1 Introduction.- 1.1 Antigen Presentation and Antigen Presenting Cells.- 1.2 Antigen Presentation within the CNS.- 1.3 Microglia Might Be the Cerebral Antigen Presenting Cells.- 1.4 Theories on the Antigen Presentation Site.- 1.5 Questions Still Open.- 1.6 Methodological Approach.- 2 Materials and Methods.- 2.1 Animals.- 2.2 Overview of Animal Experiments.- 2.3 Surgery.- 2.4 Number of Sprouting Neurons After Retrograde Tracing with HRP.- 2.5 Number of Surviving Neurons After Resection of the Facial Nerve.- 2.6 Neuronophagic Microglia Identified by Vital Labeling with Fluoro-Gold.- 3 Results.- 3.1 Resection of 10 mm of the Facial Nerve Causes a Slow Loss of Facial Motoneurons in the Adult Rat.- 3.2 No Breakdown of the Blood - Brain Barrier or Passage of Unprimed Lymphocytes into Brain Tissue After Facial Nerve Resection.- 3.3 Fluoro-Gold Labeling of Motoneurons, Phagocytic Microglia and Perivascular Cells.- 3.4 Time Course of Existence and Migration of Fluoro-Gold-Labeled Neuronophages.- 3.5 Immunocytochemistry of the Fluoro-Gold-Labeled Neuronophages.- 4 Discussion.- 4.1 Oligodendrocytes and Astrocytes Are Not the APC of the Brain.- 4.2 Microglia Are Not the APC of the Brain.- 4.3 The Perivascular Cells Are the APC of the Brain.- 5 Summary.- References.