The Animal Self: Molecular Recognition 115“9x6” b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanitythe body and function as universal “garbage collectors,” removing all
unwanted materials on the way (their large size seems to make them
suitable for this task). Upon encountering pathogens, macrophages also
send out signals to recruit other immune cells such as neutrophils to the
scene.
Neutrophils are combat soldiers at the frontline. Present in over-
whelming number in the blood, they are quickly attracted to the infec-
tion site and engage in frenzied killing. They take up the pathogens,
digest them with enzymes, and literally “burn” them to death with strong
oxidizing agents. Their job is to destroy the invaders as quickly as pos-
sible, frequently at the expense of their own life. (Pus cells found in
infected wounds are dead neutrophils after doing their job of defense.)
Dendritic cells are so-called because of their star-shaped, branched
appearance (not to be confused with the dendrites of a neuron). Rather
than directly attacking the pathogens, they gather chemical information
about the antigen and present it to the adaptive immune cells (lym-
phocytes) for a delayed but powerful response. This is accomplished
by digesting the foreign proteins and displaying the resulting peptides
on their cell surface. Considered the most effective antigen-presenting
cells, the dendritic cells through their “scouting” function serve as a link
between the innate and the adaptive immune response.
Natural killer cells (NK cells), the fourth type of cells in innate
immunity, do not recognize or attack the invading pathogens. Instead,
they can directly detect host cells that are infected with virus. They then
destroy the virus-infected cells in order to prevent the spread of infec-
tion. They also destroy certain tumor cells and are thought to be involved
in immune surveillance against malignancy.
6.4 Special Strategies Against Viruses
Since viral particles are little more than pockets of nucleic acids, they
lack proteins and other markers that can easily be distinguished by host
cells as non-self. To remedy this, the host cells develop special strategies
by focusing on viral nucleic acids. One way to single out viral DNA is the