Haematopoiesis and the Spleen Quiz

See Section. Option D is correct. T lymphocytes mature in the thymus, an organ in the upper chest behind the sternum. Their precursors arise in marrow and travel there to develop functional T-cell receptors. Thymic selection retains cells able to recognise presented antigen while removing many cells that react too strongly to the body's own tissues. This helps establish self-tolerance, the ability to avoid attacking self. Mature T cells provide cell-mediated immunity: cytotoxic T cells kill infected cells, while helper T cells coordinate responses through cytokines, chemical signals between immune cells.

Antibody secretion (A) belongs to plasma cells derived from B lymphocytes, whose early development occurs in marrow. Anucleate clotting fragments (E) are platelets, whereas T cells have nuclei. Histamine-releasing granulocytes (C) are basophils; T cells are lymphocytes and do not have that defining role. B reverses the usual antigen-presentation relationship: dendritic cells, macrophages and B cells present antigen to T cells, although helper T cells can subsequently help activate B cells.