What is a function-triggering antibody?

The term "function-triggering antibodies" is not standard, these are more widely known as cross-linking antibodies. As you may know, an antibody has multiple "arms", with the antigen binding region or F_ab (Fragment antigen binding) at the end. IgG antibodies have 2 arms, IgM can have 10. The F_ab portion is generally identical on all the arms, although it's possible to engineer them differently. This allows the antibody to bind to multiple antigens at the same time, drawing them close together.

NCAM is a homophilic binding protein, with an extracellular domain that binds to other NCAM molecules, both on neighboring cells and on the same cell. That binding induces structural changes in the intracellular domain which promotes the binding and activation of kinases, proteins that phosphorylate (and thus activate or inhibit) other proteins. This starts a signaling cascade that induces changes in the cell, typically by controlling the activity of transcription factors in the nucleus.

Receptors are generally not spread out evenly over the surface of the cell, they often exist grouped together in membrane structures known as lipid rafts. When drawn close enough together, NCAM molecules can begin their signaling cascades on their own, as adapter proteins bound or recruited to the intracellular domains get activated – the kinases and their substrates are in close proximity as well. To induce this type of signaling in the lab, cells can be treated with cross-linking antibodies that draw the receptors together. This in effect triggers the function of the receptors, beginning their downstream signaling.

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