Cell membrane

The membrane is a double layer of fat molecules studded with proteins: receptors that receive signals, channels and pumps that move molecules in and out, and adhesion molecules that hold cells to their neighbours. Only small, fat-soluble molecules cross it freely, which is why small-molecule drugs can reach targets inside the cell while antibodies (which are large) are restricted to targets on the surface. Antibody-drug conjugates exploit this: the antibody binds a surface protein, the whole complex is swallowed by the cell, and the payload is released inside.