At approximately day 1

Tonsils and spleen begin to develop.

Duration: 4 weeks

Reference [PubMed Link]

West LJ. 2002 Sep-Oct. Defining critical windows in the development of the human immune system. Hum Exp Toxicol 21(9-10):499-505.

At approximately day 1

Thymus divides into cortex and medulla.

Duration: 2 weeks

Reference [PubMed Link]

West LJ. 2002 Sep-Oct. Defining critical windows in the development of the human immune system. Hum Exp Toxicol 21(9-10):499-505.

At approximately day 1

Mature T cells (thymus) and B cells (bone marrow) migrate to secondary lymphoid tissue, the lymph nodes, mucous-membrane-associated lymphoid tissue and spleen.

Duration: 2 weeks

Reference

Williams CB. 2003. Development of the immune system. In Rudolph’s Pediatrics 21st Edition, Rudolph CD & Rudolph AM, Editors. New York: McGraw-Hill.