The Animal Self: Molecular Recognition 113“9x6” b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to HumanityInnate immunity in animals consists of three lines of defense. First,
there are physical barriers in the form of skin, epithelial lining, and cell
membrane (cell wall in plants). These barriers are frequently fortified by
chemical agents that discourage pathogen growth, such as acidity in the
stomach and lysozyme in tears. Many epithelial cells protect themselves
with a mucus layer that contains antibacterial agents, the most abundant
among which are the defensins, polypeptides of about 12-50 amino acids
long. Defensins are produced not only by epithelial cells but also by
neutrophilic leukocytes and are partly responsible for their bactericidal
mechanism. Defensins belong to a large group of innate host defense
peptides (also known as antimicrobial peptides) found in all animals and
plants. They exhibit a broad-spectral activity that kills bacteria, fungi,
protozoa, nematodes and some viruses.^6
The second line of defense is the ability of all cells, when infected
by a microbe, to attempt to degrade the invader with a variety of diges-
tive enzymes inside the cell. The third line of defense consists of special
proteins and phagocytic cells, dedicated to actively attacking the invad-
ers based on molecular recognition. Molecular features common to
pathogens are perceived by animal cells as foreign.^7 They trigger an array
of innate immune responses after binding to host receptors, collectively
called pattern recognition receptors. These receptors are found in many
places: in the blood, on the cell surface, and inside the cell. The soluble
receptors in the blood are part of the “complement system,” a cascade of
protein chain-reactions involved in direct and indirect destruction of the
pathogens. The cell-associated receptors stimulate phagocytosis (inges-
tion of foreign bodies) and also activate the gene to initiate a series of
immune responses. Both animals and plants possess these receptors, the
best known of which are the Toll-like receptors.
6.3 Cells Involved in Innate Immunity
The major cellular players in innate immunity are (see Table 6.2): mac-
rophages, neutrophils, dendritic cells and natural killer cells (NK cells).