Immunopathology is the branch of pathology dealing with the immune system and its response to disease and infection. The immune system is a complex defense system that functions to protect the body from disease. A properly functioning immune system is finely tuned to rapidly respond to invading infectious agents while ignoring benign agents and substances. The immune system relies heavily on the lymphatic system, which is made up of organs and cells that store and circulate fluids involved in immune response. The primary function of the lymphatic system is the collection and transport of lymph, a colorless fluid that contains white blood cells, which help fight infection. Due to the complexities of the immune and lymphatic systems, errors can occur, leading the immune system to overreact or underreact to various internal or external agents.The immune system consists of three lines of defense that are categorized into innate immune defenses and adaptive immune defenses. The innate immune system involves aspects of general defense that are not specifically targeted against any particular pathogen.
The Immune System's Lines of Defenses
Lymphatic and Immune Structures
Secondary immunodeficiency, also called acquired immunodeficiency, is the failure of the immune system to adequately protect the body from infections and other health problems. Secondary immunodeficiency can result from various extrinsic factors, including poor nutrition and pathogens, or it can be drug-induced by immunosuppressive therapies. Protein-energy malnutrition, or malnutrition as a result of a diet with little to no protein, leads to atrophy of the primary lymphoid organs (bone marrow and thymus, producers of B and T cells, respectively). Pathogens, such as human immunodeficiency virus (HIV), specifically attack T cells in the host immune system. This may lead to acquired immune deficiency syndrome (AIDS). People taking medications such as azathioprine to treat autoimmune diseases, such as Crohn's disease or ulcerative colitis, may suffer a suppression in the production of their white blood cells.
Immune dysfunction can also result in overreactivity. An immune system that is overly active, or hypersensitive, responds to benign threats in an exaggerated manner. In autoimmune disorders the hypersensitive immune system perceives the body's own tissues as a threat. Examples of autoimmune disorders include rheumatoid arthritis, type I diabetes mellitus, multiple sclerosis, and inflammatory bowel disease (Crohn's disease or ulcerative colitis). The immune system can also overreact to external agents in the environment, such as pet dander, plant oils (such as poison ivy and poison sumac), and pollen. A hypersensitive reaction, regardless of the inciting agent, can result in damage to a host's own tissues. It is often unknown why one individual develops hypersensitivity and another does not. People may have a genetic predisposition to developing allergies. Individuals with family members that have allergies are more likely to develop them. Some people also have atopy, a genetic predisposition to developing type I hypersensitivity.