Model of potential CEACAM dimers in signaling complexes on neutrophils. A possible model of CEACAM signaling complexes on neutrophils that is compatible with observed desensitization data is shown. In this model, CEACAMs can exist on the neutrophil surface as heterodimers or as CEACAM-1 homodimers. Signaling would require an active dimer. For example, desensitization of CEACAM-1 would allow signaling by CEACAM-8/6; 8/3; or 6/3 dimers, while desensitization of CEACAM-1 and any other two CEACAMs would leave no active dimers. In contrast, desensitization of CEACAMs-8, 6, and 3 would leave active CEACAM-1 homodimers. The existence of potential unidentified cooperative signaling molecules is denoted by the "?"