The Immune System

Overview

The immune system is a complex network of cells, tissues, and organs that protects the body from infection, illness, and disease. It operates through two complementary mechanisms: innate (nonspecific) immunity and adaptive (specific) immunity. These mechanisms work independently and together to provide a coordinated immune response.

• Innate Immunity: Present at birth, provides nonspecific resistance to a wide range of pathogens.

• Adaptive Immunity: Develops after exposure to specific antigens, providing targeted protection.

Innate (Nonspecific) Immunity

Key Features

Innate immunity is the body's first line of defense and does not distinguish between different types of threats. Its response is the same regardless of the invading agent.

• Does not distinguish one type of threat from another

• Response is the same regardless of type of invader

• Present at birth

• Prevents approach, denies entry, and limits spread of microbes or environmental hazards

Components of Innate Immunity

• Physical barriers (skin and mucous membranes)

• Phagocytes (cells that engulf pathogens and debris)

• Immune surveillance (destruction of abnormal cells by natural killer (NK) cells)

• Interferons (chemicals against viral infections)

• Complement (circulating proteins that assist antibodies)

• Inflammation (localized tissue-level response to infection)

• Fever (elevation of body temperature)

Physical Barriers

The Integumentary System

Physical barriers are the body's first defense against pathogens, primarily provided by the skin and mucous membranes.

• Secretions:

  • Sebaceous and sweat glands produce secretions that wash away microorganisms and chemical agents.

  • Secretions may contain bactericidal chemicals, destructive enzymes (such as lysozymes), and antibodies.

• Hair:

  • Protects from physical abrasion.

  • Prevents hazardous materials or insects from contacting the skin.

• Stratified Squamous Epithelium:

  • Multiple layers of epithelial cells with keratin, connected by desmosomes, provide a tough, protective barrier.

Other Epithelial Linings

Additional physical barriers are found along the digestive, respiratory, urinary, and reproductive tracts.

• Cells provide a physical barrier to pathogens.

• Secretions such as mucus, enzymes, and stomach acid ensnare, destroy, or wash away pathogenic material.

• MALT (Mucosa-associated lymphoid tissue) provides nonspecific defense in mucous membranes.

Adaptive (Specific) Immunity

Key Features

Adaptive immunity is the body's second line of defense, providing targeted protection against specific threats. It depends on the activities of specific lymphocytes and produces a state of protection known as specific resistance.

• Utilizes adaptive defenses

• Protects against particular threats

• Depends on specific lymphocytes

• Produces specific resistance

Summary Table: Components of Immunity

Key Terms and Definitions

• Innate Immunity: Nonspecific defense mechanisms present at birth.

• Adaptive Immunity: Specific defense mechanisms acquired after exposure to antigens.

• Phagocytes: Cells that engulf and destroy pathogens and debris.

• Natural Killer (NK) Cells: Lymphocytes that destroy abnormal cells through immune surveillance.

• Interferons: Proteins that interfere with viral replication and activate immune cells.

• Complement System: Group of proteins that enhance the ability of antibodies and phagocytes to clear pathogens.

• Inflammation: Localized tissue response to injury or infection.

• Fever: Elevation of body temperature as a defense mechanism.

Example: Physical Barriers in Action

When a pathogen lands on the skin, sweat and sebaceous gland secretions can wash it away or destroy it with enzymes and chemicals. If the pathogen breaches the skin, phagocytes are activated to engulf and destroy it.

Additional info:

• Adaptive immunity includes both humoral (antibody-mediated) and cellular (T cell-mediated) responses.

• Physical barriers are supported by chemical defenses such as antimicrobial peptides and acidic pH in certain body regions.