Specific antigen-binding molecules in normal serum. A quest for circulating receptors

Molecules (I) in the IgM fraction of normal serum compete with specific antibody for a single determinant on a macromolecule. Following immunization, the levels of I fall as antibody titer rises, but they later increase again during the decline of antibody. These findings are discussed in light of recent studies regarding soluble antigen-specific factors.