Viral Infections and the Immune System

Vocabulary flashcards covering key terms, molecules, immune mechanisms, and viral evasion strategies discussed in the lecture on viral infections and the immune system.

Viral Infection

Invasion of host cells by viruses that replicate using host machinery and often generate mutations.

Mode of Viral Entry

Across mucosal surfaces, broken skin, or via bites, followed by binding to specific cell-surface receptors.

Influenza Virus

Enveloped, segmented ssRNA virus causing respiratory disease in humans and animals; has 8 RNA segments coding for ~10 proteins.

Hemagglutinin (HA)

Influenza surface glycoprotein that binds host sialic acid to mediate viral attachment and entry.

Neuraminidase (NA)

Influenza surface enzyme that cleaves sialic acid to release budding virions from the host cell.

M1 Protein

Major influenza capsid protein providing structural stability to the virion.

M2 Protein

Influenza ion channel involved in uncoating and viral genome release inside the host cell.

Influenza Type A

Most common influenza type; causes major pandemics and possesses multiple HA and NA subtypes.

Antigenic Drift

Gradual accumulation of point mutations in HA/NA, producing new seasonal influenza strains.

Antigenic Shift

Sudden genetic reassortment generating a novel HA/NA combination, leading to pandemic influenza.

Natural IgM Antibodies

Pre-existing, low-affinity antibodies that bind diverse pathogens, activate complement, and aid early viral neutralization.

Complement System

Plasma protein cascade that opsonizes, agglutinates, or lyses enveloped viruses via the classical pathway.

Type I Interferons (IFN-α/β)

Cytokines produced by infected cells that induce antiviral states, inhibit protein synthesis, and activate NK cells.

Interleukin-12 (IL-12)

Cytokine produced during viral infection that promotes NK and Th1 responses.

Natural Killer (NK) Cell

Innate lymphocyte that kills virus-infected cells displaying reduced MHC I (“missing self”) via perforin/granzymes.

Cytotoxic T (Tc) Cell

CD8⁺ T lymphocyte that recognizes viral peptides on MHC I and induces apoptosis of infected cells.

Antigen-Presenting Cell (APC)

Cell such as dendritic cell or macrophage that processes and presents viral antigens on MHC molecules to T cells.

Cross-Presentation

Process whereby dendritic cells present exogenous viral antigens on MHC I to activate naive CD8⁺ T cells.

Antibody-Dependent Cellular Cytotoxicity (ADCC)

Killing of antibody-coated infected cells by NK cells through Fcγ receptor engagement.

Agglutination

Clumping of virions by antibodies (and complement) to enhance clearance in lymphoid tissues.

Opsonization

Coating of virions with antibody and/or complement to promote phagocytosis.

Memory B Cell

Long-lived cell formed after infection or vaccination that rapidly produces specific antibodies on re-exposure.

Latency

Viral survival strategy in which replication is minimal or absent, delaying immune detection (e.g., HIV).

Phagocytosis

Engulfment of viral particles or infected cells by macrophages, dendritic cells, or B cells.

Evasion Strategy

Any viral mechanism that avoids or undermines host immunity, including inhibition of complement or antigen presentation.

Complement Inhibitor Proteins

Host or viral proteins (e.g., HCMV, HSV) that block complement activation on infected cells.

TAP (Transporter Associated with Antigen Processing)

ER membrane transporter delivering peptides to MHC I; inhibited by HSV ICP47 to block antigen presentation.

MHC I Down-regulation

Reduction of MHC I surface expression by viruses (adenovirus, CMV) to escape Tc recognition.

MHC II Down-regulation

Viral suppression of MHC II (HIV, measles, CMV) to impair CD4⁺ T-cell activation.

Fc-Binding Proteins

Viral proteins that attach to the Fc portion of antibodies, neutralizing their effector functions (e.g., HSV).

PKR Inhibition

Hepatitis C strategy blocking the IFN-induced protein kinase R, preventing shutdown of viral protein synthesis.

Missing-Self Recognition

NK cell mechanism detecting cells with reduced MHC I, leading to targeted killing.

Adaptive Immunity

Specific, memory-forming responses involving antibodies and T cells that eliminate and prevent viral spread.

Humoral Immunity

Antibody-mediated defense that neutralizes viruses, blocks entry, and marks infected cells for destruction.

Cytokine

Small secreted protein (e.g., IFNs, IL-12) that modulates immune cell communication and antiviral activity.

Clathrin-Mediated Endocytosis

Cellular process exploited by influenza HA–sialic acid interaction for viral entry.

RNAse L

Enzyme activated by type I IFNs that degrades viral RNA within infected cells.

ADCC Effector Cell

Typically an NK cell that binds Fc of antibodies on infected cells and induces apoptosis.

Viral Reassortment

Exchange of genome segments between different influenza strains, underlying antigenic shift.

Complement Opsonization Entry

Exploitation of complement-coated virions (e.g., HIV) to facilitate infection via complement receptors.