Neurons and Immune Cells: A section through the area postrema in the lower brain stem. This is a structure in the lower brain stem that lacks the blood brain barrier and thus can sense blood components directly. Within this structure, many immune cells labeled by their MHC class II receptors (stained black) intermingle with neurons stained orange-brown for the dopamine-synthesizing enzyme tyrosine hydroxylase.

 
 
  

Neurons activated by immune stimulants: This image shows a part of the brain (the nucleus of the solitary tract) containing neurons that are activated by a bacterial immune stimulant. Activated nerve cells can be recognized by their dark back cell nucleus (see black arrows) appearing as little black beads. This black staining locates the c-fos protein, an activation marker that is not expressed in "quiet" neurons. The orange-brown staining marks the presence of tyrosine hydroxylase, a marker for catecholaminergic neurons. Some of these neurons show both black and brown markers together, showing particular catecholaminergic neurons responsive to abdominal immune stimulation.

Close up immune cells: This image shows a close up of an immune cell with arborizing processes (black) in close apposition with neurons (brown) that are located within the area postrema.

Sensory ganglia activated: This image shows a part of the vagal sensory ganglion located in the neck that contains the nerve cell bodies of nerve fibers that extend all the way into the thorax and abdomen, and their axons into the brain stem. Some of these cell bodies show a granular reaction product indicative of the expression of messenger RNA encoding the activation marker c-fos. This activation was induced by intraperitoneal injection of the bacterial immune stimulant SEB.

Paraganglia: Here is a image showing the abdominal vagus nerve and associated chemosensory cell clusters (forming a paraganglion), visualized by double-labeling immunofluorescence  for tyrosine hydroxylase (the dopamine -and noradrenaline down the line-synthesizing enzyme) in green, and for serotonin in red. Colocalization causes the colors to blend into yellow. The big red cells are specialized immune cells called mast cells.