Identification of IgG-binding site on E.-histolytica cell membrane

$\text{E.histolytica}$ did not bind significantly ertthrocytes (E) but formed a high percentage of rosettes with bovine E sensitized by rabbit IgG (EA_G) or IgM (EA_M) antibody as well as rosettes with human E coated by human anti-CD antibody (EA_CD). Although a weak phagocytosis of untreated E was recorded both types of E were more frequently ingested upon incubation at 37°C when coated with corresponding antibodies. Attachment of EA to amoeba membrane was visualised even after incubation at 4°C or when amoeba cells were fixed with formaldehyde. Binding of sensitized erythrocytes to amoeba surface was markedly reduced when: (i) bovine E were coated with Fab fragment of rabbit IgG antibody; (ii) EA were previously treated with protein A of $\text{S.aureus}$; (iii) amoeba were pretreated with monomeric or polymeric IgG or its Fc fragment. It was concluded that $\text{E.histolytica}$ possesses Fc-like receptors on its surface.