life11e_ch41

Innate defenses evolved before adaptive defenses

All animals have innate defenses against their enemies. For example, the arthropod Tachypleus tridentatus—the Japanese horseshoe crab—first appeared in the fossil record about 400 million years ago. It relies only on innate defenses. These defenses include barriers, defensive cells, and defensive molecules.

Barriers include physical, chemical, and biological mechanisms for resisting infections. The horseshoe crab has the hard exoskeleton that is characteristic of arthropods. This shell acts to protect the crab from invasion by pathogens.
Cells involved in innate defenses include phagocytes that bind to microbial pathogens, ingest them by endocytosis, and destroy them by hydrolysis. Amebocytes in the blood of the horseshoe crab fufill this defensive role.
Molecules that are toxic to invading pathogens are important in innate defense. The horseshoe crab has a wide array of such molecules that are released from cells in its blood. These molecules include peptides that disrupt the bacterial cell membrane, rendering it permeable; and peptides that bind to bacterial surfaces and cross-link them.

Studies of innate immunity, along with genome sequencing, have revealed that the recognition and activation phases of innate immunity evolved very early in animals. For example, animals as diverse as humans and fruit flies share a class of receptors, called Toll-like receptors (TLRs), that participate in innate defense responses. These receptors recognize nonself molecules called *pathogen-associated molecular patterns (PAMPs). In vertebrates, each Toll-like receptor recognizes and binds to a specific molecule that is found in a broad class of pathogens, such as a component of the bacterial cell wall. Binding triggers a signal transduction pathway that ends with the expression of genes for anti-pathogen molecules (Figure 41.1). This pathway exists in some form in many animal groups, including humans.

Figure 41.1 Cell Signaling and Defense Binding of a pathogenic molecule or fragment to the Toll-like receptor initiates a signal transduction pathway that results in the transcription of genes whose products are involved in adaptive and innate defenses.

*connect the concepts The recognition of PAMPs is widespread in immunity and is therefore an evolutionarily ancient mechanism. Plants have an immune response to invading pathogens that also involves PAMPs. See Key Concept 38.1.