Opsonins found on the surface of microbial cells in vivo and ex vivo are characterized. The possibility of their visualization by immune electron and fluorescent microscopy has been demonstrated. Opsonins are shown to play a role in immune-mediated adhesion of bacteria to phagocytes and erythrocytes and formation of virus-bacterium associations. Staphylococci, Streptococci, and Propionibacteria appear to actively adsorb blood proteins on their cell surface giving rise to well-apparent capsular structures tentatively called immunoglobulin coatings. Pathogen (microorganism)-associated molecular patterns of gram-positive and gram-negative bacteria show different degree of interaction with soluble proteins that in turn promote their adhesion to blood corpuscles. The role of erythrocytes in transportation and elimination of immune complexes bound to their membrane receptors is discussed.