INTRODUCTION

It has been known for a long time that arthropods requiring blood meals induce pronounced immune responses in the host on which they feed (Trager, 1939). These, together with the wide range of haemostatic mechanisms that are employed by the host to avoid blood loss, present a formidable barrier to the arthropod' essential need for regular blood meals in order to moult or produce eggs.

The immune responses first recognized were in those vector–host interactions where the arthropod remains attached to its host for a long period – days or often weeks. This is true of many ticks. The much cited work of William Trager (1939) is recognized as one of the seminal studies on anti-tick resistance and, during the past 60 years, a great deal has been learned about the numerous underlying mechanisms.

It is not surprising that such a lengthy and intimate association leads to cellular and humoral responses to vector antigens. However, it is now equally clear that such responses can also develop to haematophagous arthropods that feed rapidly and then leave the host. Notable here are insects of medical importance such as mosquitoes, sandflies and black flies.

Extensive studies have shown that, in both the slow-feeding (tick) and rapid-feeding (insect) arthropods, there is an array of immunomodulatory factors to counter the induction of immune responses, to deflect the immunity away from mechanisms that are harmful to the arthropod, or to lessen the effect of these if they do occur.