Theileria parva and T annulata provide intriguing models for the study of p
arasite-host interactions. Both parasites possess the unique property of be
ing able to transform the cells they infect; T. parva transforms T and B ce
lls, whereas T annulata affects B cells and monocytes/macrophages. Parasiti
zed cells do not require antigenic stimulation or exogenous growth factors
and acquire the ability to proliferate continuously. In vivo, parasitized c
ells undergo clonal expansion and infiltrate both lymphoid and non-lymphoid
tissues of the infected host. Theileria-induced transformation is entirely
reversible and is accompanied by the expression of a wide range of differe
nt lymphokines and cytokines, some of which may contribute to proliferation
or may enhance spread and survival of the parasitized cell in the host. Th
e presence of the parasite in the host-cell cytoplasm modulates the state o
f activation of a number of signal transduction pathways. This, in turn, le
ads to the activation of transcription factors, including nuclear factor-ka
ppa B, which appear to be essential for the survival of Theileria-transform
ed T cells.