Schematic illustration of the membrane-proximal signaling events in B (left) and T cells (right). (Î) Activation of raft-associated Src PTKs upon engagement of the immunoreceptor. (II) Recruitment of the BCR and TCR to the rafts. (III) Recruitment of Syk PTK to dually phosphorylated ITAMs. (d) Phosphorylation of members of the Ca2⁺-initiation complex. (IV) Phosphorylation-dependent assembly of the Ca2⁺-initiation complex. Note that it is not yet clear whether the Ca2⁺-initiation complex in B cells involves a LAT-like TRAP. (V) Generation of the second messengers DAG and IP₃ via cleavage of PIP₂ by activated PLCγ. (VI) Alteration of the actin cytoskeleton and regulation of adhesion and migration via SLP-76/SLP-65–associated adapter and effector molecules. (VII) Negative regulation of Src PTK activation via the TRAP PAG and the Src kinase inhibitor Csk.