The immune system defends the body against foreign invaders. Specialized cells within this network maintain health. Among these are plasma cells, immune cells with unique functions, particularly residing in the bone marrow.
What Are Plasma Cells?
Plasma cells are a specific type of white blood cell that develops from B lymphocytes, also known as B cells. These cells are a main component of the adaptive immune system, responsible for producing proteins called antibodies. When B cells encounter specific foreign substances, called antigens, they undergo a process of activation, proliferation, and differentiation to become plasma cells.
Plasma cells are characterized by their abundant cytoplasm, which is rich in rough endoplasmic reticulum and Golgi apparatus, structures indicating their high capacity for protein synthesis and secretion. This cellular machinery allows them to efficiently produce and release large quantities of antibodies. While some immature plasma cells, called plasmablasts, are short-lived, fully differentiated plasma cells can survive for extended periods.
The Bone Marrow’s Role
The bone marrow provides a unique and supportive environment for plasma cells, often referred to as a “niche,” which is crucial for their long-term survival and function. This specialized microenvironment offers the necessary cellular and molecular factors that promote the longevity of these antibody-producing cells. Stromal cells within the bone marrow are particularly important, as they attract plasma cells by producing chemicals like CXCL12.
Beyond attracting them, the bone marrow niche also provides various soluble factors that contribute to plasma cell survival and proliferation. These include cytokines such as interleukin-6 (IL-6) and ligands for B-cell maturation antigen (BCMA), like A Proliferation-Inducing Ligand (APRIL) and B-cell Activating Factor (BAFF). Eosinophils, basophils, and megakaryocytes supply these factors, ensuring sustained activity.
How Plasma Cells Protect Us
Plasma cells are the body’s primary producers of antibodies, proteins that recognize and neutralize specific antigens. Once released into the bloodstream and lymphatic system, these antibodies circulate throughout the body, providing humoral immunity. This process is specific; each antibody recognizes only one antigen.
Antibodies employ several mechanisms to protect the body. They can directly bind to and inactivate foreign substances like pathogens and toxins through a process called neutralization. For example, antitoxins are antibodies that neutralize poisons by altering their chemical composition. Antibodies also facilitate the destruction of pathogens by marking them for uptake and digestion by other immune cells, a process known as opsonization. Additionally, antibodies can activate the complement system, a cascade of proteins that directly destroy bacterial cells by creating holes in their cell walls.
Disorders Involving Plasma Cells
Malfunctions in plasma cells can lead to conditions, particularly affecting the bone marrow. Multiple myeloma is a cancer of plasma cells, characterized by their uncontrolled growth in the bone marrow. These abnormal plasma cells produce large amounts of a single type of antibody. This overgrowth can crowd out healthy blood-forming cells, leading to anemia, low platelet counts, and increased infection risk.
Monoclonal gammopathy of undetermined significance (MGUS) is a pre-cancerous condition where abnormal plasma cells produce a monoclonal protein, but without the symptoms or organ damage seen in multiple myeloma. While MGUS is not considered cancer, individuals with MGUS may progress to multiple myeloma. The risk of progression is higher in individuals with elevated levels of the monoclonal protein.
Amyloidosis, specifically light chain (AL) amyloidosis, is another disorder involving abnormal plasma cells. In this condition, abnormal plasma cells produce an excess of antibody light chains, which then misfold and clump together to form amyloid fibrils. These amyloid deposits can accumulate in various organs, such as the heart and kidneys, leading to organ enlargement and impaired function. While AL amyloidosis can occur independently, it is sometimes diagnosed alongside multiple myeloma.