An antibody that reduces or abolishes some biological activity of a soluble antigen or of a living microorganism. Thus, diphtheria antitoxin is a neutralizing antibody that, in adequate amounts, abolishes the pathological effects of diphtheria toxin in animals. This is only one characteristic; the other general properties of the antibody are those of the immunoglobulin family (IgG, IgA, or IgM) to which it belongs.