Antiviral Immunity: Innate, Adaptive & Mechanisms

Antiviral Immunity

For medical students2 min readUpdated 2026-10-10

Antiviral immunity is a complex of innate and adaptive defense mechanisms aimed at preventing viral entry, suppressing their replication, and destroying infected cells. It includes physical barriers, the interferon system, humoral factors, and a specific cellular response.

First lineMucous membranes, skin, and type I interferons.

Cellular response peakMaximum number of cytotoxic T-lymphocytes is generated on days 7–10.

Main weaponPerforins and granzymes, which induce apoptosis in infected cells.

EvasionViruses can suppress antigen presentation and mutate.

Innate Immunity: The First Barriers

Defense against viruses begins long before specific T- and B-lymphocytes are activated. Innate mechanisms react immediately.

Physical and biological barriers: Skin and mucous membranes, as well as normal microflora and the cell glycocalyx, prevent initial pathogen entry.
Response to a breach: When barriers are compromised, secretory components are activated, including the complement system and antimicrobial peptides. For example, $\alpha$-defensins inhibit viral replication, while cathelicidins disrupt their envelopes.
Early cellular migration: Neutrophils are the first to rush to the site of infection, followed by macrophages, dendritic cells, and natural killer (NK) cells. In the early days, activated macrophages and NK cells play the primary role in containing the infection.

Interferons: The Universal Alarm Signal

The interferon (IFN) system is a critical factor in early antiviral defense, blocking pathogen replication.

Type I Interferons (IFN-$\alpha$, IFN-$\beta$): Released by infected cells (primarily plasmacytoid dendritic cells). They induce neighboring cells to produce enzymes that non-specifically inhibit the synthesis of any RNA and proteins. This leads to the death of the infected cell but deprives the virus of the ability to replicate. They also enhance the expression of MHC antigens on the cell surface.
Type II Interferon (IFN-$\gamma$): Acts at the intersection of innate and adaptive immunity. It activates macrophages (triggering their microbicidal properties), NK cells, and cytotoxic T-lymphocytes.

Cellular Mechanisms and Target Destruction

The main task of cellular immunity is to find and destroy infected cells harboring the virus inside.

Natural Killer (NK) cells: Recognize cells by the "missing self" principle. If a virus suppresses MHC class I molecule expression on the target cell surface, an NK cell attacks it.
Cytotoxic T-lymphocytes (CTLs, CD8+): The main effectors of the adaptive response. They recognize foreign viral peptides presented alongside MHC class I. Specific CTLs appear on days 3–4 and peak by days 7–10.

Killing mechanism (granule exocytosis): Both NK cells and CTLs use a similar cytotoxic mechanism. They release perforins, which punch pores in the target cell membrane. Granzymes (serine proteases) enter through these pores. Granzymes activate intracellular caspases and degrade DNA, triggering programmed cell death — apoptosis.

Humoral Response: The Action of Antibodies

Antibodies are primarily effective against extracellular forms of the virus.

On mucous membranes, secretory immunoglobulins (sIgA, sIgM) coat virions, preventing them from attaching to the epithelium.
In the systemic circulation, IgG and IgM neutralize viruses and facilitate their phagocytosis.
Antibodies can trigger complement-dependent lysis of infected cells (via membrane attack complex formation).
An important role is played by antibody-dependent cellular cytotoxicity (ADCC): antibodies tag the infected cell, after which NK cells bind to them and destroy the target using perforins and granzymes.

How Viruses Evade Immunity

Infection development is always a race between the immune system and the virus. Viruses have evolved numerous evasion strategies (immune evasion):

Blockade of antigen presentation: Adenoviruses and herpes simplex virus reduce MHC class I molecule numbers on the surface, becoming invisible to T-lymphocytes.
Antigenic variability: Influenza viruses and rhinoviruses frequently mutate, changing their antigens, which allows them to escape previously generated antibodies.
Complement suppression: Poxviruses and herpesviruses synthesize proteins that block various components of the complement system.
Immunosuppression and mimicry: Epstein–Barr virus produces the BCRF1 protein, which closely resembles the human anti-inflammatory cytokine IL-10, thereby suppressing the immune response.
Latency: Hepatitis B virus and herpesviruses can integrate their genome into the host DNA and remain dormant for years until favorable conditions for activation arise.

Frequently asked questions

Which pattern-recognition receptors (PRRs) are responsible for intracellular recognition of viral nucleic acids?

Endosomal Toll-like receptors (TLRs 3, 7, 8, and 9) are responsible for intracellular recognition of viral nucleic acids.

TLR-3 recognizes double-stranded RNA (dsRNA), typical of most viruses.
TLR-7 recognizes single-stranded RNA (ssRNA) of viruses (e.g., influenza, HIV).
TLR-8 is also involved in recognizing single-stranded viral RNA.
TLR-9 recognizes DNA segments enriched with unmethylated CpG motifs, characteristic of viruses and bacteria.

What functions do macrophages perform during a viral infection?

During a viral infection, activated macrophages carry the main viricidal burden in the first days after infection. Macrophages can also serve as a reservoir for viral replication and transport throughout the body. Specifically, herpesviruses and poxviruses are capable of replicating inside macrophages.

Through which pathways does complement system activation occur during viral infections?

The complement system is activated via the classical, lectin, and alternative pathways. The classical pathway is triggered by the antigen-antibody complex; the lectin pathway by binding mannose-binding lectin to carbohydrate components; the alternative pathway by spontaneous C3 activation. In antiviral defense, IgG and IgM can participate in complement activation. The common outcome of activation is the assembly of the membrane attack complex and target cell lysis. Viruses can suppress complement: vaccinia virus blocks the classical pathway, and herpes simplex virus blocks both the classical and alternative pathways.

How do viruses hide from cytotoxic T-lymphocytes?

Antibodies combat viruses located in the extracellular space (neutralizing them and preventing cell entry). Cytotoxic T-lymphocytes (and NK cells) destroy cells of the body that are already infected.

How do viruses hide from cytotoxic T-lymphocytes?

They can suppress the expression of MHC class I molecules on the surface of the infected cell. Without this complex, T-lymphocytes simply "cannot see" the viral antigens. However, such cells lacking MHC class I become targets for natural killer cells.

What role does apoptosis play in antiviral immunity?

Apoptosis induced by granzyme action allows for the neat destruction of the infected cell along with the viral nucleic acids inside it, without causing massive inflammation or allowing the virus to escape and infect new cells.

Go deeper

Mechanisms of action of antimicrobial peptides (defensins and cathelicidins).
Features of activation and function of plasmacytoid dendritic cells.
Detailed process of the perforin-granzyme mechanism of cellular cytotoxicity.
Molecular mimicry using the Epstein–Barr virus as an example.
Mechanisms of formation and maintenance of latent viral infection.