The Torah in Parashat Emor (Vayikra 21:17) commands that a Kohen who has a physical blemish is forbidden from performing the service in the Bet Ha’mikdash. This command refers specifically to a “Mum Kabu’a” – a permanent blemish. The Sefer Ha’hinuch gives the example of broken bones which do not fully heal, and result in a permanent limp or other permanent state of dysfunction. Temporary blemishes, such as skin conditions like pimples, warts and boils, are not included in this prohibition.
The Sefer Ha’hinuch explains that the Torah forbids Ba’aleh Mum (people with physical blemishes) from performing the Aboda because the service in the Bet Ha’mikdash is meant to make an impression upon the people. The impact of the service, the Sefer Ha’hinuch writes, is stronger and more significant when it is performed by people who are healthy and strong, and have a pleasant, impressive appearance, as opposed to those with visible physical defects. In order to ensure that the service will leave a strong impression on the people, the Torah commanded that only Kohanim who are free from physical blemishes may perform the Aboda (service in the Bet Ha’mikdash). The Sefer Ha’hinuch emphasizes that as the people’s atonement depends on the service of the Kohanim, it is appropriate for this service to be performed by people who command respect through their appearance.
Additionally, the Sefer Ha’hinuch comments, seeing a Kohen who is physically unblemished has a spiritual impact upon a person, uplifting his soul, and for this reason, too, the Torah forbade Ba’aleh Mum from performing the service.
Only external wounds and blemishes disqualify a Kohen for the service. Internal defects, such as a missing kidney, do not disqualify a Kohen and are not included in this prohibition.
The Sefer Ha’hinuch lists 140 defects which disqualify a Kohen from performing the service. Out of these 140 defects, 50 are defects which, in an animal, disqualify the animal as a sacrifice; the other 90 apply specifically to human beings. A Kohen with any one of these 140 defects who performs the Aboda is liable to Malkut. If he has one of the 50 defects which apply also to animals, the service he performed is invalid, whereas performing the service with one of the other 90 blemishes does not disqualify the service, even though the Kohen is liable to Malkut.
The Rabbis enacted an extension of this prohibition to include a number of other physical defects which resemble defects which disqualify a Kohen on the level of Torah law. A Kohen who violates this Rabbinic enactment and performs the Aboda with one of these additional blemishes is not liable to Malkut, and the service is not invalidated.
Of course, this command applies only in the times of the Bet Ha’mikdash, when the Kohanim performed the service.