112 Self and the Phenomenon of Life
b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity “9x6”infection was practiced in the East as early as the tenth century.^4 It was
brought to modern medicine in 1796 when Edward Jenner started to
inoculate cowpox for protection against smallpox. The concept came
from the common observation that milkmaids who had contracted cow-
pox did not get smallpox. Immunology received a boost with the subse-
quent discovery of microorganisms and their connection to disease; and
with the discovery of special proteins called antibodies circulating in the
blood that are capable of neutralizing these pathogens.
Today, we realize that all of immunity, with its ramifications in
transplantation, transfusion, vaccination, and defense against pathogens,
boils down to the principle of “self” versus “non-self” discrimination on
the molecular level.
6.2 Innate Immunity is Evolutionarily Very Old
There are two types of animal immunity: the inborn or innate immu-
nity which is rapid in action but lacks specificity, and the acquired or
adaptive immunity which is slow but highly specific. Both are defense
against invaders. The distinction is that innate immunity is shaped by
evolution and passes down from generation to generation through the
genes, whereas adaptive immunity is developed (acquired) only during
the lifetime of an animal.^5
Innate immunity appeared very early in evolution, being present in
all multicellular organisms, including vertebrates and invertebrates, and
even plants (see Chapter 5). Innate immunity depends on recognition of
common features in pathogens, the so-called “danger signals.” The rec-
ognition is genetically programmed and does not need any prior encoun-
ter with the invaders. Its function is to provide a quick and immediate
halt of the invading microbes. Without innate immunity, a single patho-
genic bacterium in blood may double every hour and end up with 20
million progeny in a day, causing a full-blown infection. Innate immunity
keeps the infection in check while awaiting the more powerful adaptive
immunity to kick in, which takes about a week’s time.