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 and does location matter？
Muscles make an excellent vaccine （疫苗） administration site because muscle tissue contains important immune （免疫） cells.These immune cells recognize the antigen （抗原），a tiny piece of a virus or bacteria introduced by the vaccine that stimulates an immune response.
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 （淋巴结）.Injecting （注射） 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.
Croups of lymph nodes are located in areas close to vaccine administration sites.For instance,many vaccines are injected in the deltoid （三角肌） because it is close to lymph nodes located just under the armpit （腋下）.
Muscle tissue also tends to keep vaccine reactions localized at the injection site.If certain vaccines are injected into fat tissue,the chance of irritation and side effect increases because fat tissue has poor blood supply,leading to poor absorption of some vaccine components.Vaccines must be given in a muscle to avoid widespread side effect.
Yet another deciding factor in vaccine administration location is the size of the muscle as well as convenience.



Why did the author raised many questions in the first paragraph？ ______

A. People do have many questions about injecting the vaccine into arms.
B. The writer actually didn't know the answers of these questions.
C. The writer wanted to arouse readers' interest in reading the article.
D. The theory behind the vaccine injection is difficult to understand.

What's the advantage of getting vaccine shots in the muscle？ ______

A. Muscle tissue spread important immune cells easily.
B. Muscle tissue can stimulate an immune response.
C. The antigen can be recognized by muscle tissue easily.
D. Vaccine can stimulate a better immune response there.

What may the underlined word in the sixth paragraph mean？ ______

A. Common feeling of sorrow.
B. Abnormal sensitivity to stimulation.
C. Terrible wound in the muscle.
D. Absorption of vaccine components.

What may the article continue to present？ ______

A. The advantage of getting wide vaccine shots in arms.
B. How to prevent the wide spread of the COVID-19.
C. The size of muscle and convenience also location.
D. How to prevent the side effect of getting vaccine shots.