Body Defenses Against Infection

The presence and multiplication of a disease-causing agent, or pathogen (path'o-jen), causes an infection. Pathogens include simple microorganisms such as bacteria, complex microorganisms such as protozoa, and even spores of multicellular organisms, such as fungi. Viruses are pathogens, but they are not considered organisms because their structure is far simpler than that of a living cell and they must infect a living cell to reproduce. An infection may be present even though an individual feels well. People who are infected with the human immunodeficiency virus (HIV), which causes AIDS, often live for many years in good health before becoming ill.

The human body can prevent entry of pathogens or destroy them if they enter. Some mechanisms are quite general and protect against many types of pathogens, providing nonspecific defense. They function the same way regardless of the invader or the number of times a particular type of organism invades. These defenses include species resistance, mechanical barriers, enzyme action, interferon, fever, inflammation, and phagocytosis. Other protective mechanisms are very precise, targeting certain pathogens with a specific defense, or immunity. These more directed responses are carried out by specialized lymphocytes that recognize foreign molecules (nonself antigens) in the body and take action against them.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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