The infection and immune life-cycle for Influenza A and similar enveloped viruses such as SARS-CoV-2 are varied and may depend on many factors. Several quotes from various reference materials included below provide a summary of the infection and immune life-cycle.
“Three requirements must be satisfied to ensure successful infection in an individual host:
- Sufficient virus must be available to initiate infection
- Cells at the site of infection must be accessible, susceptible, and permissive for the virus
- Local host anti-viral defense systems must be absent or initially ineffective.
“To infect its host, a virus must first enter cells at a body surface. Common sites of entry include the mucosal linings of the respiratory, alimentary, and urogenital tracts, the outer surface of the eye (conjunctival membranes or cornea), and the skin”
“Following replication at the site of entry, virus particles can remain localized, or can spread to other tissues”
“Viruses that escape from local defenses to produce a disseminated infection often do so by entering the bloodstream (hematogenous spread). Virus particles may enter the blood directly through capillaries, by replicating in endothelial cells, or through inoculation by a vector bite. Once in the blood, viruses may access almost every tissue in the host. Hematogenous spread begins when newly replicated particles produced at the entry site are released into the extracellular fluids, which can be taken up by the local lymphatic vascular system (Fig. 7). Lymphatic capillaries are considerably more permeable than circulatory system capillaries, facilitating virus entry. As the lymphatic vessels ultimately join with the venous system, virus particles in lymph have free access to the bloodstream. In the lymphatic system, virions pass through lymph nodes, where they encounter migratory cells of the immune system.”
Please read the following for more details regarding viral entry.
The immune system response to the viral entry is detailed in the following materials:
How The Body Reacts To Viruses – HMX | Harvard Medical School
This free collection of online learning materials can help with understanding how the body reacts to threats like the coronavirus that causes COVID-19.
Most viruses are not viremic meaning that they do not persist for long periods of time in the blood and are generally not hematogenously spread through the body in people with generally healthy immune systems. A few viruses that are blood borne include human immunodeficiency virus (HIV), Hepatitis B and Hepatitis C. These viruses persist in the blood long-term or for life. “Many other viruses may be found briefly in blood, but they generally don’t persist and are not considered significant “blood-borne” pathogens. Any infectious agent with a blood-borne, or “viraemic” phase has the potential for blood borne transmission, and so may be important for blood transfusions. For many infections, this viraemic period persists until the immune system is able to cure the infection”.
Why are only some viruses transmissible by blood and how are they actually spread?
Why is it only some viruses are transmissible by blood, and how does the virus actually move from person to person?
“Viremia can be maintained only if there is a continuing introduction of virus into the bloodstream from infected tissues to counter the continual removal of virus by macrophages and other cells”. Because of the large amount of leukocytes in the blood circulatory system and the rapid pumping of blood by the heart, most viruses such as influenza and other non-blood-borne viruses are rapidly inactivated and phagocytosized (eaten by phagocytes). Thus, the primary infection path of viruses is directly from cell to nearby cell, via air in the respiratory system, or interstitially in the areas that drain into the lymph system. Once deeper in the lymph system and lymph nodes, viruses again tend to be rapidly inactivated and phagocytosized due to abundance of leukocytes in those areas.
“Fluid from circulating blood leaks into the tissues of the body by capillary action, carrying nutrients to the cells. The fluid bathes the tissues as interstitial fluid, collecting waste products, bacteria, and damaged cells, and then drains as lymph into the lymphatic capillaries and lymphatic vessels. These vessels carry the lymph throughout the body, passing through numerous lymph nodes which filter out unwanted materials such as [viruses], bacteria and damaged cells. Lymph then passes into much larger lymph vessels known as lymph ducts. The right lymphatic duct drains the right side of the region and the much larger left lymphatic duct, known as the thoracic duct, drains the left side of the body. The ducts empty into the subclavian veins to return to the blood circulation.”
By the time the lymph system returns the fluid to the blood circulatory system, a healthy immune system has removed any viruses. This means that people with healthy immune systems have very little viral load within their blood circulatory system even when infected (except for the few blood-borne virus types noted above). In general, the more severe the case, the more likely that some viral load will be detectable in the circulating blood. A study found that in 6 of 6 cases of COVID-19 where blood was found PCR positive, the case was severe; none of the cases where blood was PCR positive were mild. This implies that case severity may be minimized by improving viral elimination within the lymphatic system prior to lymph being returned to the blood circulatory system.
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