**3.1 Thymic failure during childhood**

Thymus is the major site of TL maturation and, therefore, essential for the adaptive immune system formation. Few clinical situations involve lack of thymus, but all they lead to serious consequences.

Thymic function *gold standard*

Accurate indirect approximation

Peripheral blood

Peripheral blood

Short proliferative

Peripheral blood

Peripheral blood

Peripheral blood

Immature naive TL

sample

sample

history

sample

sample Intrathymic generation

samples Intrathymic generation Not affected by peripheral proliferation

Table 1. Advantages and disadvantages of the different thymic function quantification

Thymus is the major site of TL maturation and, therefore, essential for the adaptive immune system formation. Few clinical situations involve lack of thymus, but all they lead to serious

Needs thymic biopsy

Irradiation (CT) Specifically trained

Cannot discriminate long-lived naive TL Controversy regarding markers

Quantifies a recent thymic emigrant (RTE)-enriched subset rather than

Quantifies a recent thymic emigrant (RTE)-enriched subset rather than

Deeply affected by

Technically difficult

RTEs

RTEs

peripheral proliferation

radiologist Coarser quantification

Quantification method Type Advantages Disadvantages

Thymic *function*related marker

related marker

related marker

Thymic *output*related marker

Thymic *output*related marker

related marker

related marker

Double positive (DP) thymocytes *ex-vivo* quantification

CD31-expressing naive

T Lymphocytes

PTK7-expressing T lymphocytes

methods

consequences.

Thymic volume Thymic *function*-

Naive T lymphocytes Thymic *output*-

sjTREC quantification Thymic *output*-

sj/β-TREC ratio Thymic *function*-

**3.1 Thymic failure during childhood** 
