The Scope Of Immunology
Of the four major causes of death – injury,
infection, degenerative disease
and cancer – only the first two regularly kill their victims before
child-bearing age, which means that they are a potential source of lost genes.
Therefore any mechanism that reduces their effects has tremendous survival
value, and we see this in the processes of, respectively, healing and immunity.
Immunity is concerned with the
recognition and disposal of foreign or ‘non-self’ material that enters the body
(represented by red arrows in the figure), usually in the form of
life-threatening infectious micro- organisms but sometimes, unfortunately, in
the shape of a life-saving kidney graft. Resistance to infection may be ‘innate’
(i.e. inborn and unchanging) or ‘acquired’ as the result of an adaptive
immune response (centre).
Immunology is the study of the
organs, cells and molecules respon-sible for this recognition and disposal (the
‘immune system’), of how they respond and interact, of the consequences – desirable
(top) or otherwise (bottom) – of their activity, and of the ways in which they
can be advantageously increased or reduced.
By far the most important type of
foreign material that needs to be recognized and disposed of is the
microorganisms capable of causing infectious disease and, strictly speaking,
immunity begins at the point when they enter the body. But it must be
remembered that the first line of defence is to keep them out, and a variety of
external defences have evolved for this purpose. Whether these are part
of the immune system is a purely semantic question, but an immunologist is
certainly expected to know about
them.
Non-self A widely used term in immunology, covering everything that is detectably different from an animal’s
own constituents. Infectious microorganisms, together with cells, organs or
other materials from another animal, are the most important non-self substances
from an immunological viewpoint, but drugs and even normal foods, which are, of
course, non-self too, can sometimes give rise to immunity. Detection of
non-self material is carried out by a range of receptor molecules (see
Figs 5, 10–14).
Infection Parasitic viruses, bacteria, protozoa, worms or
fungi that attempt to gain access to the body or its surfaces are probably the
chief raison d’être of the immune system. Higher animals whose immune
system is damaged or deficient frequently succumb to infections that normal
animals overcome.
External defences The presence of intact skin on the outside and
mucous membranes lining the hollow viscera is in itself a powerful barrier
against entry of potentially infectious organisms. In addition, there are
numerous antimicrobial (mainly antibacterial) secretions in the skin and mucous
surfaces; these include lysozyme (also found in tears), lactoferrin, defensins
and peroxidases. More specialized defences include the extreme acidity of the
stomach (about pH 2), the mucus and upwardly beating cilia of the bronchial
tree, and specialized surfactant proteins that recognize and clump bacteria
that reach the lung alveoli. Successful microorganisms usually have cunning
ways of breaching or evading these defences.
Innate resistance Organisms that enter the body (shown in the
figure as dots or rods) are often eliminated within minutes or hours by inborn,
ever-present mechanisms, while others (the rods in the figure) can avoid this
and survive, and may cause disease unless they are dealt with by adaptive
immunity (see below). These mechanisms have evolved to dispose of pathogens
(e.g. bacteria, viruses) that if unchecked can cause disease. Harmless
microorganisms are usually ignored by the innate immune system. Innate immunity
also has a vital role in
initiating the adaptive immune response.
Adaptive immune
response The
development or augmentation
of defence mechanisms in
response to a particular (‘specific’) stimulus, e.g. an infectious organism. It can result in elimination of the micro-
organism and recovery from disease, and often leaves the host with specific
memory, enabling it to respond more effectively on reinfection with the same
microorganism, a condition called acquired resistance. Because the process by
which the body puts together the receptors of the adaptive immune system is
random (see Fig. 10), adaptive immunity sometimes responds to harmless foreign
material such as the relatively inoffensive pollens, etc., or even to ‘self’
tissues leading to autoimmunity.
Vaccination A method of stimulating the adaptive immune
response and generating memory and acquired resistance without suffering the
full effects of the disease. The name comes from vaccinia, or cowpox, used by
Jenner to protect against smallpox.
Grafting Cells or organs from another individual usually
survive innate resistance mechanisms but are attacked by the adaptive immune
response, leading to rejection.
Autoimmunity The body’s own (‘self’) cells and molecules do
not normally stimulate its adaptive immune responses because of a variety of
special mechanisms that ensure a state of self-tolerance, but in certain circumstances
they do stimulate a response and the body’s own structures are attacked as if
they were foreign, a condition called autoimmunity or autoimmune disease.
Hypersensitivity Sometimes the result of specific memory is that
reexposure to the same stimulus, as well as or instead of eliminating the
stimulus, has unpleasant or damaging effects on the body’s own tissues. This is
called hypersensitivity; examples are allergies such as hay fever and some
forms of kidney disease.
Immunosuppression Autoimmunity, hypersensitivity and, above all,
graft rejection sometimes
necessitate the suppression
of adaptive immune responses by drugs or other means.
