Immunology is a branch of biology that covers
the study of immune systems in all organisms.
Immunology charts, measures, and contextualizes
the: physiological functioning of the immune
system in states of both health and diseases;
malfunctions of the immune system in immunological
disorders (such as autoimmune diseases, hypersensitivities
immune deficiency, and transplant rejection);
the physical, chemical and physiological characteristics
of the components of the immune system in
vitro, in situ, and in vivo.
Immunology has applications in numerous disciplines
of medicine, particularly in the fields of
organ transplantation, oncology, rheumatology,
virology, bacteriology, parasitology, psychiatry,
and dermatology.
The term was coined by Russian biologist Ilya
Ilyich Mechnikov who advanced studies on immunology
and received the Nobel Prize for his work
in 1908.
He pinned small thorns into starfish larvae
and noticed unusual cells surrounding the
thorns.
This was the active response of the body trying
to maintain its integrity.
It was Mechnikov who first observed the phenomenon
of phagocytosis, in which the body defends
itself against a foreign body.
Prior to the designation of immunity, from
the etymological root immunis, which is Latin
for "exempt"; early physicians characterized
organs that would later be proven as essential
components of the immune system.
The important lymphoid organs of the immune
system are the thymus and bone marrow, and
chief lymphatic tissues such as spleen, tonsils,
lymph vessels, lymph nodes, adenoids, and
liver.
When health conditions worsen to emergency
status, portions of immune system organs including
the thymus, spleen, bone marrow, lymph nodes
and other lymphatic tissues can be surgically
excised for examination while patients are
still alive.
Many components of the immune system are typically
cellular in nature and not associated with
any specific organ; but rather are embedded
or circulating in various tissues located
throughout the body.
== Classical immunology ==
Classical immunology ties in with the fields
of epidemiology and medicine.
It studies the relationship between the body
systems, pathogens, and immunity.
The earliest written mention of immunity can
be traced back to the plague of Athens in
430 BCE.
Thucydides noted that people who had recovered
from a previous bout of the disease could
nurse the sick without contracting the illness
a second time.
Many other ancient societies have references
to this phenomenon, but it was not until the
19th and 20th centuries before the concept
developed into scientific theory.
The study of the molecular and cellular components
that comprise the immune system, including
their function and interaction, is the central
science of immunology.
The immune system has been divided into a
more primitive innate immune system and, in
vertebrates, an acquired or adaptive immune
system.
The latter is further divided into humoral
(or antibody) and cell-mediated components.
The immune system has the capability of self
and non-self-recognition.
An antigen is a substance that ignites the
immune response.
The cells involved in recognizing the antigen
are Lymphocytes.
Once they recognize, they secrete antibodies.
Antibodies are proteins that neutralize the
disease-causing microorganisms.
The antibodies don’t kill the pathogens
rather phagocytes are involved in it.
The humoral (antibody) response is defined
as the interaction between antibodies and
antigens.
Antibodies are specific proteins released
from a certain class of immune cells known
as B lymphocytes, while antigens are defined
as anything that elicits the generation of
antibodies ("anti"body "gen"erators).
Immunology rests on an understanding of the
properties of these two biological entities
and the cellular response to both.
It’s now getting clear that the immune responses
contribute to the development of many common
disorders not traditionally viewed as immunologic,
including metabolic, cardiovascular, cancer,
and neurodegenerative conditions like Alzheimer’s
disease.
Besides, there are direct implications of
the immune system in the infectious diseases
(tuberculosis, malaria, hepatitis, pneumonia,
dysentery, and helminth infestations) as well.
Hence, research in the field of immunology
is of prime importance for the advancements
in the fields of modern medicine, biomedical
research, and biotechnology.Immunological
research continues to become more specialized,
pursuing non-classical models of immunity
and functions of cells, organs and systems
not previously associated with the immune
system (Yemeserach 2010).
== Clinical immunology ==
Clinical immunology is the study of diseases
caused by disorders of the immune system (failure,
aberrant action, and malignant growth of the
cellular elements of the system).
It also involves diseases of other systems,
where immune reactions play a part in the
pathology and clinical features.
The diseases caused by disorders of the immune
system fall into two broad categories:
immunodeficiency, in which parts of the immune
system fail to provide an adequate response
(examples include chronic granulomatous disease
and primary immune diseases);
autoimmunity, in which the immune system attacks
its own host's body (examples include systemic
lupus erythematosus, rheumatoid arthritis,
Hashimoto's disease and myasthenia gravis).Other
immune system disorders include various hypersensitivities
(such as in asthma and other allergies) that
respond inappropriately to otherwise harmless
compounds.
The most well-known disease that affects the
immune system itself is AIDS, an immunodeficiency
characterized by the suppression of CD4+ ("helper")
T cells, dendritic cells and macrophages by
the Human Immunodeficiency Virus (HIV).
Clinical immunologists also study ways to
prevent the immune system's attempts to destroy
allografts (transplant rejection).
== Developmental immunology ==
The body’s capability to react to antigens
depends on a person's age, antigen type, maternal
factors and the area where the antigen is
presented.
Neonates are said to be in a state of physiological
immunodeficiency, because both their innate
and adaptive immunological responses are greatly
suppressed.
Once born, a child’s immune system responds
favorably to protein antigens while not as
well to glycoproteins and polysaccharides.
In fact, many of the infections acquired by
neonates are caused by low virulence organisms
like Staphylococcus and Pseudomonas.
In neonates, opsonic activity and the ability
to activate the complement cascade is very
limited.
For example, the mean level of C3 in a newborn
is approximately 65% of that found in the
adult.
Phagocytic activity is also greatly impaired
in newborns.
This is due to lower opsonic activity, as
well as diminished up-regulation of integrin
and selectin receptors, which limit the ability
of neutrophils to interact with adhesion molecules
in the endothelium.
Their monocytes are slow and have a reduced
ATP production, which also limits the newborn's
phagocytic activity.
Although, the number of total lymphocytes
is significantly higher than in adults, the
cellular and humoral immunity is also impaired.
Antigen-presenting cells in newborns have
a reduced capability to activate T cells.
Also, T cells of a newborn proliferate poorly
and produce very small amounts of cytokines
like IL-2, IL-4, IL-5, IL-12, and IFN-g which
limits their capacity to activate the humoral
response as well as the phagocitic activity
of macrophage.
B cells develop early during gestation but
are not fully active.
Maternal factors also play a role in the body’s
immune response.
At birth, most of the immunoglobulin present
is maternal IgG.
Because IgM, IgD, IgE and IgA don’t cross
the placenta, they are almost undetectable
at birth.
Some IgA is provided by breast milk.
These passively-acquired antibodies can protect
the newborn for up to 18 months, but their
response is usually short-lived and of low
affinity.
These antibodies can also produce a negative
response.
If a child is exposed to the antibody for
a particular antigen before being exposed
to the antigen itself then the child will
produce a dampened response.
Passively acquired maternal antibodies can
suppress the antibody response to active immunization.
Similarly the response of T-cells to vaccination
differs in children compared to adults, and
vaccines that induce Th1 responses in adults
do not readily elicit these same responses
in neonates.
Between six and nine months after birth, a
child’s immune system begins to respond
more strongly to glycoproteins, but there
is usually no marked improvement in their
response to polysaccharides until they are
at least one year old.
This can be the reason for distinct time frames
found in vaccination schedules.During adolescence,
the human body undergoes various physical,
physiological and immunological changes triggered
and mediated by hormones, of which the most
significant in females is 17-β-estradiol
(an estrogen) and, in males, is testosterone.
Estradiol usually begins to act around the
age of 10 and testosterone some months later.
There is evidence that these steroids not
only act directly on the primary and secondary
sexual characteristics but also have an effect
on the development and regulation of the immune
system, including an increased risk in developing
pubescent and post-pubescent autoimmunity.
There is also some evidence that cell surface
receptors on B cells and macrophages may detect
sex hormones in the system.The female sex
hormone 17-β-estradiol has been shown to
regulate the level of immunological response,
while some male androgens such as testosterone
seem to suppress the stress response to infection.
Other androgens, however, such as DHEA, increase
immune response.
As in females, the male sex hormones seem
to have more control of the immune system
during puberty and post-puberty than during
the rest of a male's adult life.
Physical changes during puberty such as thymic
involution also affect immunological response.
== Ecoimmunology and behavioural immunity
==
Ecoimmunology, or ecological immunology, explores
the relationship between the immune system
of an organism and its social, biotic and
abiotic environment.
More recent ecoimmunological research has
focused on host pathogen defences traditionally
considered "non-immunological", such as pathogen
avoidance, self-medication, symbiont-mediated
defenses, and fecundity trade-offs.
Behavioural immunity, a phrase coined by Mark
Schaller, specifically refers to psychological
pathogen avoidance drivers, such as disgust
aroused by stimuli encountered around pathogen-infected
individuals, such as the smell of vomit.
More broadly, "behavioural" ecological immunity
has been demonstrated in multiple species.
For example, the Monarch butterfly often lays
its eggs on certain toxic milkweed species
when infected with parasites.
These toxins reduce parasite growth in the
offspring of the infected Monarch.
However, when uninfected Monarch butterflies
are forced to feed only on these toxic plants,
they suffer a fitness cost as reduced lifespan
relative to other uninfected Monarch butterflies.
This indicates that laying eggs on toxic plants
is a costly behaviour in Monarchs which has
probably evolved to reduce the severity of
parasite infection.Symbiont-mediated defenses
are also heritable across host generations,
despite a non-genetic direct basis for the
transmission.
Aphids, for example, rely on several different
symbionts for defense from key parasites,
and can vertically transmit their symbionts
from parent to offspring.
Therefore, a symbiont which successfully confers
protection from a parasite is more likely
to be passed to the host offspring, allowing
coevolution with parasites attacking the host
in a way similar to traditional immunity.
== Immunotherapy ==
The use of immune system components to treat
a disease or disorder is known as immunotherapy.
Immunotherapy is most commonly used in the
context of the treatment of cancers together
with chemotherapy (drugs) and radiotherapy
(radiation).
However, immunotherapy is also often used
in the immunosuppressed (such as HIV patients)
and people suffering from other immune deficiencies
or autoimmune diseases.
This includes regulating factors such as IL-2,
IL-10, GM-CSF B, IFN-α.
== 
Diagnostic immunology ==
The specificity of the bond between antibody
and antigen has made the antibody an excellent
tool for the detection of substances by a
variety of diagnostic techniques.
Antibodies specific for a desired antigen
can be conjugated with an isotopic (radio)
or fluorescent label or with a color-forming
enzyme in order to detect it.
However, the similarity between some antigens
can lead to false positives and other errors
in such tests by antibodies cross-reacting
with antigens that aren't exact matches.
== Cancer immunology ==
The study of the interaction of the immune
system with cancer cells can lead to diagnostic
tests and therapies with which to find and
fight cancer.
== Reproductive immunology ==
This area of the immunology is devoted to
the study of immunological aspects of the
reproductive process including fetus acceptance.
The term has also been used by fertility clinics
to address fertility problems, recurrent miscarriages,
premature deliveries and dangerous complications
such as pre-eclampsia.
== 
Theoretical immunology ==
Immunology is strongly experimental in everyday
practice but is also characterized by an ongoing
theoretical attitude.
Many theories have been suggested in immunology
from the end of the nineteenth century up
to the present time.
The end of the 19th century and the beginning
of the 20th century saw a battle between "cellular"
and "humoral" theories of immunity.
According to the cellular theory of immunity,
represented in particular by Elie Metchnikoff,
it was cells – more precisely, phagocytes
– that were responsible for immune responses.
In contrast, the humoral theory of immunity,
held by Robert Koch and Emil von Behring,
among others, stated that the active immune
agents were soluble components (molecules)
found in the organism's "humors" rather than
its cells.In the mid-1950s, Macfarlane Burnet,
inspired by a suggestion made by Niels Jerne,
formulated the clonal selection theory (CST)
of immunity.
On the basis of CST, Burnet developed a theory
of how an immune response is triggered according
to the self/nonself distinction: "self" constituents
(constituents of the body) do not trigger
destructive immune responses, while "nonself"
entities (e.g., pathogens, an allograft) trigger
a destructive immune response.
The theory was later modified to reflect new
discoveries regarding histocompatibility or
the complex "two-signal" activation of T cells.
The self/nonself theory of immunity and the
self/nonself vocabulary have been criticized,
but remain very influential.More recently,
several theoretical frameworks have been suggested
in immunology, including "autopoietic" views,
"cognitive immune" views, the "danger model"
(or "danger theory"), and the "discontinuity"
theory.
The danger model, suggested by Polly Matzinger
and colleagues, has been very influential,
arousing many comments and discussions.
== See also ==
History of immunology
Immunomics
International Reviews of Immunology
List of immunologists
Osteoimmunology
Outline of immunology
