Immune System

Technology :: Immune System

The immune system is the body’s natural defense against disease-causing agents such as bacteria, viruses and parasites. It plays an important role in identifying and eliminating abnormal cells, including cancer cells. The first line of defense is an immediate, rapid response (innate immunity) that protects the body during the days or weeks needed for a second, longer-term response (adaptive immunity) to develop.

The immune system responds to foreign substances (antigens) with a cascade of events orchestrated by specialized immune cells called dendritic cells (see diagram). Dendritic cells have two key functions in the initial, innate immune response. First, they produce cytokines that help kill viruses and bacteria. Second, they ensure that pathogens and other foreign substances are highly visible to specialized helper T cells, called Th1 and Th2 cells, which coordinate the longer-term adaptive immune response. Dendritic cells recognize different types of offending substances and guide the immune system to make the most appropriate response. When viruses, bacteria and abnormal cells are encountered, dendritic cells trigger a Th1 response, whereas when a parasite infection is detected, dendritic cells initiate a Th2 response. Th1 and Th2 responses last for an extended time in the form of Th1 and Th2 memory cells, conferring long-term immunity.

The Th1 Response

The Th1 response involves the production of the body’s most potent anti-infective weapons – specific cytokines, including interferon-alpha, interferon-gamma and interleukin 12 (IL-12), as well as killer T cells, a specialized immune cell. In addition, the Th1 response generates IgG antibodies to help rid the body of foreign antigens and allergens. Once a population of Th1 cells specific to a particular antigen or allergen is produced, it persists for a long time in the form of memory Th1 cells, enabling a more rapid and powerful immune response the next time exposure to that particular antigen or allergen occurs. An insufficient Th1 response to an infection can result in chronic disease, whereas an inappropriate Th1 response can cause diseases such as rheumatoid arthritis.

The Th2 Response

Activation of the Th2 response involves the production of other cytokines, IL-4, IL-5 and IL-13, which attract inflammatory cells, such as eosinophils, basophils and mast cells, to destroy the invading organism. The Th2 response also leads to the generation of IgE, a specialized antibody that can recognize antigens and allergens and further enhance the protective response. An inappropriate Th2 immune response to allergens, such as plant pollens, can lead to chronic inflammation resulting in allergic rhinitis, asthma and other allergic diseases. Subsequent exposures to the same allergens can reactivate memory Th2 cells, sustaining inflammation and leading to chronic disease.