Craig sent this DIY blood smear site, he writes –
“Blood smears are neat because they’re a particularly simple way of looking at your own cells. Other tissues usually require complicated preparations and specialized equipment needed to get thin sections, but since blood is a liquid you can just smear it out into a film on a microscope slide. It’s then easy to stain the slide with Wright’s stain, which is easy and relatively cheap to purchase from biochemical suppliers (unfortunately, a lot of voodoo goes into the manufacturing, so it’s not practical to make it yourself.) You can then look at the slide under a microscope and pick out the different types of white cells (neutrophils, monocytes, and so on) interspersed among the red cells. With this, you can see the immune system at work: if you suffer from hay fever, for example, your level of eosinophils will be dramatically elevated, while out of season they will settle down to a normal proportion. Eosinophils go up with parasitic infection too: the authors of the linked site were even able to detect bloodborne heartworm larvae in eosinophilic (high-eosinophil) dog blood!” – Link.