In most cases of multiple myeloma, the excess antibodies found in the blood all share an identical protein sequence (thus they have the same structure). Consider such a case where all of the antibodies are identical and think about the cell(s) of origin. How many different B cell clones gave rise to this immune cancer?

One single B cell clone gave rise to this immune cancer.

Explanation:

The body's cells divide and grow in a completely regulated manner, following an orderly cell cycle that indicates when a cell should die by apoptosis. With this regulation, it is controlled that there are no aging or mutated cells that grow and affect the organism. Cancer is a genetic disease caused by changes in these genes that control how our cells function. These changes, genetic mutations, can occur randomly in the body or can be inherited from parents. When a cell has its functions altered, they begin to grow out of control and in doing so, form masses called tumors that consume the body's resources and energy and crowd out normal cells, preventing them from fulfilling the biological functions necessary for life. Practically, any cell can become a cancer cell and migrate to other areas of the body.

Multiple myeloma is a type of plasma cell cancer. These cells are found in the bone marrow (tissue inside the bones) and are an important component of the immune system, which is made up of several types of cells that work together to fight pathogens and disease. Some cell types are B and T lymphocytes, and myeloid cells such as macrophages, dendritic cells, etc. When B cells respond to an infection with the participation of T cells that present pathogen-derived antigens to them, they mature into plasma cells. Plasma cells produce antibodies that help the body attack and destroy pathogens as well as cancer cells. These antibodies are proteins that bind to antigens that are also proteins found on the surface of the pathogens or cells to be attacked and "mark" them for destruction by other types of immune cells, such as macrophages or neutrophils.

When the plasma cells become cancerous and grow out of control, this is called multiple myeloma, and the mutated plasma cells produce abnormal antibodies. The overgrowth of plasma cells in the bone marrow can crowd out normal blood cell-producing cells, causing low blood cell counts. This can cause anemia which is characterized by a shortage of red blood cells, and thrombocytopenia which means a low level of platelets in the blood causing increased bleeding and bruising. Another condition that can occur is leukopenia (a shortage of normal white blood cells), which can cause problems fighting infections.

Then, the abnormal antibodies produced by the cancerous plasma cells will have the same amino acid sequence and therefore the same structure. Remember that an antibody is a protein, and proteins are a polymer of amino acids. This amino acid sequence is determined by the genes since each gene consists of a sequence of nitrogenous bases and every three bases read by the ribosomal apparatus (the one that synthesizes proteins), an amino acid is synthesized. Therefore, if we consider that cancer originates from a single cell that suffered a mutation in its genes and began to divide in an uncontrolled manner, the daughter cells (which are genetically identical to the mother cell) derived from this mutated cell will have the same genes (in this case, altered because they divide and grow in an uncontrolled and deregulated manner). This tells us that a single mutated B cell gave rise to this cancer.