Particle, affinity particle, and reagent, kit and detection method containing the same

An affinity particle comprising a particle including:

• a polymer including a repeating unit having a side chain A represented by formula (10) and a repeating unit having a side chain B represented by formula (10′):
• wherein, in the formula (10):
• R1 is H or CH3;
• L4 is a partial structure containing a plural number of interatomic bonds;
• and
• R3 is represented by formula (11) or (12):
• wherein, in the formula (11) and (12), * represents bonding to a terminal atom of L4;
• wherein, in the formula (10′);
• R1′ is H or CH3;
• L4′ is a partial structure containing a plural number of interatomic bonds;
• and
• R3′ is represented by formula (13):
• wherein, in the formula (13), * represents bonding to a terminal atom of L4′;
• and
• a ligand bonded to the particle via the side chain A,
• wherein the unit having the side chain A and the unit having the side chain B satisfy a relationship of expression (1′):
• 20≤(number of units having side chain A)/((number of units having side chain A)+(number of units having side chain B))×100≤30 (1′).