Since thymus is the organ for the development of T cells, failure of thymus development or its abnormal development results in the T cell production.
The precursor T lymphocytes fail to populate in the thymus for further differentiation and multiplication to become immune competent T cells and the thymus does not become a lymphoid organ.
Patients with no T cells or poor T cell function are susceptible to opportunistic infections. Since B cell function is dependent on T cells to certain extent, deficiency in T cells is associated with humoral immunity also. The following types of immune deficiencies are associated with T cells.