Chapter 21: The Immune System – Innate and Adaptive Body Defenses

Immune System Basics

The immune system is a complex network of cells, tissues, and organs that work together to defend the body against harmful invaders such as pathogens (bacteria, viruses, fungi, and parasites). It is divided into two main branches: innate (nonspecific) defenses and adaptive (specific) defenses.

Innate Defenses

• Definition: Innate defenses are the body's first and second lines of defense, providing rapid, nonspecific protection against pathogens.

Surface Barriers

• Skin: Acts as a physical barrier, preventing entry of pathogens.

• Mucous membranes: Line body cavities open to the exterior (e.g., respiratory, digestive tracts) and trap or destroy invaders with mucus, enzymes, and acidic secretions.

• Example: Stomach acid destroys swallowed microbes; tears and saliva contain lysozyme, an enzyme that breaks down bacterial cell walls.

Internal Defenses

• Phagocytes: White blood cells (e.g., neutrophils, macrophages) that engulf and digest pathogens and debris.

• Natural Killer (NK) Cells: Lymphocytes that recognize and destroy infected or cancerous cells by inducing apoptosis (programmed cell death).

• Inflammation Response: A localized response to injury or infection characterized by redness, heat, swelling, and pain. Inflammation helps isolate the affected area, recruits immune cells, and promotes healing.

• Purpose of Interferon: Interferons are proteins released by virus-infected cells that help protect neighboring cells by inhibiting viral replication.

• Role of Fever: Fever is a systemic response to infection. Elevated body temperature inhibits pathogen growth and enhances immune cell activity.

Adaptive Immunity

• Definition: Adaptive immunity is a specific defense system that recognizes and targets particular pathogens. It has memory, allowing for a faster and stronger response upon subsequent exposures.

Antigens

• Definition: Antigens are substances (usually proteins or polysaccharides) that are recognized as foreign by the immune system and trigger an immune response.

Role of B and T Lymphocytes

• B Lymphocytes (B cells): Responsible for humoral immunity; they produce antibodies that bind to antigens in body fluids.

• T Lymphocytes (T cells): Responsible for cellular immunity; they directly attack infected or abnormal cells or help regulate other immune cells.

Antigen Presenting Cells (APCs)

• Definition: APCs are cells that process and present antigens to T cells, initiating the adaptive immune response.

• Dendritic Cells: Specialized APCs found in tissues; they capture antigens and migrate to lymph nodes to activate T cells.

• Macrophages: Engulf pathogens and present their antigens to T cells; also act as phagocytes in innate immunity.

• B-Lymphocytes: Can act as APCs by presenting antigens to helper T cells to receive activation signals.

Basics of Humoral Immunity

• Definition: Humoral immunity involves B cells and the production of antibodies that circulate in body fluids ("humors").

• Mechanism: Antibodies bind to antigens, neutralizing them or marking them for destruction by other immune cells.

• Example: Vaccines stimulate humoral immunity by exposing the body to harmless forms of antigens.

Basics of Cellular Immunity

• Definition: Cellular immunity involves T cells that directly attack infected or abnormal cells.

• Types of T cells: Helper T cells (activate other immune cells), cytotoxic T cells (destroy infected cells), regulatory T cells (suppress immune responses).

• Example: Cytotoxic T cells destroy virus-infected cells by recognizing antigen-MHC complexes on their surfaces.

Homeostatic Imbalances of the Immune System

• Immunodeficiency: A condition in which the immune system's ability to fight infections is compromised or absent. Example: HIV/AIDS destroys helper T cells, leading to immunodeficiency.

• Autoimmunity: The immune system mistakenly attacks the body's own tissues. Example: Rheumatoid arthritis, type 1 diabetes.

• Allergic Reactions: Hypersensitive immune responses to harmless substances (allergens), such as pollen or food proteins. Example: Hay fever, peanut allergy.

Additional info: The immune system's ability to distinguish self from non-self is critical for preventing autoimmunity. Both innate and adaptive defenses work together to provide comprehensive protection against disease.