Immunology is a branch of biology
that covers the study of immune system
s in all organism
s. 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 disease
, immune deficiency
, and transplant rejection
); and 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
, bone marrow
, and chief lymphatic tissues such as spleen
s, lymph vessels
, lymph node
s, 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 cell
ular in nature and not associated with any specific organ, but rather are embedded or circulating in various tissue
s located throughout the body.
Classical immunology ties in with the fields of epidemiology
. It studies the relationship between the body systems, pathogen
s, and immunity. The earliest written mention of immunity can be traced back to the plague
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 vertebrate
s, an acquired or adaptive immune system
. The latter is further divided into humoral
) and cell-mediated
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. Antibodies don't directly kill pathogens, but instead, identify antigens as targets for destruction by other immune cells such as phagocytes or NK cells.
The humoral (antibody) response is defined as the interaction between antibodies and antigen
s. Antibodies are specific protein
s released from a certain class of immune cells known as B lymphocyte
s, while antigens are defined as anything that elicits the generation of antibodies (antibody generators). 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 is the study of disease
s 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:
, in which parts of the immune system fail to provide an adequate response (examples include chronic granulomatous disease
and primary immune diseases);
, 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 compound
The most well-known disease that affects the immune system itself is AIDS
, an immunodeficiency characterized by the suppression of CD4+ ("helper") T cell
s, dendritic cells
by the Human Immunodeficiency Virus
Clinical immunologists also study ways to prevent the immune system's attempts to destroy allograft
s (transplant rejection
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.
s 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 glycoprotein
s and polysaccharide
s. 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
receptors, which limit the ability of neutrophil
s to interact with adhesion molecule
s in the endothelium
. Their monocyte
s are slow and have a reduced ATP
production, which also limits the newborn's phagocytic activity. Although, the number of total lymphocyte
s is significantly higher than in adults, the cellular and humoral immunity is also impaired. Antigen-presenting cell
s in newborns have a reduced capability to activate T cells. Also, T cells of a newborn proliferate poorly and produce very small amounts of cytokine
s 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. These antibodies are transferred from the placenta to the fetus using the FcRn (neonatal Fc receptor). 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 glycoprotein
s, but there is usually no marked improvement in their response to polysaccharide
s until they are at least one year old. This can be the reason for distinct time frames found in vaccination schedule
During adolescence, the human body undergoes various physical, physiological and immunological changes triggered and mediated by hormone
s, of which the most significant in females is 17-β-estradiol
) 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 steroid
s 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 androgen
s 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. Aphid
s, 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 that 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.
The use of immune system components or antigens to treat a disease or disorder is known as immunotherapy
. Immunotherapy is most commonly used to treat allergies, autoimmune disorders such as Crohn’s disease
and rheumatoid arthritis
, and certain cancer
s. Immunotherapy is also often used in the immunosuppressed (such as HIV
patients) and people suffering from other immune deficiencies.
This includes regulating factors such as IL-2, IL-10, GM-CSF B, IFN-α.
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.
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. The immunology concerned with physiological reaction characteristic of the immune state.
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
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.
* History of immunology
* ''International Reviews of Immunology
* List of immunologists
* Outline of immunology
American Association of Immunologists
British Society for Immunology
Federation of Clinical Immunology Societies