Comparative Immunity

The field of immunology is concerned mostly with how innate and adaptive mechanisms collaborate to protect vertebrates from infection. Although many cellular and molecular actors have important roles, antibodies and lymphocytes are considered to be the principal players. Yet despite their prominence in vertebrate immune systems, it would be a mistake to conclude that these extraordinary molecules and versatile cells are essential for immunity. In fact, a determined search for antibodies, T cells, and B cells in organisms of the nonvertebrate phyla has failed to find them. The interior spaces of organisms as diverse as fruit flies, cockroaches, and plants do not contain unchecked microbial populations, however, which implies that some sort of immunity exists in most, possibly all, multicellular organisms, including those with no components of adaptive immunity.

Insects and plants provide particularly clear and dramatic examples of innate immunity that is not based on lymphocytes. The invasion of the interior body cavity of the fruit fly, Drosophila melanogaster, by bacteria or molds triggers the synthesis of small peptides that have strong antibacterial or antifungal activity. The effectiveness of these antimicrobial peptides is demonstrated by the fate of mutants that are unable to produce them. For example, a fungal infection overwhelms a mutant fruit fly that is unable to trigger the synthesis of drosomycin, an antifungal peptide (Figure 1-12). Further evidence for immunity in the fruit fly is given by the recent findings that cell receptors recognizing various classes of microbial molecules (the toll-like receptors) were first found in Drosophila.

Plants respond to infection by producing a wide variety of antimicrobial proteins and peptides, as well as small

Toll Drosomycin Fungi

FIGURE 1-12

Severe fungal infection in a fruit fly (Drosophila melanogaster) with a disabling mutation in a signal-transduction pathway required for the synthesis of the antifungal peptide drosomycin. [From B. Lemaitre et al, 1996, Cell 86:973; courtesy ofJ. A. Hoffman, University of Strasbourg.]

FIGURE 1-12

Severe fungal infection in a fruit fly (Drosophila melanogaster) with a disabling mutation in a signal-transduction pathway required for the synthesis of the antifungal peptide drosomycin. [From B. Lemaitre et al, 1996, Cell 86:973; courtesy ofJ. A. Hoffman, University of Strasbourg.]

nonpeptide organic molecules that have antibiotic activity. Among these agents are enzymes that digest microbial cell walls, peptides and a protein that damages microbial membranes, and the small organic molecules phytoalexins. The importance of the phytoalexins is shown by the fact that mutations that alter their biosynthetic pathways result in loss of resistance to many plant pathogens. In some cases, the response of plants to pathogens goes beyond this chemical assault to include an architectural response, in which the plant isolates cells in the infected area by strengthening the walls of surrounding cells. Table 1-4 compares the capabilities of immune systems in a wide range of multicellular organisms, both animals and plants.

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