The immune system defends the body from invasion by viruses, fungi, bacteria and parasites. Comprised of lymph nodes, lymphatic vessels, the thymus gland, tonsils, spleen and other immune tissues spread throughout the body, the immune system provides continuous surveillance and protection. Using complex recognition pathways, the immune system distinguishes between the body tissues and all other substances. Multistep controls regulate immune system activity. Excessive or diminished immune activity can cause serious, potentially life-threatening human disease.
Lymphatic System Transports Immune Cells
The National Cancer Institute explains that the immune system has a network of vessels similar to those of the blood circulation. The lymphatic system transports immune cells in fluid called lymph, which runs through the lymphatic vessels. The lymphatic vessels connect the lymph nodes and other lymphatic tissues of the body. Immune cells can also travel via the bloodstream.
Self-Markers Prevent Autoimmunity
Control of immune system activity depends on accurate recognition of the body's own tissues--self--compared to invading organisms or foreign substances, termed non-self. Major histocompatibility complex proteins on the surface of human cells serve as self-markers for the immune system. These proteins, commonly known as MHC, normally keep the immune system from attacking the body's own tissues. As explained in "Genes and Disease" presented online by the National Center for Biotechnology Information, immune system failure to accurately distinguish self from non-self is the underlying defect with autoimmune diseases, including lupus erythematosus, rheumatoid arthritis, autoimmune hepatitis and type 1 diabetes mellitus.
Nonspecific Immune Cells Provide First-Line Defense
Certain immune cells possess the ability to attack a broad array of bacteria and viruses. These are the nonspecific immune cells, which provide the first line of defense against invading organisms. The National Cancer Institute notes that nonspecific immune cell types include neutrophils, monocytes, macrophages and dendritic cells. These cells are termed phagocytes because they possess the capacity to ingest and kill invading organisms.
Memory Exists in the Immune System
An infection triggers an onslaught of immune cells that participate in defending the body against the invading organisms. After the immune system clears the infection, some of the participating immune cells transform into memory cells, explains the National Institute of Allergy and Infectious Diseases. Memory cells remain inactive unless the body faces another invasion by the same invading organism. A repeat attack causes rapid replication of the memory cells, creating an entire population of immune cells specifically targeted to kill the repeat offender. Many illnesses can only be contracted once because of the immune system's capacity for memory. The development of memory cells against a target organism typically provides long-term or lifelong immunity.
Immune System Provides Cancer Protection
Cancer cells often produce abnormal proteins, which may trigger an aggressive immune response. According to the National Cancer Institute, the reaction of immune cells against cancerous cells provides protection against the development of cancerous tumors. Immune cells capable of destroying cancer cells include macrophages, cytotoxic T cells and natural killer cells.
References
- National Institute of Allergy and Infectious Diseases: What is the Immune System?
- National Institute of Allergy and Infectious Diseases: The Structure of the Immune System
- National Cancer Institute: Understanding Cancer Series, The Immune System, Lymphatic System
- National Cancer Institute: Understanding Cancer Series, The Immune System, Markers of Self--Major Histocompatibility Complex
- NCBI Genes and Disease: Diseases of the Immune System
