Virology and Host Immunity Practice Flashcards

Comprehensive vocabulary flashcards covering virus architecture, classification, replication cycles, host immunity, diagnostic testing, and key historical milestones.

Virus

An infectious, obligate intracellular parasite composed of genetic material (DNA or RNA) surrounded by a protein coat, with or without an envelope derived from a host-cell membrane.

Virion

The complete extracellular viral particle.

Virus vs. Cell Comparison

Viruses are acellular entities with a DNA or RNA genome that lack ribosomes and must hijack host machinery to reproduce; cells are cellular organisms with a DNA genome, ribosomes, and the ability to synthesize proteins and reproduce independently.

Ivanovsky (1892)

Russian scientist who demonstrated in 1892 that the infectious agent of tobacco mosaic disease could pass through a $\sim 0.2\,\mu\text{m}$ filter that retained bacteria.

Beijerinck (1898)

Dutch microbiologist who confirmed the filterable nature of Tobacco Mosaic Virus and described it as contagium vivum fluidum ("contagious living fluid").

Capsid

The protein shell surrounding and protecting a viral genome, composed of repeating protein subunits called capsomers.

Capsomer

A repeating protein subunit that forms the viral capsid.

Nucleocapsid

The combined unit formed by the viral genome and its protein capsid.

Envelope

A host-derived lipid membrane surrounding the nucleocapsid in certain viruses, typically acquired during viral budding.

Matrix

A viral protein layer that connects the envelope to the nucleocapsid, providing structural rigidity and aiding in assembly and budding.

Naked Virus vs. Enveloped Virus

Naked viruses consist of only a capsid and genome, making them environmentally stable and detergent-resistant; enveloped viruses possess a lipid bilayer, rendering them fragile and sensitive to heat, desiccation, and detergents.

Tropism

The specific cell, tissue, or host species specificity that dictates which cells a virus can infect.

Susceptible Cell

A cell that possesses the functional receptor required for viral attachment and entry.

Permissive Cell

A cell that possesses the necessary intracellular machinery and environment to support viral replication.

Resistant Cell

A cell that lacks the specific host cell surface receptor required for viral attachment.

Multiplicity of Infection (MOI)

The ratio of infectious viral units per host cell in an experimental inoculum.

Baltimore Classification System

A taxonomic framework that classifies viruses into seven groups based on genome composition, strandedness, sense, and the pathway used to produce +ssmRNA.

Group I (dsDNA)

Viruses with double-stranded DNA genomes that transcribe directly to mRNA (e.g., Herpesvirus).

Group II (ssDNA)

Viruses with single-stranded DNA genomes that synthesize a complementary strand to form dsDNA before transcribing mRNA (e.g., Parvovirus).

Group III (dsRNA)

Viruses with double-stranded RNA genomes that utilize viral RNA-dependent RNA polymerase (RdRp) to transcribe +mRNA (e.g., Rotavirus).

Group IV (+ssRNA)

Viruses with positive-sense single-stranded RNA genomes that can be directly translated as mRNA by host ribosomes (e.g., Poliovirus, SARS-CoV-2).

Group V (−ssRNA)

Viruses with negative-sense single-stranded RNA genomes that must use viral RdRp to transcribe complementary +mRNA (e.g., Influenza, Ebola, Rabies).

Group VI (+ssRNA-RT)

Viruses with positive-sense single-stranded RNA genomes that convert RNA to DNA using reverse transcriptase before transcribing mRNA (e.g., HIV).

Group VII (dsDNA-RT)

Viruses with double-stranded DNA genomes that replicate through an RNA intermediate using reverse transcriptase (e.g., Hepatitis B).

RNA-dependent RNA Polymerase (RdRp)

An essential viral enzyme brought or encoded by RNA viruses to transcribe RNA from an RNA template, a process host cells cannot naturally perform.

Viral Replication Cycle Stages

The seven steps of viral replication: Attachment, Penetration/Entry, Uncoating, Genome replication + gene expression, Assembly, Maturation, and Egress.

Internal Ribosome Entry Site (IRES)

A specialized viral RNA sequence that allows host cell ribosomes to initiate translation internally without needing a normal 5' cap (e.g., Poliovirus).

Cap Snatching

A viral process (e.g., used by Influenza) where the virus cleaves and steals 5' caps from host cell mRNAs to decorate viral transcripts.

Viral Metagenomics

A sequencing technique that identifies all nucleic acids present in a sample, enabling discovery of known and unknown viruses without prior targeted selection.

Innate vs. Adaptive Immunity Characteristics

Innate immunity provides rapid, non-specific recognition of general patterns (PAMPs) present from birth without memory; adaptive immunity develops slowly upon exposure, utilizes BCRs/TCRs for specific antigens, and creates immunologic memory.

Pathogen-Associated Molecular Pattern (PAMP)

Conserved molecular structures associated with pathogens, such as viral dsRNA, that are recognized by host innate receptors.

Pattern Recognition Receptor (PRR)

Host cell receptors, such as Toll-Like Receptors (TLRs), that sense PAMPs to initiate innate immune responses.

Interferons (IFNs)

Antiviral cytokines secreted by host cells to induce an antiviral state in neighboring cells, inhibiting protein synthesis and degrading viral RNA.

Cellular Altruism

The mechanism where an infected host cell shuts down its own protein synthesis and degrades RNA to prevent viral replication and protect surrounding cells.

Natural Killer (NK) Cells

Innate immune cells that recognize and kill infected target cells displaying low or absent MHC Class I surface molecules using perforin and granzymes.

MHC Class I vs. MHC Class II Comparison

MHC Class I presents intracellular antigens to CD8+ cytotoxic T cells to kill infected cells, while MHC Class II presents extracellular antigens to CD4+ helper T cells to coordinate immune responses.

CD8+ Cytotoxic T Cells

T cells that recognize intracellular viral peptides presented on MHC Class I and destroy infected cells via perforin and granzymes.

CD4+ Helper T Cells

T cells that recognize extracellular viral peptides presented on MHC Class II and release cytokines to coordinate B cell and macrophage responses.

IgM

The early antibody isotype produced during the initial primary response to a pathogen exposure.

IgG

The dominant antibody isotype produced during secondary immune responses, providing long-term systemic protection.

Passive Immunity

Immunity acquired by directly receiving preformed antibodies, providing immediate but temporary protection without forming memory.

Active Immunity

Immunity generated when the host's own immune system responds to an antigen, producing long-lasting immunological memory.

Cytopathic Effect (CPE)

Visible cellular morphological change resulting from viral infection, such as cell rounding, detachment, inclusion bodies, or syncytia.

Syncytium

A multinucleated giant cell formed by the fusion of adjacent infected cells, facilitating viral spread without extracellular exposure.

Antigen ELISA vs. Antibody ELISA

Antigen ELISA directly detects viral proteins in patient samples using virus-specific antibodies, whereas Antibody ELISA detects host antibodies directed against the virus.

Reverse Transcription PCR (RT-PCR)

A molecular diagnostic technique that uses reverse transcriptase to synthesize DNA from viral RNA before PCR amplification.

Cycle Threshold (Ct) Value

The number of PCR cycles required for a fluorescent signal to cross the threshold; lower values indicate higher starting viral nucleic acid amounts.

Key Virology Terms (Part 1)

A list of fundamental virology definitions covering structural parts, host cell tropism parameters, pattern recognition molecules, MHC classes, and immunoglobulins.

Key Virology Terms (Part 2)

A list of definitions covering forms of immunity, viral-induced cellular changes (CPE), molecular detection techniques (PCR/RT-PCR), and Ct value interpretations.