: Identify the toxin profile
Enterotoxigenic Escherichia coli (ETEC) is the classic organism that produces two distinct toxins:
Step 2: Eliminate other options
Vibrio cholerae produces only choleragen (CT), a heat-labile toxin acting via adenylate cyclase — it does not produce ST. Shigella dysenteriae produces Shiga toxin (cytotoxic, inhibits protein synthesis) — not LT/ST. Campylobacter jejuni produces cytolethal distending toxin, not LT/ST.
Step 3: Confirm the mechanism
ETEC is a major cause of traveler's diarrhea and infantile diarrhea in developing countries. The dual toxin system (LT + ST) is a hallmark feature used in exam questions to distinguish ETEC from other diarrheal pathogens.
Answer:
The organism producing both LT (via adenylate cyclase) and ST (via guanylate cyclase) is Enterotoxigenic Escherichia coli (ETEC).
Quick Tip:
Memory aid — ETEC has Two Toxins (LT + ST); Vibrio cholerae has only ONE toxin (CT/LT-like). The ST toxin is unique to ETEC among common exam pathogens.
A Gram-negative diplococcus is isolated from a urethral swab of a 22-year-old male presenting with purulent urethral discharge. The organism is oxidase-positive and grows on Thayer-Martin medium. Which of the following virulence factors of this organism allows it to undergo antigenic variation, thereby evading host immunity?
Understanding:
This question describes Neisseria gonorrhoeae (Gram-negative diplococcus, oxidase-positive, grows on Thayer-Martin selective medium). The question asks about the virulence factor responsible for antigenic variation.
Step 1: Identify the organism
The clinical and microbiological features — purulent urethral discharge, Gram-negative intracellular diplococci, oxidase-positive, Thayer-Martin medium growth — all point to Neisseria gonorrhoeae.
Step 2: Understand antigenic variation in N. gonorrhoeae
Pili (Type IV fimbriae) of N. gonorrhoeae undergo phase variation (on/off switching) and antigenic variation through a gene conversion mechanism involving multiple silent pilin gene copies (pilS loci) and one expression locus (pilE). This generates diverse pilin proteins that are not recognized by previously formed antibodies, allowing the organism to evade humoral immunity.
Step 3: Evaluate other options
Answer:
Pili (Type IV fimbriae) are responsible for the antigenic variation in Neisseria gonorrhoeae through gene conversion at the pilE locus.
Quick Tip:
Antigenic variation of pili is the reason a protective vaccine against gonorrhea has been difficult to develop — a high-yield fact for competitive exams.
Which of the following culture media is specifically used for the isolation of Bordetella pertussis, the causative agent of whooping cough?
Understanding:
This question asks about the selective culture medium used for isolating Bordetella pertussis, the bacterium responsible for whooping cough (pertussis).
Step 1: Identify the correct medium
Bordet-Gengou medium (also called glycerol-potato-blood agar) is the classic selective medium for Bordetella pertussis. It contains glycerol, potato extract, and 20–30% sheep blood. Colonies appear pearly, mercury-drop like (bismuth sulfite colonies) with a narrow zone of hemolysis, giving a characteristic 'bisected pearl' appearance.
Step 2: Eliminate other options
Step 3: Additional note on Regan-Lowe medium
Regan-Lowe charcoal agar is a modern alternative to Bordet-Gengou for B. pertussis, but among the options given, Bordet-Gengou is the classic and most tested answer.
Answer:
Bordet-Gengou medium is the classic culture medium for isolating Bordetella pertussis.
Quick Tip:
Remember the mnemonic: 'Bordet-Gengou for Bordetella' — both named after the same discoverer Jules Bordet, making this an easy association.
A microbiologist is studying a bacterium that is an obligate intracellular parasite, lacks peptidoglycan in its cell wall, and has a biphasic life cycle consisting of an infectious elementary body (EB) and a replicating reticulate body (RB). Which organism does this describe?
Understanding:
This question describes characteristic features of an obligate intracellular organism with a unique biphasic life cycle (EB and RB) and absence of peptidoglycan in its cell wall.
Step 1: Identify the organism from its unique biphasic life cycle
The biphasic life cycle — elementary body (EB) and reticulate body (RB) — is the hallmark of Chlamydia species:
Step 2: Confirm the cell wall characteristic
Chlamydia lacks a classic peptidoglycan layer in its cell wall (despite having genes for peptidoglycan synthesis), making it resistant to beta-lactam antibiotics in clinical practice. This is a distinguishing feature.
Step 3: Eliminate other options
Answer:
The described organism with an EB–RB biphasic life cycle and absent peptidoglycan is Chlamydia trachomatis.
Quick Tip:
EB = Elementary = Extracellular and infEctious; RB = Reticulate = Replicating (intracellular). This EB–RB cycle is unique to Chlamydia and appears repeatedly in exams.
Which of the following statements correctly describes the mechanism by which Clostridium botulinum toxin causes flaccid paralysis?
Understanding:
This question asks about the precise molecular mechanism by which botulinum toxin produces flaccid paralysis.
Step 1: Understand botulinum toxin structure and entry
Clostridium botulinum produces a zinc-dependent metalloprotease toxin (the most potent biological toxin known). It consists of a heavy chain (binding and translocation) and a light chain (enzymatic activity). The light chain is a zinc protease.
Step 2: Identify the molecular target — SNARE proteins
The light chain of botulinum toxin cleaves SNARE (Soluble NSF Attachment protein REceptor) proteins, which are essential for fusion of acetylcholine-containing vesicles with the presynaptic membrane:
Cleavage of these SNARE proteins prevents vesicle docking and fusion, blocking acetylcholine (ACh) release → flaccid paralysis (muscle cannot contract).
Step 3: Eliminate other options
Answer:
Botulinum toxin causes flaccid paralysis by cleaving SNARE proteins, thereby blocking presynaptic acetylcholine release.
Quick Tip:
Contrast with tetanus toxin: both are Clostridial toxins, but tetanus blocks inhibitory neurotransmitter (glycine/GABA) release → spastic paralysis; botulinum blocks ACh release → flaccid paralysis.
The CAMP test is used to identify which of the following organisms, based on its synergistic hemolysis with Staphylococcus aureus on blood agar?
Understanding:
This question asks about the organism identified by the CAMP test, which relies on synergistic hemolysis with Staphylococcus aureus.
Step 1: Define the CAMP test
The CAMP test (Christie, Atkins, Munch-Petersen test) involves streaking a test organism perpendicular to a streak of beta-hemolysin-producing Staphylococcus aureus on a sheep blood agar plate. A positive result shows an arrowhead-shaped (flame/leaf-shaped) zone of enhanced beta-hemolysis at the junction of the two streaks.
Step 2: Identify the CAMP-positive organism
Streptococcus agalactiae (Group B Streptococcus, GBS) produces the CAMP factor — a diffusible protein that synergizes with the beta-lysin (sphingomyelinase C) of S. aureus to produce enhanced hemolysis. This is a key identifying feature of GBS, which is a major cause of neonatal meningitis and sepsis.
Step 3: Eliminate other options
Step 4: Classic teaching point
In standard bacteriology teaching, the CAMP test is the definitive test for S. agalactiae (GBS). Listeria is sometimes noted as CAMP-positive but it is not the primary organism the test is designed to identify.
Answer:
The CAMP test is classically used to identify Streptococcus agalactiae (Group B Streptococcus) based on synergistic hemolysis with S. aureus.
Quick Tip:
GBS CAMP test positivity is particularly important in screening pregnant women at 35–37 weeks gestation to prevent neonatal GBS disease — a high-yield clinical correlation.
A sputum sample from a 45-year-old immunocompromised patient is sent for microbiological examination. On Ziehl-Neelsen staining, the organism does NOT stain as acid-fast. However, on modified acid-fast staining (using 1% H2SO4 as decolorizer instead of 3% HCl-alcohol), the organism stains pink. Which organism is most likely responsible?
Understanding:
This question asks about an organism that is partially acid-fast — negative on standard Ziehl-Neelsen (ZN) staining but positive on modified acid-fast staining (using weaker decolorizer 1% H₂SO₄).
Step 1: Understand acid-fastness and its modification
Standard ZN staining uses 3% HCl-alcohol (strong decolorizer) — only strongly acid-fast organisms (e.g., Mycobacterium tuberculosis) retain the carbol-fuchsin stain. Modified acid-fast staining uses 1% H₂SO₄ (weak decolorizer) — partially acid-fast organisms also retain the stain.
Step 2: Identify the partially acid-fast organism
Nocardia asteroides is a partially (weakly) acid-fast organism. It is a Gram-positive, branching filamentous bacterium (aerobic actinomycete) that retains carbol-fuchsin with a weak decolorizer but is decolorized by the stronger acid-alcohol used in standard ZN staining. It is an important opportunistic pathogen causing pulmonary nocardiosis in immunocompromised patients.
Step 3: Eliminate other options
Answer:
Nocardia asteroides is the partially acid-fast organism, staining pink only with the modified acid-fast technique (1% H₂SO₄ decolorizer).
Quick Tip:
Key distinction: Nocardia = aerobic, partially acid-fast; Actinomyces = anaerobic, NOT acid-fast. Both form branching filaments, but they differ on these two critical points in every competitive exam.
Which of the following is the correct sequence of events in the lytic cycle of a bacteriophage?
Understanding:
This question asks about the correct chronological sequence of steps in the lytic (virulent) life cycle of a bacteriophage.
Step 1: Outline the lytic cycle steps in order
The lytic cycle of a bacteriophage consists of five sequential steps:
1. Adsorption: The phage attaches to specific receptors on the bacterial cell surface via its tail fibers.
2. Penetration (Injection): The phage injects its nucleic acid into the bacterial cell; the protein coat remains outside.
3. Biosynthesis (Replication): The phage nucleic acid hijacks bacterial machinery to synthesize phage components — early proteins (enzymes) first, then late proteins (structural components) and phage DNA.
4. Maturation (Assembly): Phage components are assembled into complete, mature virions.
5. Lysis: Phage-encoded lysozyme (endolysin) degrades the bacterial cell wall, releasing hundreds of new phage particles.
Step 2: Identify integration as a lysogenic — not lytic — event
Integration of phage DNA into the bacterial chromosome (as a prophage) occurs in the lysogenic cycle, not the lytic cycle. Options B and D incorrectly include integration in the lytic cycle.
Step 3: Confirm the correct sequence
Adsorption → Penetration → Biosynthesis → Maturation → Lysis is the correct sequence for the lytic cycle.
Answer:
The correct sequence of the bacteriophage lytic cycle is Adsorption → Penetration → Biosynthesis → Maturation → Lysis.
Quick Tip:
Mnemonic: 'A Pretty Big Mean Lion' — Adsorption, Penetration, Biosynthesis, Maturation, Lysis. Integration belongs to the LYsogenic cycle, not the LYtic cycle — do not confuse the two.
Quellung reaction (capsular swelling reaction) is a standard method used to type and identify which of the following bacteria?
Understanding:
This question asks about the bacterium for which the Quellung reaction is a standard identification and serotyping method.
Step 1: Define the Quellung reaction
The Quellung reaction (German: 'Quellung' = swelling) is a capsular precipitin test in which type-specific anticapsular antibodies bind to the polysaccharide capsule of an encapsulated organism. Under the microscope, the capsule appears to swell and become more refractile — this is actually an optical phenomenon caused by antibody binding, not true swelling. It is used for serotyping capsulated bacteria.
Step 2: Identify the organism most associated with Quellung reaction
Streptococcus pneumoniae (pneumococcus) has more than 90 serotypes based on its polysaccharide capsule. The Quellung reaction using type-specific antisera (Neufeld's Quellung reaction) is the classic method for identifying and serotyping pneumococcal isolates. The capsule is the principal virulence factor of S. pneumoniae.
Step 3: Eliminate other options
Answer:
The Quellung reaction is classically used to identify and serotype Streptococcus pneumoniae.
Quick Tip:
Quellung reaction is also applicable to other encapsulated bacteria (Klebsiella, H. influenzae, N. meningitidis), but pneumococcus is the organism most strongly associated with this test in exam questions.
Which of the following best describes the mechanism of horizontal gene transfer in bacteria where a bacteriophage carries bacterial DNA from one bacterium to another?
Understanding:
This question asks about the specific mechanism of horizontal gene transfer (HGT) in which a bacteriophage serves as the vector for transferring bacterial DNA between bacteria.
Step 1: Define the three major mechanisms of HGT
Bacteria transfer genetic material by three major mechanisms:
1. Transformation: uptake of naked (free) DNA from the environment by a competent bacterium (no vector required).
2. Conjugation: direct cell-to-cell contact via a sex pilus (F pilus); DNA transferred through a conjugation bridge — requires living donor cell.
3. Transduction: a bacteriophage (virus) accidentally packages bacterial DNA and transfers it to a new bacterial host upon infection.
Step 2: Identify transduction as the phage-mediated mechanism
In transduction, during the lytic cycle, the phage occasionally packages host bacterial DNA instead of (or in addition to) phage DNA into the capsid:
The phage then injects this bacterial DNA into a new host, achieving gene transfer.
Step 3: Clarify transposition
Transposition is the movement of mobile genetic elements (transposons) within or between DNA molecules within the same cell — it is not a mechanism of intercellular gene transfer.
Answer:
The mechanism of HGT mediated by a bacteriophage carrying bacterial DNA is called transduction.
Quick Tip:
Memory aid for HGT: Transformation = Naked DNA; Conjugation = Contact (pilus); Transduction = phage (Think: 'Transduced by a phage'). This trio is tested in nearly every microbiology exam.
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).