Immunity: Overview and Types

Definition and Importance

Immunity refers to the mechanisms by which the body defends itself against disease-causing microbes and foreign substances. It is essential for maintaining health and preventing infections.

• Susceptibility: The lack of immunity to a particular pathogen or substance.

• Immunity: The ability to eliminate or resist disease-causing microbes and protect against environmental agents (e.g., pollen).

Types of Immunity

• Innate (Nonspecific) Immunity: Routine protection present at birth, providing immediate defense against a broad range of pathogens.

• Adaptive (Specific) Immunity: Components that adapt over time and protect against specific pathogens, involving memory and specificity.

Although these systems are studied separately, they complement each other in protecting the host.

Innate Immunity: General Features

Definition and Characteristics

Innate immunity consists of protective mechanisms present at birth that act quickly (within hours) to prevent or eliminate infections. It does not require previous exposure to the pathogen.

• Inflammation: A coordinated set of events that occur when the body detects foreign or tissue damage, characterized by swelling, heat, altered function, redness, and pain.

Lines of Defense in Innate Immunity

• First-Line Defenses: Prevent microbes from entering tissues (e.g., skin, mucous membranes).

• Second-Line Defenses: Cells and mechanisms that eliminate microbes that bypass first-line defenses, including scanning systems (sentinel cells) and effector actions (elimination of invaders).

First-Line Defenses: Barriers to Infection

Overview

The first-line defenses are part of innate immunity and prevent microbes from entering the body. These barriers can be found on external surfaces (e.g., skin) or internal surfaces (e.g., mucous membranes lining the respiratory tract).

• Physical Barriers: Skin and mucous membranes that block entry of pathogens.

• Chemical Barriers: Substances like antimicrobial peptides and acidic pH that inhibit microbial growth.

• Microbiome: The community of normal microbiota that competes with pathogens and provides protection.

Physical Barriers: Skin and Mucous Membranes

• Skin: The largest organ and a major physical barrier, composed of two main layers:

  • Epidermis: Outermost layer made of tightly packed epithelial cells containing keratin, a water-repelling protein. The shedding of dead cells removes microbes from the surface.

  • Dermis: Thicker, deeper layer of connective tissue that provides strength and flexibility.

• Mucous Membranes: Line body cavities open to the environment (e.g., respiratory, digestive, and urogenital tracts). They secrete mucus, which traps microbes and contains antimicrobial substances.

Role of Keratin

• Keratin: A hydrophobic protein in the epidermis that keeps skin dry, inhibiting microbial growth. It also makes the skin tough and resistant to tearing.

Microbiome as a Barrier

• Normal Microbiota: Non-pathogenic microorganisms that colonize body surfaces and outcompete potential pathogens for nutrients and space.

Antigens and Antibodies

Definitions

• Antigen: Any foreign substance that induces an immune response, typically by being recognized by antibodies or immune cells.

• Antibody: A Y-shaped protein produced by B cells that specifically recognizes and binds to antigens.

Practice Questions and Applications

• Skin and mucous membranes are mostly involved in innate immunity, serving as physical and chemical barriers.

• The first line of defense includes physical barriers (skin), chemical barriers (saliva, stomach acid), and the microbiome.

• Keratin is important because it is hydrophobic, keeping the skin dry and less hospitable to microbes, and it is tough, making the skin hard to penetrate.

• Examples of first-line defenses include unbroken skin, digestive enzymes in saliva, destructive acids in gastric juices, and the normal human microbiome. Phagocytosis by immune cells is a second-line defense.

Summary Table: First-Line Defenses

Key Terms

• Innate Immunity: Immediate, nonspecific defense present at birth.

• Adaptive Immunity: Specific, acquired defense that develops after exposure to antigens.

• Antigen: Substance that triggers an immune response.

• Antibody: Protein that specifically binds to an antigen.

• Keratin: Water-repelling protein in the skin's outer layer.

• Microbiome: Community of normal, non-pathogenic microorganisms on body surfaces.

Example

Example: If a person consumes food contaminated with bacteria but does not become ill due to the acidic environment of the stomach, this is an example of innate immunity (chemical barrier).