natural antibody

例句：

1. 1. The immune system is the body's natural defense against disease.
   免疫系统是身体对抗疾病的自然防御机制。
2. 2. Natural antibodies are produced by the body to fight off infections.
   自然抗体是由身体产生的，用以抵抗感染。
3. 3. Researchers have discovered that certain viruses can mutate and produce new natural antibodies.
   研究人员发现某些病毒可以变异并产生新的自然抗体。