Systemic acquired resistance is a plant-wide expression of defense genes. It is nonspecific because it provides the whole plant protection against a wide variety of pathogens. It starts at an infection site, where cells produce methyl salicylic acid before they die. This signaling molecule is distributed to remote parts of the plant where it is then converted to salicylic acid, which activates the signal transduction pathway that induces the production of pathogenesis-related (PR) proteins. These proteins protect the plant against a variety of pathogens for several days.
In the innate immune systems of animals, recognition and response rely on traits common to groups of pathogens. In phagocytosis (illustrated above), cells identify fragments of molecules characteristic of a set of pathogens with Toll-like receptors on the cell membrane and then ingest the recognized bacteria and other foreign substances. Lysozymes are enzymes in the cell then break down bacterial cell walls and release the debris using exocytosis. Immune cells that go through this process release antimicrobial peptides as a result of this encounter, which can travel to other areas of the body through body fluids and disrupt the plasma membranes of fungi and bacteria elsewhere in the animal.
Similarities&Differences:
In both plants and animals, cells rely on innate genes that can recognize common pathogen fragments. In plants, PR proteins derived from resistant genes in the plant’s genome are responsible for the hydrolysis of the cell walls of pathogens. In animals, toll-like receptors are proteins derived from the animal's genome that bind to fragments of molecules characteristic of a set of pathogens. In response to the recognition of pathogens, both plants and animals are capable of alerting the rest of the organism of an attack via chemical signals (methyl-salicylic acid and antimicrobial peptides) that heighten the innate immune system in other remote parts of the organism. Plants lack phagocytes, so the common response to pathogen attacks is cell death, which effectively stops the spread of the pathogen to other parts of a plant. However, in animals the loss of a body part can be fatal, so they take advantage of their phagocytic cells. Bodily fluids circulate in the body in order to use phagocytosis and peptides to pick up and destroy foreign antigens throughout the body, keeping important cells alive.
