Chapter Overview

The body has a complex overlapping series of defenses that protect it against invasion by harmful microbes and other foreign matter.
Defenses exist at several levels of development and specificity.
First line defenses are inborn physical barriers such as skin, and second line defenses are non-specific, protective reactions in the fluid compartments such as phagocytosis.
Third line defenses are aimed at a specific pathogen and give a long-term form of protection that will come into play if that pathogen is ever encountered again.
The immune system relies on a vast network of cells that circulate through the tissues to search for, detect, and destroy foreign invaders such as bacteria and viruses.
The systems of the body that are most involved in immune function include the blood, lymphoid organs and tissues, and the reticuloendothelial system. These systems freely communicate among one another.
White blood cells, or leukocytes, are formed in the red bone marrow and released into circulation. They migrate out of the blood into the tissues to carry out some of their complex functions relating to phagocytosis, inflammation, antibody production, and pathogen killing.
The lymphoid organs such as the spleen and lymph nodes receive and transport tissue fluids and white blood cells, and are important sites of surveillance and immune reactions.
Inflammation is a sequential protective response to injury that stimulates the influx of beneficial cells and chemicals that can protect against infection and further damage at that site.
Several types of chemical substances, called mediators, are released by cells during inflammation and other immune responses. These include chemicals that change the size of blood vessels, stimulate the migration of white blood cells, and initiate fever.
Interferon is a nonspecific immune mediator that inhibits the replication of viruses and regulates a variety of immune responses.
The complement system is an organized chain reaction of chemicals acting sequentially to lyse cells and viruses.
Phagocytes are specialized cells that function in engulfment and clearance of foreign molecules, cells, viruses, and particles. They contain numerous enzymes and toxic chemicals to carry out this function.
Acquired specific immunities provided by B and T lymphocytes provide specific protection against infection and are essential to survival.

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