Science Focus ( Issue 21)

By Henry Lau 劉以軒 Picture this: there are 500 numbered balls in front of you. If you were asked to pick out the even-numbered balls in one second, how would you do so? To further complicate this, what if the balls were only 0.01mm in size, which is smaller than the width of a hair? How would you even begin to identify and sort these balls? Sounds like a trick question, doesn’t it? The reality is, biologists have to deal with the same problem all the time. Only they’re not handling balls, they’re handling cells. Each of our organs are composed of many different types of cells. What if we are only interested in studying one type? The first step is to dissociate an organ into individual cells and “pick” the cells you 流式細胞技術： 如何識別您的細胞 Flow Cytometry: Sorting Cells by Their Appearance like. The method to solve this problem was invented in the late 1960s [1], when biologists came up with an elegant solution to this riddle: flow cytometry. Before we go into the mechani sm of f low cytometry, we first have to understand the targets in question, namely the cells. Similar to how different people have unique identifying features, different cell types also have unique characteristics. The simplest identifying features of a cell are its relative size and complexity: some cells are larger than others and some cells have higher internal complexity (that is,