Which condition is marked by a significant decrease in white blood cells?

Leucopenia is specifically characterized by a significant decrease in white blood cells (WBCs). This condition can result from various factors, including autoimmune diseases, certain medications, viral infections, and bone marrow disorders, leading to an increased susceptibility to infections due to a compromised immune response.

Other conditions mentioned relate to different aspects of blood cells. For instance, leukemia involves an increase in abnormal white blood cells, which can sometimes crowd out healthy cells, but it does not primarily define a decrease in white blood cells. Thrombocytopenia is associated with a low platelet count, affecting blood clotting rather than white blood cells. Anemia focuses on a deficiency in red blood cells or hemoglobin, impacting oxygen transport in the body, rather than on white blood cell count. Thus, leucopenia clearly defines the condition in which there is a significant decrease in white blood cells.