Functional plasticity of microglial cells in transgenic rat brain expressing human truncated tau protein. Intraneuronal expression of human truncated tau induces neurofibrillary degeneration. Injured neurons drive resting microglia to become reactive microglia that transform into brain phagocytic microglia: brain macrophages. At this stage, microglia lose contact inhibition and begin to fuse with each other, forming small clusters. Morphological activation of microglia is accompanied with upregulation of several immunological markers, including integrins CD11a, CD11b, CD11c and CD18; lymphocytic antigen CD4; leukocyte common antigen CD45; and lysosomal glycoprotein CD68 (colour dots). In the late stage of neurodegeneration, bloodborne leukocytes (mainly monocytes and partially dendritic cells) infiltrate the brain parenchyma and participate in the brain's immune response.