The body is constant in contact with bacteria, fungi and viruses. The body has two defense systems for foreign materials that form the immune system.
- Innate (nonspecific) Defense System
- Adaptive (specific) Defense System
Innate (nonspecific) Defense System
Structures and tissues protect against a variety of invaders. It responds immediately to protect the body from foreign materials. Included in Innate Body Defenses are body surface covering like intact skin and mucous membranes, specialized human cells and chemical produced by the body.
Surface membrane barriers provide the first line of defense against the invasion of microorganisms.
Secretions on these surfaces also offer protection:
- acidic pH of the skin inhibits bacterial growth
- acidic pH of vaginal secretions
- secretions from mucous lining the stomach kill pathogens
- saliva and lacrimal fluid contain lysozyme, an enzyme that destroys bacteria
- mucus traps microorganisms in digestive and respiratory pathways
A second line of defense is provided by cells and other chemical processes.
- natural killer cells
- inflammatory response
- antimicrobial proteins
Adaptive (specific) Defense System
Specific defense is required for each type of invader.
Adaptive body defenses are the body's specific defense system, also known as the third line of defense.
Adaptive immune response responds to a specific threat which has gotten past the first and second lines of defense. There are three aspects of the adaptive defense:
- antigen specific - recognizes and acts against particular foreign substances
- systemic - not restricted to the initial infection site
- memory - recognizes and mounts a stronger attack on previously encountered pathogens
Abnormally high body temperature is a systemic response to invasion by microorganisms. The hypothalamus (which regulates body temperature) can be reset to a higher temperature by pyrogens, secreted by white blood cells. High temperatures inhibit the release of iron and zinc (needed by bacteria) from the liver and spleen, which inhibits their growth. Fever also increases the speed of repair processes.
Developmental Aspects of the Lymphatic System
and Body Defenses
In the developing fetus, lymphatic vessels are formed by budding off from veins. The thymus and the spleen (where blood cell production takes place in the fetus) are the first lymphoid organs to appear in the embryo. Other lymphoid organs remain relatively undeveloped until after birth. The immune response develops around the time of birth.
The ability of immunocompetent cells to recognize foreign antigens is genetically determined. Stress appears to interfere with normal immune response. Efficiency of immune response wanes in old age, and infections, cancer, immunodeficiencies, and autoimmune diseases become more prevalent.