Innate Immunity: Structure and Function (Chapter 16 Study Notes)

Innate Immunity

Introduction to Immunity

Immunity refers to the ability of an organism to resist infection and disease. The immune system is divided into two main branches: innate (nonspecific) and adaptive (specific) immunity. Innate immunity provides immediate defense against pathogens, while adaptive immunity develops more slowly and has memory for specific pathogens.

• Immunity: Ability to ward off disease.

• Susceptibility: Lack of resistance to disease.

• Innate immunity: Defenses against any pathogen; rapid, present at birth.

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

White Blood Cell (WBC) Counts and Immunity

White blood cells (leukocytes) are crucial for immune defense. Their numbers and types can indicate health status.

• Normal WBC count: 4,000 – 11,000/μL

• Differential WBC count: Indicates relative numbers of each type of WBC

• High WBC counts: May indicate bacterial infections, autoimmune diseases, or side effects of medications

• Low WBC counts: May indicate viral infections, pneumonia, autoimmune diseases, extremely severe bacterial infections, side effects of medications, or cancers

Overview of Innate Immunity

Recognition of Pathogens

Cells of the innate immune system recognize pathogens using pattern recognition receptors, such as Toll-like receptors (TLRs), which bind to pathogen-associated molecular patterns (PAMPs).

• Toll-like receptors (TLRs): Proteins on host cells that attach to PAMPs.

• PAMPs: Examples include lipopolysaccharide (LPS), flagellin, peptidoglycan, bacterial DNA, viral DNA and RNA.

• TLRs binding to PAMPs induces release of cytokines (chemical messengers) that regulate immune responses.

• Cytokines recruit other immune cells and activate both innate and adaptive responses.

Physical and Chemical Barriers

Skin

The skin acts as a formidable barrier to microbial invasion due to its structure and secretions.

• Dermis: Inner portion made of connective tissue.

• Epidermis: Outer portion made of tightly packed epithelial cells containing keratin, a protective protein.

• Shedding and dryness of skin inhibits microbial growth.

• Intact skin is rarely penetrated by microbes.

Mucous Membranes

Mucous membranes line body tracts and provide protection by trapping microbes and preventing tissue dehydration.

• Epithelial layer: Lines gastrointestinal, respiratory, and genitourinary tracts.

• Mucus: Viscous glycoproteins that trap microbes.

• Lacrimal apparatus: Drains tears and washes the eye.

Other Physical Factors

• Ciliary escalator: Transports microbes trapped in mucus away from the lungs.

• Epiglottis: Prevents microorganisms from entering the lower respiratory tract.

• Earwax: Prevents microbes from entering the ear.

• Urine: Cleanses the urethra via flow.

• Vaginal secretions: Move microorganisms out of the vaginal tract.

• Peristalsis, defecation, vomiting, diarrhea: Expel microbes from the gastrointestinal tract.

Chemical Factors

• Sebum: Forms a protective film and lowers the pH (3–5) of skin.

• Lysozyme: Enzyme in perspiration, tears, saliva, and urine that destroys bacterial cell walls.

• Low pH (1.2–3.0) of gastric juice: Destroys most bacteria and toxins.

• Low pH (3–5) of vaginal secretions: Inhibits microbes.

Normal Microbiota

Normal microbiota protect the host by competing with pathogens and producing substances harmful to invaders.

• Compete for space and nutrients (competitive exclusion)

• Produce substances harmful to pathogens

• Alter conditions that affect pathogen survival

• Prevent overgrowth of harmful microbes

• Play a role in immune system development

Formed Elements in Blood

Classification and Functions

Blood contains several types of cells and fragments, each with specific roles in immunity and homeostasis.

• Erythrocytes (red blood cells): Transport oxygen.

• Leukocytes (white blood cells): Defend against infection.

• Platelets: Involved in blood clotting.

• All formed elements are created in red bone marrow stem cells via hematopoiesis.

Granulocytes

• Neutrophils: Highly phagocytic; most active in early stages of infection; can leave blood and enter infected tissue.

• Basophils: Release histamine; work in allergic responses.

• Eosinophils: Phagocytic; toxic against parasites and helminths.

Agranulocytes

• Monocytes: Mature into macrophages in tissues; phagocytic.

• Dendritic cells: Found in skin, mucous membranes, and thymus; phagocytic.

• Lymphocytes: T cells, B cells, and NK cells; play a role in adaptive immunity.

Immune Response Mechanisms

Activation of the Immune System

Binding of TLRs to PAMPs triggers several key immune responses:

1. Cytokine release

2. Phagocytosis

3. Inflammation

4. Complement activation

Summary Table: Physical and Chemical Barriers

Barrier Type	Examples	Function
Physical	Skin, mucous membranes, ciliary escalator, epiglottis, earwax, urine, vaginal secretions, peristalsis	Prevent entry and remove microbes from body surfaces
Chemical	Sebum, lysozyme, gastric juice, vaginal secretions	Destroy or inhibit growth of microbes
Biological	Normal microbiota	Compete with pathogens, produce antimicrobial substances

Key Equations and Processes

Hematopoiesis

Hematopoiesis is the process by which all blood cells are formed from stem cells in the bone marrow.

$\text{Stem Cell} \rightarrow \text{Erythrocyte, Leukocyte, Platelet}$

Stages of Inflammation

1. Vasodilation and increased permeability of blood vessels

2. Phagocyte migration and phagocytosis

3. Tissue repair

Summary

Innate immunity provides the first line of defense against pathogens through physical, chemical, and biological barriers. White blood cells play a central role in recognizing and eliminating invaders. Understanding these mechanisms is essential for comprehending how the body resists infection and maintains health.