Immune System: Overview in Humans
There are two types of immune response: innate immunity and adaptive
immunity. Both types work together to keep you healthy. The cells of
the human immune system originate from stem cells in important key organs
in the body: early in life from the liver, and later the bone marrow.
The bone marrow continues to produce new cells throughout a person’s
lifetime. These organs provide a place for specialized cells of the
immune system to mature and develop. From here they go out into the
blood stream and to other organs of the body to where they will spring
into action when needed.
Innate immunity is a very quick response to danger signals that are
common among microbes (usually bacteria, viruses, and parasites). This
is the first line of defense when a microbe enters the body. When you
cut yourself, or have food poisoning, your innate immunity responds
right away. Some cells, like phagocytic ones, “eat” bacteria
and other organisms. Phagocytic cells migrate throughout the body or
stay in organs and tissues to wait for invading bacteria or other invading
microbes. When they find one, they trap it on their surface, then ingest
it (bring it into the cell) and destroy it. Some microbes have developed
special outer surfaces that make it hard for the phagocytic cell to
trap them. Humans share innate immune components with Drosophila.
Phagocytic cells neutralize or destroy invading bacteria and other
microbes. They do this by trapping the microbe on their surface, then
ingesting it (bringing it into the cell and eating it), and destroying
it with enzymes. Some microbes have developed special outer surfaces
that make it hard for the phagocytic cell to trap them. The phagocytic
cells in Drosophila are called haemocytes; in humans they are called
macrophages and neutrophils.
In humans, and vertebrates in general, immune cells are also able to
learn and improve immune defenses when they encounter the same microbe
several times. This part of the immune system is called the adaptive
immune response and this ability is called a “memory response”.
Specialized cells work together to recognize a disease-causing microbe
and they create targeted responses to the organism. The key to this
response is the production of specific antibodies by the cells that
are designed to specifically attack the microbe causing the threat.
These responses will be “remembered” by the cells so that
they can respond quickly the next time this threat is encountered.
The key to the adaptive humoral response is the production of specific
antibodies by cells that are designed to attack a certain microbe. The
production of specific antibodies involves complex cooperation between
immune cells. First, the macrophage traps virus particles. It processes
these and becomes an antigen presenting cell (APC). The APC then presents
the processed virus to a Helper T cell, which becomes activated and
undergoes clonal expansion. This means that it makes more of itself
(Memory Helper T cells that will remember the virus target) and causes
the B cell to become a plasma cell that makes the antibody against the
virus. The antibodies attach to the virus to destroy it. The B cell
will also clonally expand to make Memory B cells that will remember
which antibody to make against this virus. Memory cells wait for future
invasions by this virus and respond quickly.
Application of Adaptive Immunity: Vaccines
The memory response is how vaccines work (target viruses) and also why
we usually do not get the same cold or flu twice. The cartoon showing
how specific antibodies are made demonstrates how vaccines work too.
Vaccines are made from killed virus or parts of viruses that are injected
into the body so that your immune system cells can start making antibodies
against them. Several immune cells work together to create the memory
response that results in specific antibodies. That way, if you are infected
with the real virus your immune system will be ready to respond. Vaccines
have been critical in reducing the amount of diseases, like polio and
small pox, in the world’s population. Cold and flu viruses are
very good at mutating, however, and they can fool the immune cells to
cause a re-infection.
What if the system doesn’t work very well or is missing specialized
cells? Illness results. Exposure to stressors can cause your immune
system to malfunction. Not getting enough sleep or exercise can leave
your immune system more vulnerable to attack. Astronauts are exposed
to the stress of microgravity during spaceflight, and it is important
that we fully understand how their immune response changes as a result
of space travel. What we learn may be beneficial for people on Earth,
including those who are born with Severe Combined Immunodeficiency,
have AIDS, and cancer therapy patients.