What is a plasma cell? Plasma cells are a mature type of B lymphocyte that usually make up less than 5% of the cells in the bone marrow. The bone marrow is a spongy material found primarily in the center of long bones. Bone marrow is comprised of a variety of cells, which gradually mature to form: Red blood cells (also known as erythrocytes), which carry inhaled oxygen from the lungs to other organs and carry carbon dioxide from the organs to the lungs to be exhaled. Platelets (also known as thrombocytes), which form clots. White blood cells (also known as leukocytes), which are comprised of granulocytes, lymphocytes, and monocytes, each of which has a different role in the immune system. Lymphocytes are made up of B and T lymphocytes. Plasma cells are a mature form of B-lymphocytes, which produce antibodies. There are a large number of different pathogens, such as bacteria or viruses, ... read more on Multiple Myeloma: The Basics