Lewis/Life - Review of Key Concepts

Review of Key Concepts - Chapter 39

All animals have immune defenses that distinguish self from nonself and make survival possible in an environment with parasitic or infectious organisms. Immunity also fights cancer. Simple animals have amebocytes, and more complex animals have circulating phagocytes that engulf and destroy invaders. An immune response is fast, specific, diverse, and "remembers."
Nonspecific human defenses prevent infectious agents from entering the body. These include barriers such as skin, mucous membranes, secretions, and cilia. If microbes penetrate the barriers, phagocytes engulf and digest them. Inflammation, antimicrobial substances, and fever create an environment hostile to pathogens.
Macrophages quickly phagocytize infectious microbes. The macrophage displays a piece of the microbe's surface on its own surface, with an HLA glycoprotein, which alerts and activates helper T cells, which in turn activate B cells.
Activated B cells carry out the humoral immune response. They mature into plasma cells and secrete antibodies. A first reaction to a particular foreign antigen is a primary immune response; memory cells formed from some of the activated B cells provide a quick secondary immune response when an antigen is encountered again.
An antibody is a Y-shaped protein composed of two heavy chains and two light chains. Each chain has a region with a constant amino acid sequence and a region of variable sequence. The tips of the Y-shaped antibody form an antigen binding site, where a foreign antigen binds. Antibodies bind antigens and form complexes that attract other immune system components. The human body can produce a tremendous variety of antibody molecules because DNA segments shuffle during early B cell development.
T cells carry out the cellular immune response. They are educated in the thymus gland to recognize self. Helper T cells activate other T cells and B cells. A helper T cell's CD4 antigen binds to macrophages presenting antigens. Cytotoxic T cells release biochemicals that bore into bacteria, kill them, and also destroy body cells covered with viruses. T cells secrete cytokines, which control communication within the immune system.
The fetal immune system learns to recognize self but does not mature until about 18 months after birth. The fetus and newborn receive passive immunity via the placenta and breast milk. Active immunity builds during the first 2 years. The immune system begins to lose function in adolescence.
Immune system breakdown may be acquired or inherited. HIV overtakes helper T cells, preventing them from activating B cells. HIV replicates using reverse transcriptase; the T cells burst, releasing many viruses. Immunity is suppressed and opportunistic infections set in. HIV is affecting more diverse population groups, but long-term survivors may provide information that can lead to a cure. People who are resistant to HIV infection have abnormal HIV receptors on CD4 helper T cells. In severe combined immune deficiency (SCID), a person is born lacking T and B cells. Chronic fatigue syndrome reflects an overactive immune system.
In an autoimmune disorder, the body manufactures autoantibodies against its own cells. Autoimmunity may result from a virus that has borrowed a self antigen, from T cells that cannot distinguish self from nonself, or from bacterial or cancer cells that have antigens that resemble a self antigen.
In an allergy, immunity attacks a harmless substance, called an allergen. When a person contacts an allergen, IgE antibodies bind mast cells, which release allergy mediators that cause symptoms. Anaphylactic shock is a systemic, severe allergic reaction.
Several technologies alter immune system function. Vaccines are killed, weakened, or partial pathogens that stimulate antibody production without causing disease. Transplants can sometimes succeed if immune attack against nonself tissue is shut off. Individual activated B cells fused with cancer cells form hybridomas, which secrete monoclonal antibodies. Cytokines and monoclonal antibodies are useful in diagnosing and treating some diseases.