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?

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.

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.

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. 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.

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.

Clusters of lymph nodes are located in areas close to vaccine administration sites. For instance, many vaccines are injected in the deltoid because it is close to lymph nodes located just under the armpit. When vaccines are given in the thigh, the lymph vessels don’t have to travel far to reach the cluster of lymph nodes in the groin.

Injecting a vaccine into the deltoid muscle may result in local inflammation or soreness at the injection site.

If certain vaccines are injected into fat tissue, the chance of irritation and inflammation reaction increases because fat tissue has poor blood supply, leading to poor absorption of some vaccine components.

Vaccines that include the use of adjuvants — or components that enhance the immune response to the antigen — must be given in a muscle to avoid widespread irritation and inflammation.

Yet another deciding factor in vaccine administration location is the size of the muscle.

Adults and children ages 3 and older tend to receive vaccines in their upper arm in the deltoid. Younger children receive their vaccines mid-thigh because their arm muscles are smaller and less developed.

All things considered, when it comes to the flu shot and the COVID-19 vaccine, for most adults and kids, the arm is the preferred vaccination route.

Words to Learn 相关词汇

【三角肌】sānjiǎojī deltoid a thick, triangular shoulder muscle

【抗原】kàngyuán antigen any substance foreign to the body that evokes an immune response either alone or after forming a complex with a larger molecule

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？

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。

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。

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。

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。

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。

成簇状分布的淋巴结就在疫苗注射部位附近。许多疫苗都打在三角肌处，因为三角肌和腋下的淋巴结距离很近。如果疫苗是打在大腿上，距离腹股沟淋巴结聚集处也不远。

将疫苗注射进三角肌可能会引发注射处的局部红肿或酸痛。

如果疫苗被打在脂肪组织上，那么出现刺激和红肿反应的几率会增大，因为脂肪组织的血液供应不足，从而导致对某些疫苗成分的吸收不良。

含有佐剂的疫苗必须打在肌肉上，以避免大范围的刺激和红肿反应。佐剂的使用可以提高对抗原的免疫反应。

决定疫苗注射部位的另一个因素是肌肉的大小。

成年人和三岁以上儿童接种疫苗的部位通常是胳膊上方的三角肌。更年幼的小孩是在大腿中部接种疫苗，因为他们的胳膊肌肉组织较小、尚未发育完全。

综上所述，大多数成年人和孩子在接种新冠疫苗和流感疫苗时，胳膊是最佳的疫苗注射部位。

(chinadaily.com.cn)