Initial stage: adhesion and tropism
The gateway for all ARVI pathogens is the mucous membranes of the upper respiratory tract. The pathogenesis of the disease is triggered when the virus attaches to a susceptible cell — a mechanism known as adhesion.
The ability of a virus to bind to specific target cells (tropism) is driven by the interaction of its active sites with specific receptors on the host cell surface. Each family of respiratory viruses uses its own "key" for entry:
- Rhinoviruses: capsid proteins bind to the cell adhesion molecule ICAM-1, allowing them to infect fibroblasts and other susceptible cells.
- Parainfluenza viruses: envelope surface proteins interact with cell membrane glycosides.
- Coronaviruses: bind to specific glycoprotein receptors.
- Adenoviruses: use integrins for attachment.
Pathogenesis and morphological changes
Following successful adhesion, active replication of viral particles begins, occurring predominantly locally—directly within the cells of the respiratory tract. The release of the virus into the systemic circulation (viremia) is usually short-lived. In some infections, the process has distinct localization features: for instance, the spread of adenoviruses is often limited to regional lymph nodes.
The development of characteristic local symptoms is linked not only to the virus itself, but also to the immune system response. Inflammation is accompanied by the active release of mediators, specifically bradykinins, which provoke local edema and catarrhal symptoms.
Cytopathic effect of viruses
Pathogens affect infected cells in different ways:
- Rhinoviruses cause only minor, superficial damage to the nasal mucosa.
- Adenoviruses exhibit pronounced cytotoxicity, provoking a rapid cytopathic effect and subsequent shedding of affected cells.
- Respiratory syncytial virus (RSV) is characterized by a potent destructive action leading to epithelial necrosis.
Overall, the pathomorphological picture of ARVI consists of tissue edema, massive cellular infiltration, and desquamation (sloughing) of the superficial epithelium. These mucosal defects are critically important for understanding the pathogenesis of complications: they form new "entry gates" and create ideal conditions for secondary bacterial microflora to take hold.
Clinical presentation
Despite the variety of etiological agents, the clinical picture of ARVI shares many common features. They are characterized by a short incubation period and a short-lived course: in individuals with normal immunity, recovery occurs in about a week without the need for intensive therapy. Intoxication syndrome is usually mild to moderate, and body temperature often remains normal or rises to subfebrile values.
The leading manifestations are respiratory tract syndromes: catarrh (in the form of laryngitis, pharyngitis, or tracheitis), as well as rhinitis with pronounced rhinorrhea. However, some viruses have specific clinical "markers":
- Rhinovirus infection: presents predominantly as isolated rhinitis accompanied by a dry cough.
- Adenovirus infection: classically combined with the development of pharyngoconjunctivitis and lymphadenopathy.
- RSV infection: poses a particular danger to children. It runs a severe course as it affects the lower respiratory tract, causing bronchiolitis, acute pneumonia, and pronounced asthmatic syndrome.
During the infectious process, from an immunological perspective, the development of organism sensitization is frequently observed.
Complications and severe forms
The worsening of the ARVI course and the development of complications are based on two main mechanisms: the formation of temporary post-infectious immunodeficiency and the superimposition of a secondary bacterial infection. Bacterial flora colonize the damaged epithelium, causing sinusitis, otitis, and purulent bronchitis, which significantly prolongs the illness.
The most formidable "respiratory" complication is recognized to be viral-bacterial pneumonia. They are characterized by an extremely aggressive course: massive destruction of the pulmonary epithelium occurs, multiple hemorrhages arise, and lung abscesses form. Such pneumonias are associated with a high risk of mortality.
In addition to respiratory tract damage, systemic (extrapulmonary) complications involving the nervous and cardiovascular systems, liver, kidneys, and gastrointestinal tract are possible. Such lesions occur both due to the direct cytopathic effect of certain viruses and as a result of the pronounced toxic effect of breakdown products from massively dying infected cells.
Briefly: answers to topical questions
Which virus families are the main causative agents of ARVI?
The main causative agents of ARVI include influenza viruses, parainfluenza viruses, adenoviruses, respiratory syncytial viruses, rhinoviruses, coronaviruses, metapneumoviruses, human bocaviruses, and enteroviruses.
- Orthomyxoviruses (Orthomyxoviridae) include influenza viruses A, B, and C.
- Paramyxoviruses (Paramyxoviridae) include parainfluenza viruses and respiratory syncytial virus (RSV).
What local immune factors protect mucous membranes from respiratory viruses?
Mucous membranes are protected from respiratory viruses by non-specific defense factors and local cellular immunity.
- Mucociliary clearance ensures the counterflow of mucous secretion through ciliary movement.
- Secretory antibodies (IgA) prevent pathogen adhesion to cells and mediate the destruction and elimination of viruses.
- Interferons (including $\alpha$-interferon) protect neighboring intact epithelial cells.
- Cellular immunity includes phagocytosis and the cytotoxic activity of natural killer (NK) cells.
What complete blood count changes are characteristic of uncomplicated ARVI?
For uncomplicated acute respiratory viral infection, the complete blood count is typically characterized by:
- Leukopenia or normocytosis — decreased or normal white blood cell count.
- Relative lymphocytosis — an increased proportion of lymphocytes.
- Monocytosis — an increased percentage of monocytes.
To memorize adhesion receptors, associate virus names with their targets: Rhinoviruses — adhesion molecules (ICAM-1), Adenoviruses — Integrins (A-I), Parainfluenza — Glycosides (P-G), Coronaviruses — Glycoproteins (C-G).
Class and exam questions
Why do severe mucosal edema and runny nose occur in ARVI?
Local manifestations of the infection (edema, rhinorrhea) are caused not only by the replication of the virus itself, but also by the active release of tissue inflammatory mediators, primarily bradykinins.
Do respiratory viruses circulate in the patient's blood?
Yes, however, viremia (the presence of virus in the bloodstream) in a classic ARVI course is usually short-lived, as the main replication occurs locally in the respiratory epithelium.
What is the main danger of the respiratory syncytial virus?
Unlike rhinoviruses, RSV has a pronounced cytopathic effect and predominantly affects the lower respiratory tract. In children, it frequently provokes epithelial necrosis, severe bronchiolitis, pneumonia, and asthmatic syndrome.
What is the mechanism behind extrapulmonary complications in acute respiratory viral infections (ARVI)?
Damage to the heart, kidneys, liver, or nervous system is associated both with the direct action of viruses that have penetrated there and with severe intoxication from the breakdown products of the body's own infected cells.
What else to study on this topic
- Mechanisms of post-infectious immunodeficiency development
- Specific adhesion molecules and receptors: ICAM-1 and integrins
- Pathogenesis of viral-bacterial pneumonia and abscess formation
- Morphological differences in the cytopathic effects of various viruses
- The role of bradykinins in the local inflammatory response
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