Normal Bone Marrow, Blood, and Lymphoid Tissue

Different types of leukemia start in different types of blood cells. It helps to understand some basics about blood cells.

Bone marrow

Blood cells are made in your bone marrow.

Bone marrow is the soft inner part of some bones, like the skull, shoulder blades, ribs, pelvis, and backbones. Bone marrow is made up of:

• A small number of blood stem cells
• More mature blood-forming cells
• Fat cells
• Supporting tissues that help cells grow

Inside your bone marrow, blood stem cells divide and mature to make new blood cells. During this process, the cells become either lymphocytes (a kind of white blood cell) or other blood-forming cells.

These other blood-forming cells mature into red blood cells, white blood cells (other than lymphocytes), or platelets.

Types of blood cells

The different types of blood cells each have different jobs in the body.

Red blood cells

Red blood cells carry oxygen from your lungs to all other tissues in your body and take carbon dioxide back to your lungs to be removed.

Having too few red blood cells (anemia) can make you feel tired, weak, and short of breath because your body tissues aren't getting enough oxygen.

Platelets

Platelets are small pieces of cells that break off of a type of large bone marrow cell called the megakaryocyte. Platelets are important in plugging up holes in blood vessels caused by cuts or bruises.

Having too few platelets (thrombocytopenia) may cause you to bleed or bruise easily.

White blood cells

White blood cells help your body fight infections.

Having too few white blood cells lowers your immune system and can make you more likely to get an infection.

Types of white blood cells

There are several types of white blood cells.

Lymphocytes

Lymphocytes are mature, infection-fighting cells that develop from lymphoblasts, a type of blood stem cell in your bone marrow. Lymphocytes are the main cells that make up lymphoid tissue, a major part of the immune system.

Chronic lymphocytic leukemia (CLL) most often starts from B lymphocytes (see below).

Lymphoid tissue is found in your lymph nodes, thymus, spleen, tonsils, and adenoids. It's also scattered throughout your bone marrow and your digestive and respiratory systems.

There are 2 main types of lymphocytes:

• B lymphocytes (B cells) protect your body from invading germs (bacteria, viruses, and fungi) by maturing into plasma cells, which make proteins called antibodies. The antibodies attach to the germs. This helps other white blood cells recognize and destroy them. B lymphocytes are the cells from which (CLL) most often starts.
• T lymphocytes (T cells) can recognize cells infected by viruses and directly destroy these cells. They also help regulate your immune system.

Granulocytes

Granulocytes are mature, infection-fighting cells that develop from myeloblasts, a type of blood-forming cell in the bone marrow.

Granulocytes have granules that look like tiny spots under a microscope. These granules contain enzymes and other substances that can destroy germs, such as bacteria.

There are 3 types of granulocytes:

• Neutrophils
• Basophils
• Eosinophils

These are distinguished under a microscope by the size and color of their granules.

Monocytes

Monocytes are related to granulocytes. They develop from blood-forming monoblasts in your bone marrow. After circulating in your bloodstream for about a day, monocytes enter your body tissues to become macrophages.

Macrophages can destroy some germs by surrounding and digesting them. They also help lymphocytes recognize germs and start making antibodies to fight them.