Anti-endothelial cell antibodies (AECA) belonged to the IgG, IgA and IgM classes and included complement-activating IgG1 antibodies. A: AECA of the IgA class. B: AECA of the IgM class, C: AECA of the IgG1 subclass. Fluorescein isothiocyanate (FITC)-labeled goat anti-human IgM or IgA antibodies (green) and a sheep anti-human IgG1 antibody (detected by means of an Alexa Fluor™568-labelled donkey anti-sheep IgG antibody; red) were used to detect binding of patient immunoglobulin to in situ primate brain endothelial cells. See Results section for details.