Immune System Defenses

Differentiation Between Innate and Adaptive Defenses

The immune system protects the body from pathogens through two main types of defenses: innate (nonspecific) and adaptive (specific) immunity. Innate defenses provide immediate, general protection, while adaptive defenses are slower but highly specific to particular pathogens.

• Innate Defenses: Nonspecific; include surface barriers and internal defenses.

• Adaptive Defenses: Specific; involve lymphocytes and the production of antibodies.

Innate Defenses

Physical and Chemical Barriers

These barriers form the first line of defense, preventing pathogen entry.

• Physical Barriers: Skin (keratinized, resistant to weak acids and bases, bacterial enzymes, toxins), mucous membranes (trap and expel microbes).

• Chemical Secretions: Acidic pH of skin and secretions (sweat, stomach acid) inhibits microbial growth.

• Enzymes: Lysozyme in saliva, respiratory mucus, and tears destroys bacteria.

• Mucin: Thick mucus traps pathogens.

• Defensins: Antimicrobial peptides inhibit microbial growth.

• Other Modifications: Cilia in respiratory tract sweep dust and bacteria-laden mucus away.

Internal Defenses

When pathogens bypass surface barriers, internal defenses are activated.

• Cells: Natural killer (NK) cells, phagocytes (neutrophils, macrophages), and other leukocytes.

• Processes: Inflammation, antimicrobial proteins, fever.

Cellular Components of Innate Immunity

Natural Killer (NK) Cells

NK cells are lymphocytes that target and destroy virus-infected and cancerous cells by inducing apoptosis (programmed cell death). They are part of the innate immune response and do not require prior activation by antigens.

• Function: Kill abnormal cells, release interferons, and enhance phagocytosis.

Phagocytes

Phagocytes are cells that engulf and digest pathogens and debris.

• Neutrophils: Most abundant, respond quickly to infection.

• Macrophages: Derived from monocytes; reside in tissues and are highly effective at phagocytosis.

• Dendritic Cells: Present antigens to T cells, bridging innate and adaptive immunity.

Other Innate Defenses

• Inflammation: Local response to injury or infection; characterized by redness, heat, swelling, and pain. Increases blood flow and recruits immune cells.

• Antimicrobial Proteins: Interferons (block viral replication), complement proteins (enhance phagocytosis, lyse pathogens).

• Fever: Elevated body temperature inhibits microbial growth and enhances immune activity.

Phagocytosis Process

Steps of Phagocytosis

Phagocytosis is the process by which phagocytes ingest and destroy pathogens.

1. Chemotaxis: Phagocytes are attracted to the site of infection by chemical signals.

2. Adherence: Phagocyte binds to the pathogen.

3. Ingestion: Pathogen is engulfed into a phagosome.

4. Digestion: Lysosomes fuse with the phagosome, releasing enzymes that digest the pathogen.

5. Exocytosis: Debris is expelled from the cell.

Adaptive (Specific) Immunity

Overview

Adaptive immunity is characterized by specificity and memory. It involves lymphocytes (B cells and T cells) and the production of antibodies that target specific antigens.

• Humoral Immunity: Mediated by B cells and antibodies; targets extracellular pathogens.

• Cellular Immunity: Mediated by T cells; targets infected or abnormal cells.

Lymphocytes

• Origin: Both B and T lymphocytes originate in red bone marrow.

• Maturation: B cells mature in bone marrow; T cells mature in the thymus.

• Activation: Lymphocytes are activated by encountering their specific antigen.

• Proliferation and Differentiation: Activated lymphocytes proliferate and differentiate into effector and memory cells.

Humoral Immunity

B cells produce antibodies that bind to antigens and neutralize or mark them for destruction.

• Primary Immune Response: First exposure to an antigen; slower, produces memory cells.

• Secondary Immune Response: Subsequent exposure; faster and stronger due to memory cells.

Cellular Immunity

T cells directly attack infected or abnormal cells.

• Cytotoxic T Cells (CD8+): Destroy infected cells.

• Helper T Cells (CD4+): Activate B cells, other T cells, and macrophages.

• Regulatory T Cells: Suppress immune responses to prevent autoimmunity.

Types of Immunity

Active vs. Passive Immunity

• Active Immunity: Results from direct exposure to an antigen; body produces its own antibodies. Can be natural (infection) or artificial (vaccination).

• Passive Immunity: Results from receiving antibodies from another source; provides immediate, short-term protection. Can be natural (maternal antibodies) or artificial (antibody injection).

Antibody Structure and Function

Antibody Targets and Functions

Antibodies bind to antigens and facilitate their destruction by various mechanisms.

• Neutralization: Antibodies block pathogen binding sites.

• Agglutination: Antibodies cause pathogens to clump together.

• Precipitation: Antibodies cause soluble antigens to precipitate.

• Complement Activation: Antibody-antigen complexes activate complement proteins, leading to cell lysis and enhanced phagocytosis.

Summary Table: Innate vs. Adaptive Immunity

Key Terms and Definitions

• Antigen: Any substance that can induce an immune response.

• Antibody: Protein produced by B cells that binds to specific antigens.

• Phagocytosis: Process by which cells engulf and digest particles.

• Apoptosis: Programmed cell death.

• Chemotaxis: Movement of cells toward chemical signals.

• Complement System: Group of proteins that enhance immune responses.

Important Equations

• Rate of Phagocytosis:

• Antibody-Antigen Binding:

Additional info: Some explanations and definitions have been expanded for clarity and completeness.