How can antibodies act like ligands and be agonists/antagonists?

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I know drugs and peptides can activate/antagonize cells but how can antibodies do this?

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Antibodies bind to proteins on the surface of a cell/pathogen, usually to disable and mark those pathogens. But the receptors on your cells are also proteins and specific (possibly engineered) antibodies can bind to those receptors, acting either as antagonists by blocking the receptor (for example [Trastuzumab](https://en.wikipedia.org/wiki/Trastuzumab?wprov=sfti1) blocks the HER2 receptor in HER2-positive cancer cells) or as agonists by mimicking the receptors’ natural substrate (for example in [Grave’s disease](https://en.wikipedia.org/wiki/Graves'_disease?wprov=sfti1) the body produces antibodies that mimic thyrotropin and activate the thyrotropin receptor).