Concept:Leucocytes are white blood cells, which are distinct from red blood cells and platelets.Explanation:Leucocytes are nucleated cells and generally larger than red blood cells.They are not respiratory pigments; that describes haemoglobin in red blood cells.They are not the most numerous blood cells; red blood cells are more numerous.They are not involved in blood clotting; blood clotting involves platelets.Answer:They are large and nucleated.