Box 1 | Haematopoietic cell classifications

Three main classifications of blood cells derive from haematopoietic stem cells (HSCs). In all lineages, extensive proliferation occurs during differentiation.

• Myeloid: This lineage includes macrophages (monocytes) and granulocytes or polymorphonuclear cells (neutrophils, basophils and eosinophils). Macrophages are regarded as the most primitive phagocytes; however, they also have a role in adaptive immunity, cooperating with T and B cells through antigen presentation and the production of cytokines. Macrophages and dendritic cells are intimately related with respect to their origin. Neutrophils are more professional phagocytes.
• Erythroid-megakaryocyte: Erythrocytes (red blood cells) carry oxygen through blood vessels, whereas platelets derived from megakaryocytes work to maintain the vascular system.
• Lymphoid: T-cell and B-cell lineages are usually combined and called the lymphoid lineage. However, in this article we show that T and B cells are more distant relatives than generally believed; their functions and differentiation processes are distinct. Two classes of T cell exist: T cells and T cells. Natural killer (NK) cells are thought to be the prototype of T cells. Thymic, as well as pre-thymic, T-cell progenitors are able to generate dendritic cells, thereby showing the close evolutionary relationship between T cells and myeloid lineage cells. B cells secrete antibodies and are able to function as antigen-presenting cells. B cells can be converted into macrophage-like cells.