Innate Immunity: Nonspecific Defenses of the Host

Innate Immunity: Nonspecific Defenses of the Host

Overview of Immunity

Immunity is the ability of an organism to resist infection or disease. It is divided into two main types: innate immunity (nonspecific, present at birth, rapid response) and adaptive immunity (specific, slower response, memory component). Innate immunity provides the first line of defense against pathogens and activates adaptive responses.

• 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.

• Toll-like receptors (TLRs): Host cell receptors that recognize pathogen-associated molecular patterns (PAMPs) and trigger cytokine release.

• Cytokines: Signaling proteins that regulate immune responses and recruit immune cells.

White Blood Cell Counts and Clinical Significance

White blood cell (WBC) counts are used to assess immune status. Differential counts indicate the relative numbers of each type of leukocyte. Abnormal counts can signal infection, autoimmune disease, or other health issues.

• Normal WBC count: 4000–11,000/μL

• High WBC count: May indicate bacterial infection, autoimmune disease, or medication side effects.

• Low WBC count: May indicate viral infection, pneumonia, severe bacterial infection, medication side effects, or cancer.

Physical Barriers of Innate Immunity

Skin

The skin acts as a formidable barrier to microbial invasion. Its structure includes the dermis (connective tissue) and epidermis (tightly packed epithelial cells containing keratin). Shedding and dryness inhibit microbial growth.

• Keratin: Protective protein in the epidermis.

• Shedding: Removes microbes from the skin surface.

• Dryness: Inhibits microbial growth.

Mucous Membranes

Mucous membranes line the gastrointestinal, respiratory, and genitourinary tracts. Mucus traps microbes and prevents drying. The lacrimal apparatus washes the eye with tears.

• Mucus: Viscous glycoproteins that trap microbes.

• Lacrimal apparatus: Drains tears and washes the eye.

Other Physical Factors

Additional physical mechanisms help remove microbes from the body.

• 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: Cleans the urethra via flow.

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

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

Chemical Barriers of Innate Immunity

Chemical Factors

Chemical barriers include substances that inhibit or destroy pathogens.

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

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

• Gastric juice: Low pH (1.2–3.0) destroys most bacteria and toxins.

• Vaginal secretions: Low pH (3–5) inhibits microbes.

Normal Microbiota and Innate Immunity

Role of Normal Microbiota

Normal microbiota compete with pathogens for space and nutrients, produce substances harmful to pathogens, and alter conditions affecting pathogen survival. They play a crucial role in immune system development.

• Microbial antagonism: Competitive exclusion of pathogens.

• Commensalism: One organism benefits, the host is unharmed.

• Opportunistic pathogens: Normal microbiota that can cause disease under certain conditions (e.g., E. coli, S. aureus).

• Probiotics: Live microbial cultures administered for beneficial effects.

• Prebiotics: Chemicals that selectively promote growth of beneficial bacteria.

Formed Elements in Blood

Blood Cells and Hematopoiesis

Blood contains cells and cell fragments suspended in plasma. These include erythrocytes (red blood cells), leukocytes (white blood cells), and platelets. All are produced in red bone marrow via hematopoiesis.

• Erythrocytes: Red blood cells, transport oxygen.

• Leukocytes: White blood cells, involved in immunity.

• Platelets: Cell fragments involved in clotting.

• Hematopoiesis: Formation of blood cells from stem cells in bone marrow.

Leukocyte Classification

Leukocytes are classified as granulocytes (with visible granules) or agranulocytes (without visible granules).

• Granulocytes: Neutrophils, basophils, eosinophils.

• Agranulocytes: Monocytes (mature into macrophages), dendritic cells, lymphocytes (T cells, B cells, NK cells).

Leukocyte Functions

Each leukocyte type has specialized functions in immunity.

Cell Type	Description	Function
Basophil	Granulocyte	Releases histamines that cause inflammation
Eosinophil	Granulocyte	Kills parasites with oxidative burst
Mast cell	Granulocyte	Antigen-presenting cell; produces antibacterial peptides
Neutrophil	Granulocyte	Phagocytoses bacteria and fungi
Monocyte	Agranulocyte	Precursor to macrophages; phagocytic
Dendritic cell	Agranulocyte	Phagocytosis; initiates adaptive immune responses
Natural killer (NK) cell	Agranulocyte	Kills cancer cells and virus-infected cells
Plasma cell, B cell	Agranulocyte	Recognizes antigens and produces antibodies
T cells	Agranulocyte	Cell-mediated immunity

The Lymphoid System

Structure and Function

The lymphoid system consists of lymph plasma, lymphatic vessels, lymphoid tissues, and red bone marrow. Lymphoid tissue contains lymphocytes and phagocytic cells. Lymph transports microbes to lymph nodes, where immune cells encounter and destroy pathogens.

• Lymph: Fluid that carries microbes and immune cells.

• Lymphatic vessels: Transport lymph throughout the body.

• Lymphoid organs: Include lymph nodes, spleen, thymus, tonsils, and bone marrow.

Summary Table: Leukocytes (White Blood Cells)

Type	Function	Normal Range
Neutrophils	Phagocytosis	60–70%
Lymphocytes	Adaptive immunity	20–25%
Monocytes	Phagocytosis (as macrophages)	3–8%
Eosinophils	Defense against parasites	2–4%
Basophils	Release histamine	0.5–1%

Key Equations

White blood cell count calculation:

$\text{WBC count} = \frac{\text{Number of WBCs}}{\text{Volume of blood (μL)}}$

Conclusion

Innate immunity provides the first line of defense against pathogens through physical and chemical barriers, normal microbiota, and specialized blood cells. The lymphoid system coordinates immune responses, ensuring rapid and effective protection against disease.