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You may try to use Hough circle transform.

This image processing page also mentions the Hough circle transfrom as a possibility on cell detection.

Here is a short overview on approaching a problem similar to yours using MatLab.

Here is an article from last year on blood cell detection using Hough circle. The authors report that the automatic counting was quite accurate.

Here is an even more fresh study on white blood cells. The introduction enumerates some extensions of Hough circle transform and even other approaches like genetic algorithms.