While this process causes lung injury, most flu symptoms are
actually caused by the immune response to the virus. The initial immune
response involves cells of the innate body's immune system, such as macrophages
and neutrophils. These cells express receptors that can detect the presence of
the virus. They trigger the alarm by producing small hormone-like molecules
called cytokines and chemokines. These indicate to the body that an infection
has been detected.
Cytokines orchestrate other components of the immune system to
adequately control the invading virus, while chemokines direct these components
to the site of infection. One of the candidate types of cells is T lymphocytes,
a type of white blood cell that fights infection. Sometimes they are even
called "soldier" cells. When T cells specifically recognize influenza
virus proteins, they begin to multiply in the lymph nodes around the lungs and
throat. This causes swelling and pain in these lymph nodes.
After a few days, these T cells move into the lungs and kill
the virus-infected cells. This process causes large lung damage similar to
bronchitis, which can worsen existing lung disease and make breathing
difficult. In addition, the accumulation of mucus in the lungs as a result of
this immune response to infection induces coughing as a reflex to try to
cleanse the airways. Normally, this damage caused by the arrival of T-cells in
the lungs is reversible in a healthy human, but if it progresses, it is bad
news and can lead to death.
The proper functioning of influenza-specific T cells is
critical to the efficient removal of the virus from the lungs. When T-cell
function decreases, for example, with increasing age or during the use of
immunosuppressants, virus clearance is delayed. This leads to a longer
infection and major lung damage. This can also set the stage for complications,
including secondary bacterial pneumonia, which can often be fatal.
Infectious conjunctivitis is the leading cause of pink eyes
around the world. The causes of infectious conjunctivitis are numerous and can
usually be classified as viral, bacterial or fungal.
The most common causes of a serious eye infection include:
Eye histoplasmosis syndrome (OHS): Histoplasmosis is a
fungal infection of the lung that is caught by inhaling spores. It is distributed
in river valleys around the world. It is much more prevalent in the US than in
Canada, and focuses on the area known as the Bible Belt (called by the
ophthalmologists "Histo Belt"). Over 90% of adults in the
southeastern US had histoplasmosis, which usually causes no symptoms. In a
small fraction of cases, the fungus migrates to the retina many years or
decades later. Once there, it damages the retina, especially the macula (the
vital central part in which the visual cells are most concentrated). It causes
symptoms and retinal decay that are very similar to macular degeneration, and
can destroy the central part of the visual field. People of African descent are
largely immune. Although only a tiny minority of people with histoplasmosis
suffer from OHS, the fungus is so common that OHS is a major infectious cause
of blindness in Americans between the ages of 20 and 40 years.
Chlamydia and Gonorrhea: These extremely common sexually
transmitted infections (STIs) can cause conjunctivitis, although they usually
do not cause serious eye damage in adults. The infection either enters the eye
directly through genital fluids such as semen or when infected persons rub
their eyes after handling infected genital areas. Babies born from genitally
infected women are at particularly high risk for eye infections. Neisseria
gonorrheae is one of the few bacteria that can invade the protective layers of
the eye and cause infection of the inner eye.
Herpes simplex: This widespread virus can be caught as a
skin disease (cold sore) or as an STI. Herpes viruses can infect the eye in the
same way as chlamydia or gonorrhea. As with these diseases, herpes can lead to
corneal formation and ulcers. Chronic herpes infection, which is rare, can
cause acute retinal necrosis (ARN), especially in males. This leads to a
significant destruction of the retinal tissue and to dramatic visual damage.
About 15% of people with chronic herpes simplex lose vision.
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