半抗原（hapten）是指本身分子量过小，无法刺激免疫反应，必须和载体（carrier）结合成较大的才能刺激发生免疫反应。半抗原能与对应抗体结合出现抗原-抗体反应，又不能单独激发人或动物体产生抗体的抗原，它只有抗原性，不具免疫原性，又称不完成抗原。大多数多糖和所有类脂都属于半抗原，如果用化学方法把半抗原与某种纯蛋白的分子载体（protein carrier）结合，纯蛋白会获得新的免疫原性，并能刺激动物产生相应的抗体。半抗原一旦与纯蛋白质结合，就构成该蛋白质的一个抗原簇。

半抗原的概念是由卡尔·兰德施泰纳所提出 。他还提出了可以用人工合成半抗原来研究免疫化学现象的想法。

Haptens are small molecules that elicit an immune response only when attached to a large carrier such as a protein; the carrier may be one that also does not elicit an immune response by itself. (In general, only large molecules, infectious agents, or insoluble foreign matter can elicit an immune response in the body.) Once the body has generated antibodies to a hapten-carrier adduct, the small-molecule hapten may also be able to bind to the antibody, but it will usually not initiate an immune response; usually only the hapten-carrier adduct can do this. Sometimes the small-molecule hapten can even block immune response to the hapten-carrier adduct by preventing the adduct from binding to the antibody, a process called hapten inhibition.