Innate Immunity: Nonspecific Defenses of the Host (Chapter 16 Study Notes)

Innate Immunity: Nonspecific Defenses of the Host

Overview of Immunity

Immunity refers to the body's ability to defend against disease-causing organisms and substances. The immune system is divided into two main branches: innate (nonspecific) immunity and adaptive (specific) immunity. Innate immunity provides immediate, general protection, while adaptive immunity develops more slowly and targets specific pathogens.

• Immunity: The ability to ward off disease.

• Susceptibility: Lack of resistance to a disease.

• Innate Immunity: Defenses against any pathogen; rapid and present at birth.

• Adaptive Immunity: Immunity or resistance to a specific pathogen; slower to respond and has a memory component.

• Key Defenses: Skin, mucous membranes, antimicrobial substances, inflammation, fever, phagocytes, humoral and cellular immunity.

Layers of Host Defense

The body employs multiple layers of defense against pathogens such as viruses, bacteria, and fungi. These layers act sequentially to prevent infection and disease.

• Physical Barriers: Skin and mucous membranes block pathogen entry.

• Chemical Barriers: Antimicrobial substances neutralize pathogens.

• Cellular Defenses: Phagocytes and other immune cells destroy invaders.

• Inflammatory Response: Inflammation and fever help eliminate pathogens.

• Adaptive Immunity: Humoral (antibody-mediated) and cellular (T cell-mediated) responses target specific pathogens.

White Blood Cells (Leukocytes) and Immunity

White blood cells (WBCs) are crucial components of the immune system. Their numbers and types can indicate the presence of infection or disease.

• Normal WBC Count: 4,000–11,000/μl (or 5,000–10,000 per mm3).

• Differential WBC Count: Measures the relative numbers of each type of WBC.

• High WBC Count: May indicate bacterial infections, autoimmune diseases, or medication side effects.

• Low WBC Count: May indicate viral infections, pneumonia, autoimmune diseases, severe bacterial infections, medication side effects, or cancers.

Normal Ranges and Percentages of WBC Types

Cell Type	Percentage of Total WBCs
Neutrophils	60% to 70%
Lymphocytes	20% to 25%
Monocytes	3% to 8%
Eosinophils	2% to 4%
Basophils	0.5% to 1%

Types and Functions of Immune Cells

Different immune cells play specialized roles in both innate and adaptive immunity. The following table summarizes key cell types and their functions.

Immunity Type	Cell Type	Description	Function
Innate	Basophil	Granulocyte	Releases histamine that causes inflammation
Innate	Eosinophil	Granulocyte	Kills parasites with oxidative burst
Innate	Mast cell	Granulocyte	Antigen-presenting cell; produces antibacterial peptides
Both	Neutrophil	Granulocyte	Phagocytizes bacteria and fungi
Both	Monocyte	Agranulocyte	Precursor to macrophages; performs phagocytosis and inflammation
Both	Dendritic cell	Agranulocyte	Antigen-presenting cell; phagocytizes bacteria and viruses
Both	Natural killer (NK) cell	Agranulocyte (lymphocyte)	Kills cancer cells and virus-infected cells
Adaptive	Plasma cell, T cells (Helper, Cytotoxic, Regulatory)	Agranulocyte (lymphocyte)	Recognizes antigens and produces antibodies; coordinates adaptive immune responses

Key Terms and Concepts

• Pathogen: Any microorganism that can cause disease (e.g., viruses, bacteria, fungi).

• Antigen: A substance that induces an immune response, often a component of pathogens.

• Phagocytosis: The process by which certain cells (phagocytes) engulf and digest microorganisms and cellular debris.

• Inflammation: A local response to tissue damage characterized by redness, heat, swelling, and pain.

• Fever: An elevated body temperature that can enhance immune function and inhibit pathogen growth.

Example: Immune Response to Infection

When a pathogen breaches the skin or mucous membranes, innate immune cells such as neutrophils and macrophages are rapidly recruited to the site of infection. These cells engulf and destroy the invader, while chemical signals trigger inflammation and fever. If the pathogen persists, adaptive immunity is activated, involving T cells and B cells that specifically target the pathogen and provide long-term protection.

Additional info: The study notes above expand on the brief points and tables from the slides, providing definitions, context, and examples suitable for college-level microbiology students.