The Healthy Immune System
The Healthy Immune System
The immune system protects the body by recognizing antigens (bacteria, viruses, fungi, and parasites) on invading bacteria and viruses and reacting to them. An antigen is any substance that induces a state of sensitivity and immune responsiveness. These antigens interact with antibodies and immune cells, initiating an immune response. Types of antigens include. When the immune system is weakened or destroyed by a virus such as HIV, the body is left vulnerable to infections. Despite the heavy artillery that the immune system has against foreign predators, HIV defeats it over time.
The immune system (Figure 9.1) consists of lymphoid organs and tissues, including the bone marrow, thymus gland, lymph nodes, spleen, tonsils, adenoids, appendix, blood, and lymphatic vessels. All components of the immune system are vital in the production and development of lymphocytes, or white blood cells. B lymphocytes (or B cells) and T lymphocytes (or T cells) are produced from stem cells in the bone marrow. B cells stay in the bone marrow to complete the maturation process, but T lymphocytes travel to the thymus gland to complete their maturation. There T lymphocytes become immunocompetent, multiply, and become more differentiated.
B Lymphocytes
Each B cell can recognize specific antigen targets and can secrete specific antibodies. Antibodies function by coating antigens, which makes the antigens more vulnerable to phagocytosis (engulfing and ingestion of invading organisms by leukocytes and/or macrophages), or by triggering the complement system, leading to an inflammatory response. Antibodies are highly specialized serum protein molecules.
T Lymphocytes
T lymphocytes have two major functions: regulation of the immune system and killing of cells that bear specific target antigens. Each T cell has a surface marker, such as CD4+, CD8+, and CD3+, that distinguishes it from other cells. CD4+ cells are helper cells that activate B cells, killer cells, and macrophages when a specific target antigen is present. There are two main types of CD8+ cells. The first type, cytotoxic CD8+ cells, kills cells infected by viruses or bacteria, as well as cancer cells. The second type of CD8+cells, T-suppressor cells, inhibits or suppresses immune responses. Normal CD8+ cell count is between 300 and 1,000 cells in adults and children. The normal CD4+:CD8+ ratio is between 1.0 and 2.0.
T cells can secrete cytokines (chemicals that kill cells), such as interferon (are proteins made and released by host cells in response to the presence of pathogens such as viruses, bacteria, parasites or tumor cells). Cytokines can bind to target cells and activate the inflammatory process. They also promote cell growth, activate phagocytes, and destroy target cells.
Phagocytes
A cell, such as a white blood cell (Table 9.1), that engulfs and absorbs waste material, harmful microorganisms, or other foreign bodies in the bloodstream and tissues. Found throughout the body, initiate the immune response by presenting antigens to lymphocytes, are important in immune response regulation and inflammation, and carry receptors for cytokines. Dendritic cells, another type of phagocyte, also are antigen-presenting cells. They have long, threadlike extensions that help trap lymphocytes and antigens and are found in the spleen and lymph nodes. Neutrophils are granulocytic phagocytes that are important in the inflammatory response.
Complement
The complement system consists of 25 proteins. Complement can induce an inflammatory response when it functions with antibodies to facilitate phagocytosis or weaken the bacterial cell membrane. The complement proteins interact with one another in a sequential activation cascade, promoting the inflammatory process.