Ontogeny of cellular immunity in man

Area of Science:

• Immunology
• Developmental Biology
• Fetal Development

Background:

• Understanding the ontogeny of fetal immune responses is crucial for assessing immune competence.
• The sequential development of phytohemagglutinin (PHA) and mixed lymphocyte reaction (MLR) responsiveness in human fetuses remains incompletely defined.

Purpose of the Study:

• To precisely determine the developmental sequence of phytohemagglutinin (PHA) and allogenic cell responsiveness (MLR) acquisition in the human fetus.
• To investigate the timeline of immune cell reactivity in various fetal lymphoid tissues.

Main Methods:

• Quantitative assessment of immune cell reactivity using 3H-thymidine incorporation.
• Analysis of lymphoid tissues from human fetuses at various developmental stages (from 5 weeks fetal age).

Main Results:

• Mixed lymphocyte reaction (MLR) responsiveness was the earliest detectable immune response, first observed in fetal liver lymphoid cells at 5 weeks.
• Phytohemagglutinin (PHA) reactivity emerged later, appearing in thymocytes at 10.5 weeks and in spleen and blood at 14 weeks.
• MLR reactivity was detected in the thymus at 12.5 weeks and in spleen and blood at 14.5 weeks; fetal marrow lymphocytes showed no reactivity up to 18 weeks.

Conclusions:

• The fetal thymus demonstrates immunocompetence by approximately 10.5 weeks of gestation.
• Early MLR responsiveness appears to be regulated by non-thymic (hepatic) factors during fetal development.