FIGURE 1.

Comparison of immune systems across invertebrate and vertebrate species. The main constituents of the immune systems of the invertebrate (worms, flies) and vertebrate (mice, humans) species discussed in this review are depicted. A universal innate immune response (left column) is conserved from worms to flies, whereas major elements of adaptive immunity (middle and right columns) are absent in invertebrates (though dedicated immune cells have been identified in flies). The major features of each arm of the immune system are recapitulated along with layers of complexity added at each evolutionary level (shown by nested rectangles of progressively darker shades of green). Ab, antibody; Ag, antigen; HSR, heat shock response; PGRP, peptidoglycan pattern recognition receptor; UPR, unfolded protein response