PARTICLES, AFFINITY PARTICLES HAVING LIGAND FOR TARGET SUBSTANCE, IN VITRO DIAGNOSTIC REAGENT AND KIT THAT INCLUDE SAME, AND METHOD FOR DETECTING TARGET SUBSTANCE

Provided are a particle, which causes small nonspecific adsorption, has a reactive functional group for chemically bonding a ligand, and is suitable for a latex agglutination method, and an affinity particle, an in vitro diagnostic reagent and kit each including the affinity particle as a particle for a latex agglutination method, and a method of detecting a target substance.The particle is a particle including, in a surface layer thereof, a copolymer having a repeating unit A having a side chain A having, at a terminal thereof, a carboxy group to be bonded to a ligand and a repeating unit B having a side chain B having a hydroxy group at a terminal thereof, wherein when the particle is dispersed in ion-exchanged water, the surface layer of the particle is hydrated to form a swollen layer, wherein the density of the carboxy groups to be incorporated into the swollen layer satisfies a value of from 0.04 group/nm3 to 0.15 group/nm3, and wherein the ratio of a particle diameter in water to be measured when the particle is dispersed in ion-exchanged water to a dry particle diameter to be measured when the particle is dried satisfies a value of from 1.10 to 1.40.