1. (in medicine) An individual who harbours a particular disease-causing microorganism without ill-effects and who can transmit the microorganism to others. Compare vector.
2. (in genetics) An individual with an allele for some defective condition that is masked by a normal dominant allele. Such individuals therefore do not suffer from the condition themselves but they may pass on the defective allele to their offspring. In humans, women may be carriers of such conditions as red-green colour blindness and haemophilia, the alleles for which are carried on the X chromosomes (see sex linkage).
3. (in immunology) A protein that covalently binds to a hapten to form a hapten-carrier conjugate. Unlike the hapten alone, the conjugate can cause the immune system to produce antibodies against the hapten, against the carrier, and against the hapten-carrier combination.
4. (in biochemistry) See carrier molecule; hydrogen carrier.