一氧化氮是一种重要的巨噬细胞免疫效应分子，它参与免疫调节和宿主防御反应。

Nitric oxide is an important effector molecule of macrophages that is involved in immune regulation and host defense.

youdao

So, antigens are molecules or pieces of molecules that stimulate an immune response.

所以 抗原是可以激发免疫反应的,分子或分子碎片

耶鲁公开课 - 生物医学工程探索课程节选