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Immunity is generally understood to mean protection
from infectious disease. The greatest health achievement of this century has been the
control of devastating, epidemic, infectious diseases by immunization (vaccination).
Edward Jenner invented immune therapy when he inoculated an eight year old boy with
scrapings of cowpox lesions.
Jenner had noted the similarity of cowpox and small pox lesions, and was observant
enough to notice that milkmaids, exposed to cow pox lesions on the teats of cows, did not
get smallpox. Two hundred years later one major viral disease, smallpox, has been
eradicated completely from the planet. This masterful success of immunization was achieved
by the World Health Organization (WHO) by the relentless vaccination of all people who
came in contact with the disease, until the smallpox virus had no vulnerable hosts to
infect. The smallpox virus is incapable of an independent existence. With no remaining
human hosts, the virus is unable to reproduce. Diphtheria, whooping cough, tetanus,
measles, mumps, typhoid, cholera, yellow fever, hepatitis, and polio are among the
diseases now controlled by immunization.
The principal function of immune defense is protection against infection and invasion
of the body space by foreign substances of all kinds. When we are ill with a viral
infection such as a cold, we expect to get better, as a result of successful immune
defense strategies. Immune defense stops infection with several subdivisions, specializing
in attacking one of the many micro-organisms which threaten us - bacteria, fungi, viruses,
and parasites. Another role of the immune system is the defense against molecules which
invade body space from the outside.
Immune defense covers all body surfaces exposed to the environment: skin, respiratory
tract, gastrointestinal tract, and genitourinary tract. Immune cells are found circulating
in the blood stream or patrolling in the intercellular spaces of all body tissues. Immune
networks are also distributed in discrete organs of the lymphatic system which includes
lymph nodes, tonsils, liver and spleen. Bone marrow is the major manufacturing area of
immune cells.
The thymus is the organ which originates one of two populations of immune cells,
T-lymphocytes, at least in early life. In later life the thymus continues to exert
regulatory influences on the immune system. Immunity means that immune cells remember the
identity of an antigen challenge and initiate a successful defensive response. Immunizing
injections contain antigens which belong to the infecting organisms. The first response to
the injected antigen is the activation of antigen-specific lymphocytes who proliferate
into clones of antigen-specific cells. These immune clones can later identify and attack
the infecting organism. Several exposures to the vaccine (antigen) boost immune memory to
an effective level of vigilance against the infecting organism.
Immune Networks - Not System
Medical textbooks refer to the "immune system", although this is
probably not the best description of immune function. Immune activity is distributed
through all body systems and involves large, diverse populations of migratory cells. A
more meaningful description might be immune networks (IN) which are collections of
different, diverse, often unstable, components.
When the term "system" is used we may get the wrong image of a well-defined,
orderly device, perhaps similar to a new car or a computer with an instruction manual. We
are not quite the coherent entity that we like to think we are. We are really a community
of cells in prodigious array. Lymphocytes are important immune players and to get an idea
of the size of immune populations, think of a young tadpole as containing about one
million lymphocytes. Human immune networks contain about 10 trillion cells.
Some of the immune cells stay in place and do more or less predictable things. Most
immune cells tend to wander around and, like bees, forage in various body parts
looking for items of interest. An appropriate image of immune networks would be the
foraging and swarming of bees or ants, each cell moving about, with different job
descriptions in the colony and a meta order achieved by the collective behavior of many
individuals.
The overall activity of the hive or colony decides how the society or system looks and
acts. They have the property of getting excited, recruiting their peers and attacking
interlopers. A second analogy is to think in terms of military organizations with many
divisions and a diverse array of weapons and strategies. The main purpose of immune
networks is to wage war against invading aliens. Once engaged, orderly behavior tends to
become chaotic and destruction of normal tissue structures are properties of these cell
networks. Fantasies about boosting your friendly, cooperative immune system have little
biological basis.
A third analogy that helps us relate to the changeable patterns of immune response is
the weather. We regularly observe periodic and chaotic changes in the symptom patterns of
patients, especially if we observe them over years. As Alan Perelson, an immune system
theorist, suggested:
"The system never settles down to a steady-state, but rather, constantly changes
with local flare ups and storms, and with periods of relative quiescence."
Hypersensitivity & Allergy
Immune activity has a benefit and a cost. The negative side of immunity is the
production of distressing symptoms, acute and chronic diseases. The term hypersensitivity
describes increased, damaging immune response. If you read a textbook of immunology,
you get the impression that immune activity only happens occasionally when an infection
threatens or an obvious allergic response occurs. You may not realize that immune activity
is continuous and is likely to generate symptoms in every person on a regular basis. You
do learn from the immunology text that there are many hypersensitivity diseases. You do
not learn that non-specific hypersensitivity states are common and produce a variety of
ill-defined illnesses.
Allergy is a form of hypersensitivity.
Autoimmune diseases such as rheumatoid arthritis or multiple sclerosis are also
expressions of hypersensitivity.
Our basic theory of disease is that many patterns of illness can be explained if we
recognize that substances inhaled in air and ingested as foods can trigger a variety of
immune responses in any part of the body.
Allergic reactions to drugs are easily recognized and epitomize the allergic response
to foreign molecules. Patients are questioned about their allergic history, specifically
drug reactions. Food allergy is a greater problem, but seldom is a food allergy inquiry
included in medical history-taking.
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