Travel to Health

When a virus enters the body, it is picked up by certain cells of the immune system. They transport the virus to the lymph nodes where they present its fragments, known as antigens, to CD8+ T cells responsible control of viral infections. Each of these cells carries a unique T cell receptor  ( TCR ) on the surface that can recognize certain antigens. However, only very few T cell receptors match a given viral the antigen. To bring the infection under control and maximize the defenses against the virus, these few antigen-specific T cells start dividing rapidly and develop into effector T cells . These kill virus-infected host cells and then die off themselves once the infection is cleared.  Some of these short-lived effector cells —according to the generally accepted theory— turn into memory T cells , which persist in the organism long term. In case the same pathogen enters the body again, memory T cells are already present and ready to fight the invader more swiftly and eff...

Any virus that can cause disease in humans must have at least one immune evasion mechanism—at least one immune evasion “trick.” Without the ability to evade the immune system , a virus is usually harmless .  In the case of SARS-CoV-2, the virus is clearly unusually effective at evading the triggering of early innate immune responses , such as type 1 interferons (IFNs) (see below). It is plausible that much of the nature of COVID-19 as an illness is a consequence of this one big trick of SARS-CoV-2 . Transient lymphopenia is a common feature of many respiratory viral infections, such as with influenza A H3N2 virus, human rhinovirus or respiratory syncytial virus, but lymphopenia in these other infections typically occurs for only 2–4 days around symptom onset and rapidly recovers. By contrast, COVID-19-associated lymphopenia may be more severe or persistent than in these other infections and seems to be more selective for T cell lineages . [20] Thus, although the mechanisms of l...

Cytokines are a broad and loose category of small proteins (~5–20 kDa ) important in cell signaling . They are key players in the regulation of the immune response , particularly during infections, inflammatory joint, kidney, vessel and bowel diseases, or neurological and endocrinological autoimmune diseases .  Cytokines include: Lymphokine Cytokines made by lymphocytes Monokine Cytokines made by monocytes and macrophages Chemokine Cytokines with chemotactic activities Interleukin (ILs) Cytokines made by one leukocyte and acting on other leukocytes Interferons (IFNs) Interferons are a group of signaling proteins made and released by host cells in response to the presence of several viruses . Tumour necrosis factors but not hormones or growth factors . Unlike hormones, cytokines are not stored in glands as preformed molecules, but are rapidly synthesized and secreted by different cells mostly after stimulation. Figure 1. Cytokines act in networks or cascades.  They...