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1.1 introduction to basic concepts in immunology
Immunology is the science that is concerned with immune response to foreign challenges. Immunity
(derived from Latin term immunis, meaning exempt), is the ability of an organism to resist infections
by pathogens or state of protection against foreign organisms or substances. The array of cells,
tissues and organs which carry out this activity constitute the immune system.
The immune system is remarkably versatile defence system that has evolved to protect animals from
invading pathogenic microorganisms and cancer. It is able to generate an enormous variety of cells
and molecules capable of specifically recognizing and eliminating an apparently limitless variety of
foreign invaders. These cells and molecules act together in a dynamic network whose complexity
rivals that of the nervous system. Functionally, an immune response can be divided into two related
activities—recognition and response. Immune recognition is remarkable for its specificity. The
immune system is able to recognize subtle chemical differences that distinguish one foreign
pathogen from another. Furthermore, the system is able to discriminate between foreign molecules
and the body’s own cells and proteins. Once a foreign organism has been recognized, the immune
system recruits a variety of cells and molecules to mount an appropriate response, called an effector
response, to eliminate or neutralize the organism. In this way the system is able to convert the initial
recognition event into a variety of effector responses, each uniquely suited for eliminating a
particular type of pathogen. Later exposure to the same foreign organism induces a memory
response, characterized by a more rapid and heightened immune reaction that serves to eliminate
the pathogen and prevent disease.
Thucydides wrote in his History of the Peloponnesian War that persons who had been exposed to
plague previously could care for the sick without danger. In the 19th century, variolation was
commonplace; this was the removal of smallpox (variola virus) skin pustules which were
subsequently put into small cuts in the skin of healthy people. This was itself a crude form of
vaccination, with the crusty dry pustules acting as an incubator of attenuated virus. Edward Jenner
would later use the cowpox virus to vaccinate (from vacca, Latin for "cow") patients against
smallpox, and Louis Pasteur attenuated rabies and injected it into a small boy, naming this substance
a vaccine in honor of Jenner's earlier studies in the science of immunology.
As immunology progressed, many people began to question how these vaccines worked. Why
should exposure to plague in Thucycides' time confer protection only against plague and not all
disease? Why should cowpox, a similar disease to smallpox but clearly a less severe virus, give milk
maids sufficient immunity to resist full smallpox infection? In short, what has caused this memory
response to be relatively (yet not absolutely) specific as well as selective
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