116 Self and the Phenomenon of Life
b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity “9x6”abundance of the unmethylated “CpG” motif, a property shared by both
viruses and bacteria, which is recognized by host cell’s Toll-like recep-
tors. Another way to deal with viruses is through a process called RNA
interference. In this process, mammalian cells take advantage of the fact
that viral RNA inside the infected cells goes through a double-stranded
stage, whereas mammalian RNA are always single-stranded. Thus,
double-stranded viral RNAs are recognized as foreign by host cells and
are degraded into small pieces. These viral RNA fragments then bind
to the majority of intact viral RNA (which are single-stranded) if their
sequences match. The binding triggers a destructive mechanism to get
rid of the virus. Furthermore, viral RNA fragments stimulate the infected
cells to secrete interferons, powerful signals that alert many cells in the
body to mount an all-out antiviral response, through induction of more
than 300 different genes. (Note that the anti-viral mechanism through
RNA interference is also found in plants.) Animal cells also recognize
foreign DNA (usually from a virus) in the cytoplasm, through a sensor
called cGAS (cyclic GMP–AMP synthase). cGAS is an enzyme that pro-
duces the dinucleotide cyclic GMP–AMP, which sends a message to the
nucleus to activate the production and secretion of interferons, leading
to an all-out anti-viral response, as described above for viral RNA.^8
6.5 Adaptive (Acquired) Immunity Came in Late but Robust
Adaptive immunity appeared in evolution less than 500 million years
ago, and is present only in vertebrates. Adaptive immunity deals with
antigens encountered during the lifetime of an animal, needs a lag
period of one to two weeks to act, is highly specific in terms of molecular
recognition, and has a long-lasting effect. Each repeated exposure to the
same antigen strengthens the immune response, suggesting the pres-
ence of immune “memory.”
The major players of adaptive immunity are the lymphocytes
(see Table 6.2), which are of two types: the B cells which exclusively
make antibodies and release them into the blood stream, creating a