A general feature of many parasitic infections by protozoa or helminths is their chronicity and several reasons contribute to this, e.g., weak innate immunity and the capacity of parasites to withstand or to evade destruction by specific immune responses of the vertebrate host. General aspects of the various host immune responses are described in the following. Peculiarities of the immunological response to specific parasites are described in detail under the headwords of the respective diseases.
The skin and the linings of the respiratory, gastrointestinal, and urogenital tract present formidable physical and chemical barriers to infective organisms and represent a first line of defense. These barriers provide a natural resistance, also called innate immunity, to infection, but they are not perfect. Protozoan and helminthic parasites, have evolved in such a way that they either are able to penetrate the body’s barriers directly or are transmitted by insect bites.