Viral Immunology

Viral immunology is the principles layer beneath the rest of clinical virology. It explains why some infections are cleared while others persist for life, why a virus that is harmless in a healthy person can kill a child with severe combined immunodeficiency (SCID), and why vaccines work at all. Antiviral defence runs as a division of labour between two systems: an innate response that engages within hours, broad and pattern-based and without memory, and an adaptive response that follows over about a week, antigen-specific and leaving the durable memory that answers faster on re-exposure.

Immunity is double-edged. The same effectors that clear a virus also injure tissue, so that much viral disease, from the cytokine storm of severe pneumonia to post-infectious autoimmunity, is inflicted by the host response rather than by the virus. Immunodeficiency is the mirror image: when one arm is missing it leaves a characteristic gap, and the virus that fills that gap reveals what the arm normally did. The topic is treated in four articles.

→ See Innate antiviral immunity for the barriers, the pattern-recognition sensors and the interferon system that engage in the first hours, with natural killer (NK) cells, restriction factors and programmed cell death.

→ See Adaptive antiviral immunity for antigen presentation and major histocompatibility complex (MHC) restriction, the helper and cytotoxic T cell responses, antibody effector functions, and the immunological memory that vaccination exploits.

→ See Viral immune evasion for the countermeasures persistent viruses deploy against both arms, and the latency and persistence these buy.

→ See Viral immunopathology for the diseases caused by the immune response itself, from the cytokine storm and antibody-dependent enhancement to post-infectious autoimmunity.

The two arms of antiviral immunity

Innate immunity buys the time that adaptive immunity needs to assemble a specific, lasting response.

Feature	Innate immunity	Adaptive immunity
Speed	Minutes to hours	Days (~1 week on first encounter)
Specificity	Broad, pattern-based	Antigen-specific
Memory	None	Long-lived, the basis of vaccination
Recognition	Germline pattern-recognition receptors (PRRs): Toll-like receptors (TLRs), RIG-I, cGAS–STING	Rearranged B cell and T cell receptors
Key effectors	Interferons, NK cells, phagocytes, complement, restriction factors	Antibody, CD8 cytotoxic T cells, CD4 helper T cells

Immunodeficiency and viral susceptibility

The clearest demonstration of what each component does is what happens when it is missing. Immunodeficiency is either primary or secondary.

The primary (inherited) immunodeficiencies are uncommon single-gene defects of one component, among them severe combined immunodeficiency, DiGeorge syndrome, common variable immunodeficiency (CVID) and the inborn errors of interferon immunity; because each removes a defined component, the infections that follow show what that component normally does.

The secondary (acquired) immunodeficiencies are far more common, arising from HIV infection, immunosuppressive and biologic drugs, transplantation, and cancer and its treatment. Either way, the arm that fails predicts the virus that exploits it.

Immune component	Antiviral role	Consequence when it fails
Type I interferon	Switches on hundreds of antiviral genes; the first systemic alarm	Severe viral disease; anti-interferon autoantibodies and inborn errors underlie some severe COVID-19
TLR3 pathway	Senses viral double-stranded RNA, especially in the central nervous system	Herpes simplex encephalitis
NK cells	Kill infected cells that have shed MHC class I	Severe herpes simplex, varicella-zoster and cytomegalovirus disease
CD8 cytotoxic T cells	Kill virus-infected cells	Fatal disseminated herpesvirus disease; giant-cell measles pneumonia (SCID, DiGeorge syndrome)
CD4 helper T cells	Coordinate the entire adaptive response	The opportunistic viral cascade of advanced HIV
B cells and antibody	Neutralise virions; guard mucosal surfaces	Chronic enteroviral meningoencephalitis (Bruton agammaglobulinaemia, CVID)

Managing secondary immunodeficiency, from the transplant recipient to the patient with advanced HIV, is the clinical work of caring for the immunocompromised patient.

Key terms

The vocabulary that recurs across the topic, grouped by theme.

Innate sensing and the interferon system:

Term	Definition
Pattern-recognition receptor (PRR)	A germline-encoded sensor that detects conserved microbial signatures; the trigger for the innate response.
Toll-like receptor (TLR)	A membrane-bound PRR family; TLR3, 7, 8 and 9 sense viral RNA and DNA inside endosomes.
RIG-I	A cytoplasmic sensor (retinoic acid-inducible gene I) that detects viral RNA and induces interferon.
cGAS–STING	The cytoplasmic DNA-sensing pathway: cGAS detects viral DNA and activates STING to drive interferon.
Type I interferon	Interferon alpha and beta, the principal antiviral cytokines, secreted by an infected cell to warn its neighbours.
Interferon-stimulated gene (ISG)	One of hundreds of genes switched on by interferon, each blocking a step of viral replication.
JAK–STAT pathway	The cascade (Janus kinase and signal transducer and activator of transcription) that carries the interferon signal from the receptor to the nucleus, switching on the ISGs.
Interferon-alpha/beta receptor (IFNAR)	The type I interferon receptor; its loss causes severe, sometimes fatal, viral disease.

Innate effectors:

Term	Definition
Natural killer (NK) cell	A lymphocyte that kills infected cells which have lost MHC class I to hide from T cells (the missing-self response).
Antibody-dependent cellular cytotoxicity (ADCC)	Antibody coats an infected cell and flags it for an NK cell to destroy.
Complement	A plasma-protein cascade that neutralises virions, opsonises them for phagocytosis, and lyses enveloped virus and infected cells.
Restriction factor	A constitutively expressed host protein that blocks a specific viral step.
Tetherin	A restriction factor that holds budding virions on the cell surface so they cannot be released.
APOBEC3	A restriction factor that hypermutates the viral genome during reverse transcription (countered by HIV Vif).
MX proteins	Interferon-induced GTPases (myxovirus-resistance proteins) that trap incoming viral nucleocapsids.

Adaptive immunity:

Term	Definition
MHC class I	Displays peptides from proteins made inside the cell to CD8 T cells; on all nucleated cells, it shows the immune system what a cell is producing.
MHC class II	Displays peptides taken up from outside to CD4 T cells; on professional antigen-presenting cells (dendritic cells, macrophages, B cells).
Antigen presentation	Loading a viral peptide onto an MHC molecule for display to a T cell; the step viruses most often sabotage.
CD8 cytotoxic T cell	Recognises viral peptide on MHC class I and kills the infected cell with perforin and granzymes.
CD4 helper T cell	Recognises peptide on MHC class II and licenses B cells and CD8 T cells; its subsets (T-helper-1, follicular helper) steer the response.
B cell and antibody	The B cell matures into a plasma cell secreting antibody that binds virus; the basis of humoral immunity.
Neutralising versus binding antibody	Neutralising antibody blocks a virus from entering cells; binding (non-neutralising) antibody attaches without blocking entry but can still flag the cell for killing.
Affinity maturation	The germinal-centre process that progressively sharpens antibody binding over the course of a response.
Immunological memory	The long-lived memory B and T cells that respond faster and more strongly on re-exposure; the substrate of vaccination.
Original antigenic sin	The tendency to recall the response to a first viral strain when meeting a related one, sometimes at the cost of responding well to the new strain.

References and recommended reading

• Burrell CJ, Howard CR, Murphy FA. Innate Immunity; Adaptive Immune Responses to Infection; Pathogenesis of Virus Infections. In: Fenner and White’s Medical Virology, 5th edition, Chapters 5, 6 and 7. Academic Press / Elsevier; 2017. The foundational account of antiviral immunity, immune evasion and immunopathology.
• Iwasaki A, Schoggins JW, Hur S, et al. Innate Immunity to Viruses; The Adaptive Immune Response to Viruses. In: Fields Virology, 7th edition, Chapters 9 and 10. Wolters Kluwer; 2023. The current depth-of-field reference for antiviral immunity.
• Sompayrac LM. How the Immune System Works, 6th edition. Wiley-Blackwell; 2019. A plain-language primer on the immune system and how its parts fit together.