A screening method suitable for screening for a blood IgA production inhibitor, a blood IgA production promoter, or a preventive or therapeutic agent for a disease caused by excess blood IgA, a blood IgA production inhibitor, and a blood IgA production promoter A method suitable for controlling the amount of IgA in the blood and a method for assessing the risk, degree or condition of the disease caused by excess IgA in the blood, and removal for preventing or treating the disease caused by excess IgA in the blood Provided is a method for presenting at least one selected from the group consisting of enterobacteria to be administered and enterobacteria to be administered. Inhibition of expression of antibacterial protein gene of intestinal innate immunity system, promotion of expression or activity of said antibacterial protein, or increase in reaction of said antibacterial protein as an index For screening for preventive or therapeutic agents for diseases caused by the disease.血中IgA産生抑制剤、血中IgA産生促進剤又は血中の過剰IgAに起因する疾患の予防又は治療剤のスクリーニングに好適なスクリーニング方法及び血中IgA産生抑制剤、血中IgA産生促進剤の提供、血中IgA量の制御に好適な方法並びに血中の過剰IgAに起因する疾患のリスク、程度ないし状態を評価する方法及び血中の過剰IgAに起因する疾患を予防又は治療するために除去すべき腸内細菌及び投与すべき腸内細菌よりなる群から選択される少なくとも1種を提示する方法を提供する。 腸内の自然免疫系の抗菌タンパク質遺伝子の発現の促進、前記抗菌タンパク質の発現若しくは活性の促進又は上記抗菌タンパク質の反応増加を指標として、血中のIgAの産生抑制剤、又は血中の過剰IgAに起因する疾患の予防又は治療剤をスクリーニングする方法。