胸腺是动物的初级淋巴器官。人体内的其中一种腺体，部分骨髓免疫细胞（例如T细胞）在此成熟。

胸腺在人体内的胸骨上端，左右两肺叶之间，甲状腺下方，由两叶不对称的淡红色或略带黄色的薄片样组织构成。以前人们把胸腺和阑尾（盲肠下面的管子）一样看待，认为是一个演化过程中的痕迹器官。随着近半世纪以来免疫学的进展，才认识到胸腺在人体免疫功能中的重要作用，把它誉为「免疫大王」。

人胸腺的大小和结构随年龄的不同而有明显差异。胸腺出现于胚胎第9周，在胚胎第20周发育成熟，已具有正常胸腺的结构，是发生最早的免疫器官。新生期胸腺约重15~20g，至青春期可达30~40g。青春期以后，胸腺随年龄增长而逐渐萎缩，表现为胸腺细胞减少，间质细胞增多，并含有大量脂肪细胞。老年期胸腺萎缩，多被脂肪组织取代，功能衰退，造成细胞免疫力下降，机体容易发生感染和肿瘤。

The thymus is a specialized primary lymphoid organ of the immune system. Within the thymus, T cells or T lymphocytes mature. T cells are critical to the adaptive immune system, where the body adapts specifically to foreign invaders. The thymus is composed of two identical lobes and is located anatomically in the anterior superior mediastinum, in front of the heart and behind the sternum. Histologically, each lobe of the thymus can be divided into a central medulla and a peripheral cortex which is surrounded by an outer capsule. The cortex and medulla play different roles in the development of T-cells. Cells in the thymus can be divided into thymic stromal cells and cells of hematopoietic origin (derived from bone marrow resident hematopoietic stem cells). Developing T-cells are referred to as thymocytes and are of hematopoietic origin. Stromal cells include epithelial cells of the thymic cortex and medulla, and dendritic cells.