Human T cells express major histocompatibility complex (MHC) class II
antigens and adhesion molecules characteristic of antigen-presenting c
ells (APCs), and recent in vitro and in vivo evidence supports an anti
gen-presenting function for T cells. In this guise, T cells provide do
wnregulatory signals for the immune response by inducing anergy in T c
ells that have already been activated and cytotoxicity in resting T ce
lls. Here, Werner Pichler and Tony Wyss-Coray suggest that this may re
present art important negative mechanism for T-cell homeostasis.