A small amount of bone marrow is removed during a bone marrow aspiration. The procedure is uncomfortable, but can be tolerated by both children and adults. The marrow can be studied to determine the cause of anemia, the presence of leukemia or other malignancy, or the presence of some "storage diseases" in which abnormal metabolic products are stored in certain bone marrow cells.

This picture shows the darkly-stained lymph cells (lymphoblasts) seen in acute lymphocytic leukemia (ALL), the most common type of childhood leukemia.

Note multiple Auer rods which are found only in acute myeloid leukemias, either myeloblastic or monoblastic. These rods consist of clumps of azurophilic granule material.

This is a microscopic view of bone marrow from a person with chronic lymphocytic leukemia; it shows predominantly small, mature lymphocytes.

This high-power microscopic view of a blood smear from a person with classical CML shows predominantly normal-appearing cells with intermediate maturity.

Oil immersion field demonstrating myeloid cells of all degrees of maturity.

Low power view showing marked hypercellularity with a broad-spectrum of myeloid and erythroid cell types and marked myeloid hyperplasia.

Cryoglobulinemia is caused by an abnormal protein that is occasionally found in the blood of people with multiple myeloma, leukemia, and certain forms of pneumonia. It causes blood to gel at low temperatures. In this picture, cryoglobulinemia has reduced blood flow in the fingers so much the fingers have turned dark; the black areas are gangrene resulting from lack of blood flow.

Antigens are large molecules (usually proteins) on the surface of cells, viruses, fungi, bacteria, and some non-living substances such as toxins, chemicals, drugs, and foreign particles. The immune system recognizes antigens and produces antibodies that destroy substances containing antigens.