Tick–host interactions and their immunological implications in tick-borne diseases

The family of ticks are important vectors of pathogens which can cause a variety of diseases in different species, including humans. In recent years the incidence of diseases transmitted by ticks has risen. This seems to be partly due to the expansion of the tick vector into urban areas. Pathogens transmitted by ticks include viruses, bacteria and protozoans. Ticks serve not only as transmitters of pathogens, they also allow development of the infectious agent inside. In some diseases, like Lyme disease, this can be important for the transmission of the pathogen. Together with the pathogen, the tick injects saliva components into the host which can modulate or impair the local immune response of the host, and which can facilitate pathogen evasion into the host tissue. As a reaction of the host immune system to the tick bite and to the transmission of potential pathogens, the tick itself is exposed to the innate and adaptive immune system of the host. This action of the host immune system against the transmitted pathogen can be utilized in immunization strategies aiming to sterilize the pathogen-infested tick. The complex interactions of the pathogen with the tick vector and the host, and the tick with the host provide a basis for the development of disease-preventive strategies aiming to control the emerging diseases transmitted by ticks.