The immune system of high vertebrates consists of two components: inna
te and adaptive. The innate immune system relies on a set of germ-line
encoded receptors that recognize conserved molecular patterns found o
nly in microorganisms. The adaptive immune system uses somatically gen
erated antigen receptors which are clonally distributed on the two typ
es of lymphocytes: T cells and B cells. These antigen receptors are ge
nerated by random processes and, as a consequence, the general design
of the adaptive immune system is based on clonal selection of lymphocy
tes expressing receptors with particular specificities. Here we discus
s the essential role of the innate immune system in the clonal selecti
on of lymphocytes and activation of the adaptive immune responses.