The insect immune system serves as a key defense against attack by par
asitoids. Incompatible hosts often eliminate parasitoids by encapsulat
ion, a process in which hemocytes form a multilayered envelope around
the invading organism. Capsule formation involves cooperation between
one or more classes of hemocytes and is likely mediated by cytokines a
nd adhesion molecules. Reciprocally, parasitoids have evolved a variet
y of strategies for overcoming host immune responses. Some parasitoids
passively avoid elimination by developing in locations inaccessible t
o host hemocytes or by possessing surface features that fail to elicit
an immune response. Other species actively disrupt the host immune sy
stem by injecting specific factors into the host at oviposition. In pa
rticular, polydnaviruses associated with several taxa of parasitoids d
isrupt capsule formation by killing hemocytes or altering their abilit
y to adhere to foreign surfaces. These symbionts have likely played a
critical role in evolution of host range and in defining parasitoid-ho
st compatibility.