Mild TBI activates microglia 24 hours after impact. IB4-positive cells were observed in different areas of brain 24 hours after TBI. Some of these cells were CD11b-positive. This labelling was absent in the sham animals and significantly less on the contralateral side or 48h after TBI. A. Confocal microscopic images showing IB4-positive (Alexafluor 488-conjugated, green fluorescence), CD11b-positive (red fluorescence) or IB4/CD11b-positive (red-green overlap) microglia in representative sections of ipsilateral dentate gyrus 24 hours after moderate TBI. The left column shows CD11b immunostaining, the middle column IB4 labelling and the right column is an overlay of CD11b and IB4 double labelling. Arrows indicate the CD11b or IB4 or CD11b-IB4 positive cells. Scale bar 30μ.B. Histograms show the quantitation of IB4-positive microglia in the ipsilateral cortex, hippocampus and thalamus 24 or 48 hours after TBI. In all three regions, the number of IB4-positive cells was significantly increased 24 h after TBI compared to sham animals. ** p < 0.001; * p < 0.05; compared to sham; # p < 0.05, ## p < 0.001 compared to 24H TBI.