Model for competition for space between myelopoiesis and lymphopoiesis during emergency myelopoiesis. In steady state, early myelopoiesis and lymphopoiesis occupy the same niche space in BM, resulting in a stable myeloid to lymphoid ratio on the basis of the intrinsic fitness of the respective lineage to compete for growth and retention signals (first panel). Systemic infectious signals cause a reduced expression of lymphoid-supportive growth and retention signals and lead to lymphocyte mobilization from BM and recruitment to secondary lymphoid organs (second panel), thus creating vacant niche space (third panel). Consequently, BM myelopoietic precursors gain access to enhanced retention and growth stimuli, resulting in expansion of myelopoiesis and increased myeloid cell output (fourth panel).