你有没有想过，为什么疫苗的接种部位通常是胳膊而不是屁股或大腿？尽管撸起袖子确实要方便得多，但是疫苗接种部位的选择没有你想的那么简单。

Hundreds of millions have rolled up their sleeves for the COVID-19 vaccine, but why haven't they rolled up their pants legs instead? Why do we get most shots in our arms? What's the science behind why we get most vaccines in our arm?

数以亿计的人已经接种了新冠疫苗，为什么打疫苗都是撸起袖子而不是卷起裤腿？为什么多数疫苗都是打在胳膊上？这背后有什么科学原理呢？

It's worth noting that most, but not all, vaccines are given in the muscle – this is known as an intramuscular injection.

值得注意的是，多数人（但不是所有人）接种疫苗都是打在胳膊上，号称“肌肉注射”。

Some vaccines, like the rotavirus vaccine, are given orally. Others are given just beneath the skin, or subcutaneously – think of the measles, mumps, and rubella vaccine. However, many others are given in the muscle.

有些疫苗是口服的，比如轮状病毒疫苗。其他一些疫苗是皮下注射的，比如麻疹、腮腺炎和风疹疫苗。但是，许多其他疫苗都是肌肉注射的。

But why is the muscle so important, and does location matter?

那么，为什么肌肉注射占比这么大，注射的肌肉部位是否有讲究？

肌肉含有免疫细胞

Muscles make an excellent vaccine administration site because muscle tissue contains important immune cells. These immune cells recognize the antigen, a tiny piece of a virus or bacteria introduced by the vaccine that stimulates an immune response.

肌肉是绝佳的疫苗注射部位，因为肌肉组织含有重要的免疫细胞。这些免疫细胞能识别疫苗中引发免疫反应的抗原。

In the case of the COVID-19 vaccine, the immune cells in the muscle tissue pick up these antigens and present them to the lymph nodes.

以新冠疫苗为例，肌肉组织中的免疫细胞会识别这些抗原，然后让抗原进入淋巴结。

Injecting the vaccine into muscle tissue keeps the vaccine localized, allowing immune cells to sound the alarm to other immune cells and get to work.

将疫苗注射进肌肉组织可以让疫苗先停留在局部，肌肉部位的免疫细胞会向其他免疫细胞发出警报，通知它们开始工作。

Once a vaccine is recognized by the immune cells in the muscle, these cells carry the antigen to lymph vessels, which transport the antigen-carrying immune cells into the lymph nodes.

一旦疫苗被肌肉内的免疫细胞识别，这些细胞会携带抗原进入淋巴管，然后淋巴管会将这些带有抗原的免疫细胞输送给淋巴结。

Lymph nodes, key components of our immune system, contain more immune cells that recognize the antigens in vaccines and start the immune process of creating antibodies.

作为人体免疫系统的关键组成部分，淋巴结所含的免疫细胞更多，这些免疫细胞能够识别疫苗中的抗原，从而启动制造抗体的免疫程序。