monovalent

1. Chemistry. Having a valence of 1; univalent.
2. Immunology.
   1. Containing antigens from a single strain of a microorganism: a monovalent vaccine; a monovalent serum.
   2. Having only one site of attachment. Used of an antibody or antigen.

Having a valency of one.

1. Able to form only one covalent or ionic bond.
2. Having a valence of one; univalent.
3. Of or relating to an antiserum containing an antibody or antibodies specific for one antigen.
4. Containing antigens from a single strain of a microorganism.
5. Having only one site of attachment. Used of an antibody or antigen.

1. having a valency of one.
2. capable of binding with only one antigenic or antibody specificity.

The adjective has 2 meanings:

Meaning #1: (immunology) containing only one kind of antibody
  Antonym: polyvalent (meaning #1)

Meaning #2: (chemistry) having a valence of 1
  Synonym: univalent
  Antonym: polyvalent (meaning #2)