Acquired Immunity

Acquired immunity is the cumulative development over a lifetime of the body's ability to fight off disease caused by invading agents with antibodies and specialized lymphocytes, one type of white blood cells. Each person's immune system is unique, defined by genetic coding with an ability to distinguish self-cells from foreign invaders, with some exceptions, including autoimmune diseases or immunodeficiency. The immune response can be activated through either active or passive means.

Active immunity is the development of an immune response by exposure to either the disease or by vaccination. Exposure to the invading agent and developing the illness activates the immune system for recovery and stores the information in memory cells. Immunizations stimulate body tissues to form [Page 9]antibodies and develop memory cells to store information without causing the illness.

Passive immunity provides protection after exposure to a pathogen by direct injection of specific antibodies and generalized human-derived immune globulins. Also, for some cancers, specific T-lymphocytes can be grown in a lab from a person's own cells and then reinjected. Mothers can provide passive immunity to newborns and infants; immune globulin (IgG) crosses the placenta to provide some initial immunity to newborns and immune globulin (IgA) passes to the infant in the colostrum of breast milk.

Immunity is also categorized by source, either natural or artificial. Natural sources include exposure to disease or pathogen or conferred immunity from mother to child. Artificial sources include immunization or direct injection of antibodies from a source outside the body.

The body uses two methods of specific defense against invading agents: The humoral defense with the creation of antibodies, and a cell-mediated response where the immune system activates specialized T-lymphocytes, also known as natural killer cells, to destroy the infected cells.

B-lymphocytes (B-cells) make and secrete antibodies with a few acting as memory cells to ensure lasting immunity is produced in the bone marrow and in the fetal liver. Each lymphocyte is programmed to recognize only one specific antigen, yet B-lymphocytes can respond to millions of different antigens, both naturally occurring and artificially created. T-lymphocytes are produced in the thymus and directly attack and destroy infected cells.

When the body is exposed to an invading agent, helper T-cells (also known as CD4 cells) send a signal to the B-cells to initiate the humoral response and begin antibody production. The immune system utilizes B-cells to recognize and identify the invading agent. The B-cells produce and release antibodies through the humoral response, and some B-cells commonly called memory cells record and retain the information specific to the antigen to provide long-lasting protection against each specific invading agent.

The Y-shaped antibodies have receptor binding sites that attach to the antigen or invading agent. Antibodies work in three similar, though different, ways: by binding to several antigens to form a clump to be devoured by phagocytes, by the complement system that brings water into the cells causing the cells to burst, and by opsonization coating the invading cells with antibodies with a constant region providing a receptor site matching a receptor on the phagocyte, allowing the phagocyte to engulf and destroy the invading agent.

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