Insect immune systems face a great diversity of challenges from parasites, parasitoids, and pathogens. Parasitoid insects alone are estimated to comprise about 15 percent of the approximately one million known species of insects. Pathogens and parasites include vast numbers of viruses, bacteria, microsporidia, fungi, nematodes, and protozoa such as sporozoa, gregarines, and coccidia; the number of described species of these organisms grows each year. The success of these parasitoids and pathogens has depended to a great extent on their ability to either physically avoid encounters with cells of the insect immune system (Kathirithamby, Ross, and Johnston 2003; Manfredini et al. 2007) or to produce substances that can circumvent the defenses mounted by a host’s blood cells. These substances can be (1) viruses or molecules that suppress the immune response, (2) molecules that mimic host antigens and are mistaken for self antigens by the host, or (3) molecules that mask the nonself antigens of the intruders and result in the pathogens’ or parasitoids’ not being recognized as foreign or nonself by the immune cells of the host (e.g., Salt 1970; Rizki and Rizki 1984; Davies and Vinson 1986; Schmidt et al. 1990; Strand and Noda 1991; Strand and Wong 1991; Hoek et al. 1996; Lavine and Beckage 1996; Asgari et al. 1998; Galibert et al. 2003; Beck and Strand 2005; Wang and St. Leger 2006).