The Immune System: Overview

Introduction to the Immune System

The immune system is responsible for providing resistance to disease and protecting the body from pathogens. It is composed of two main intrinsic systems: the innate (nonspecific) defense system and the adaptive (specific) defense system.

• Innate (nonspecific) defense system: Provides the first and second lines of defense against pathogens.

• Adaptive (specific) defense system: Provides the third line of defense, targeting specific foreign substances.

Functional Nature of the Immune System

• The immune system is a functional system, not a discrete organ system.

• Innate and adaptive defenses are intertwined and often work together.

• Both systems release and recognize many of the same defensive molecules.

• Innate defenses have specific pathways for certain substances, and can alert the adaptive system to the presence of foreign molecules.

Lines of Defense

First Line of Defense: Surface Barriers

The first line of defense consists of external body membranes, such as the skin and mucosae, which act as physical barriers to most microorganisms.

• Skin: Provides a tough, keratinized barrier resistant to weak acids, bases, bacterial enzymes, and toxins.

• Mucous membranes: Line body cavities open to the exterior and trap pathogens.

Second Line of Defense: Internal Defenses

If pathogens breach surface barriers, the second line of defense is activated. This includes antimicrobial proteins, phagocytes, and other cells that inhibit the spread of invaders. Inflammation is a key mechanism in this line of defense.

• Antimicrobial proteins: Attack microorganisms or hinder their ability to reproduce.

• Phagocytes: Engulf and digest pathogens.

• Inflammation: Localizes infection and recruits immune cells.

Third Line of Defense: Adaptive Immunity

The adaptive (specific) defense system targets particular foreign substances and provides a more powerful and specific response. However, it takes longer to react than the innate system.

• Specificity: Recognizes and targets specific antigens.

• Memory: Mounts a stronger attack upon subsequent exposures to the same antigen.

Key Learning Objectives

• Describe mechanisms of innate defensive strategies, including the roles of skin and mucous membranes, fever, antimicrobial proteins, and inflammation.

• Describe antigens and antibodies, focusing on specificity and memory.

• Explain B lymphocytes, their function, and their role in humoral immunity.

• Discuss immunological memory, including the cells and molecules involved.

• Explain T lymphocytes, their function, and their role in cellular immunity, including major classes and their relationship to MHC (major histocompatibility complex).

Summary Table: Innate vs. Adaptive Immunity

Example:

When a pathogen enters the body, the skin and mucous membranes act as the first barrier. If the pathogen breaches these barriers, phagocytes and inflammation respond rapidly. If the pathogen persists, the adaptive immune system is activated, producing a targeted response and memory for future protection.