Five molecules are known to bind the Fab fragments of human immunoglobulins (Ig). Microbial protein A and protein G are primarily Fc-binding molecules but can also bind other structures of the heavy chain, which are located in the variable domain of the third subgroup (VH3) and in the first constant domain of IgG (CH1 gamma), respectively. In contrast, the two other microbial receptors have a sole Ig-binding site, directed to chi chains (protein L) or to Ig polymers (protein P). Protein Fv is synthesized by human liver cells and released in the digestive lumen, where it forms large complexes with secretory Ig after binding to the VH domains. These five molecules, in the main, bind cleaved Ig and most of them recognize all classes of antibodies. Bacterial molecules are, or can be, used as reagents to purify and detect Ig and fragments. Furthermore, a possible use in human therapy or vaccination is envisaged, and the human protein Fv is a key-factor in immune protection against intraluminal pathogens of the gut.