In an SSB/CW receiver, what is the purpose of the antenna?

An antenna acts as a transducer between free space and a radio receiver. When electromagnetic waves travelling through space strike the conductive metal elements of a receiving antenna, they induce tiny, alternating electrical currents in the metal. These minute currents are an exact electrical replica of the original electromagnetic wave, just scaled down in strength. They are then fed into the receiver's front end for amplification and processing. The distractors are incorrect because an antenna does not inherently separate signals from atmospheric noise (which is the job of filters) or protect against overload. Furthermore, while an antenna has a specific polarization, it does not actively 'polarize' incoming signals; it simply responds best to waves of matching polarization.