Understanding the Development of the Thymus Gland Across Species Through a Psychoneuroimmunological Perspective: A Modern Attempt to Approach and Fill the Gaps in the Literature

In the last few decades, there has been much research advancement in the understanding of the functions of the immune system and the central nervous system (CNS) (i.e., neuroimmunity) that has led to a new subfield that integrates cognitive and behavioral systems known as psychoneuroimmunology. However, as more work has been dedicated to the microglia as the resident immune cells within the Central Nervous System (CNS), less attention has been directed towards the more peripheral thymusThymus gland where mature immune cells are produced to serve the body and brain. This chapter examines psychoneuroimmunology through the lens of the thymus, from development, cross-species activity, and cross-reactivity with the brain to exert behavioral control. This review also draws upon prior and recent work highlighting the relationship between the thymusThymus gland with the sympathetic, parasympathetic, and CNS through thymus hormone and acetylcholineAcetylcholine, noradrenalineNoradrenaline (NA), catecholamines, and dopamineDopamine neurotransmitterNeurotransmitter signaling. The findings suggest that the dopaminergic pathways in the thymusThymus gland serve a unique interface that requires more study to fully comprehend its reciprocal role in the feedback loops between the brain and thymus gland as it relates to psychoneuroimmunology.