Immune System

Three Lines of Defense

The immune system protects the body through three main lines of defense, each with distinct mechanisms and components.

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

• Innate Immunity: Non-specific, immediate response involving phagocytes, inflammation, fever, and complement proteins.

• Adaptive Immunity: Specific, slower response involving lymphocytes (B and T cells) and antibodies.

Innate vs. Adaptive Immunity

Innate and adaptive immunity differ in specificity, memory, and response time.

• Innate Immunity: Rapid, non-specific, no memory. Includes neutrophils, macrophages, dendritic cells, eosinophils, basophils, and NK cells.

• Adaptive Immunity: Slower, highly specific, develops memory. Involves B and T lymphocytes, antibodies.

Immune Cell Types and Functions

Various immune cells play specialized roles in defense and regulation.

• Helper T Cells (CD4+): Coordinate immune responses by activating other immune cells.

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

• B Cells: Produce antibodies to neutralize pathogens.

• Plasma Cells: Differentiated B cells that secrete large amounts of antibodies.

• Memory Cells: Long-lived cells that provide rapid response upon re-exposure to the same antigen.

Major Histocompatibility Complex (MHC)

MHC molecules present antigens to T cells, crucial for immune recognition.

• MHC Class I: Found on all nucleated cells; presents endogenous antigens to cytotoxic T cells.

• MHC Class II: Found on antigen-presenting cells (APCs); presents exogenous antigens to helper T cells.

Self-Tolerance and Autoimmunity

The immune system distinguishes self from non-self to prevent attacking the body's own tissues.

• Self-Tolerance: Mechanisms that prevent immune response against self-antigens.

• Autoimmunity: Failure of self-tolerance, leading to immune attack on self-tissues (e.g., type 1 diabetes, lupus).

Antigen Presentation and Activation

Antigen-presenting cells (APCs) process and present antigens to T cells, initiating adaptive responses.

• APCs: Macrophages, dendritic cells, B cells.

• Process: APCs ingest antigen → process it → present peptide on MHC → activate T cells.

Inflammation

Inflammation is a protective response to infection or injury, characterized by four cardinal signs.

• Signs: Redness, heat, swelling, pain.

• Causes: Increased blood flow, capillary permeability, and leukocyte migration.

Immunity Types and Examples

Immunity can be classified by how it is acquired and its duration.

• Innate Immunity: Present at birth, non-specific.

• Adaptive Immunity: Acquired after exposure to antigens.

• Natural Active: Infection-induced immunity.

• Natural Passive: Maternal antibodies transferred to fetus.

• Artificial Active: Vaccination.

• Artificial Passive: Injection of antibodies (e.g., antivenom).

Antibodies and Immunoglobulins

Antibodies are proteins produced by B cells that bind specific antigens.

• Classes: IgG, IgM, IgA, IgE, IgD.

• Functions: Neutralization, opsonization, complement activation.

General Sequence of Immune Response

The immune response follows a general sequence from pathogen entry to elimination.

• Pathogen entry → Detection → Antigen presentation → Activation of lymphocytes → Effector response → Memory formation

Respiratory System

Site of Gas Exchange and Influencing Factors

Gas exchange occurs in the alveoli, influenced by pressure gradients, membrane thickness, and surface area.

• Alveoli: Tiny air sacs where oxygen and carbon dioxide are exchanged.

• Factors: Partial pressure differences, thin respiratory membrane, large surface area.

Respiratory Membrane Structure and Function

The respiratory membrane facilitates efficient gas exchange between air and blood.

• Components: Alveolar epithelium, capillary endothelium, fused basement membrane.

Oxygen and Carbon Dioxide Transport

Oxygen and carbon dioxide are transported in blood in different forms.

• Oxygen: Mostly bound to hemoglobin; small amount dissolved in plasma.

• Carbon Dioxide: Transported as bicarbonate (majority), carbaminohemoglobin, and dissolved gas.

Hemoglobin and Oxygen Dissociation Curve

The oxygen-hemoglobin dissociation curve shows the relationship between oxygen partial pressure and hemoglobin saturation.

• Right Shift: Decreased affinity (increased CO2, temperature, 2,3-BPG, lower pH).

• Left Shift: Increased affinity (decreased CO2, temperature, 2,3-BPG, higher pH).

Ventilation and Perfusion

Effective gas exchange requires matching of ventilation (airflow) and perfusion (blood flow).

• Mismatch: Can lead to hypoxemia or hypercapnia.

Homeostatic Responses and Acid-Base Balance

The respiratory system helps maintain acid-base balance by regulating CO2 exhalation.

• Acidosis: Increased CO2 (hypoventilation).

• Alkalosis: Decreased CO2 (hyperventilation).

Digestive System & Metabolism

Major Digestive Organs and Functions

The digestive system consists of organs that process food, absorb nutrients, and eliminate waste.

• Mouth: Mechanical and chemical digestion begins.

• Pharynx & Esophagus: Transport food to stomach.

• Stomach: Protein digestion, food storage, mixing.

• Small Intestine: Main site of digestion and absorption.

• Large Intestine: Water absorption, feces formation.

Accessory Organs and Secretions

Accessory organs produce secretions that aid digestion.

• Salivary Glands: Secrete saliva (amylase for carbohydrate digestion).

• Liver: Produces bile for fat emulsification.

• Gallbladder: Stores and releases bile.

• Pancreas: Secretes digestive enzymes and bicarbonate.

Digestion of Macronutrients

Enzymes break down carbohydrates, proteins, and fats into absorbable units.

• Carbohydrates: Amylase, maltase, sucrase, lactase.

• Proteins: Pepsin, trypsin, peptidases.

• Fats: Lipase, bile salts.

Swallowing and Motility

Swallowing involves coordinated action of the epiglottis and uvula to direct food into the esophagus.

• Peristalsis: Wave-like muscle contractions move food through the GI tract.

Stomach and Intestinal Function

The stomach and intestines perform mechanical and chemical digestion, absorption, and motility.

• Stomach: Acidic environment activates pepsin, churns food.

• Small Intestine: Enzymatic digestion, nutrient absorption.

• Large Intestine: Water and electrolyte absorption, feces formation.

Metabolic Pathways

Metabolism includes catabolic and anabolic pathways for energy production and biosynthesis.

• Glycogenolysis: Breakdown of glycogen to glucose.

• Gluconeogenesis: Formation of glucose from non-carbohydrate sources.

• Beta Oxidation: Breakdown of fatty acids for energy.

• Anabolism: Synthesis of complex molecules (e.g., proteins, glycogen).

• Catabolism: Breakdown of molecules for energy.

Digestive Enzymes

Key digestive enzymes originate from various organs and act on specific substrates.

Disorders and Clinical Correlations

Common digestive disorders include celiac disease, lactose intolerance, peptic ulcers, and GERD.

• Celiac Disease: Autoimmune reaction to gluten.

• Lactose Intolerance: Deficiency of lactase enzyme.

• Peptic Ulcers: Erosion of stomach lining, often due to H. pylori infection.

• GERD: Acid reflux into the esophagus.

Additional info:

• Ketone body formation (ketogenesis) and ketoacidosis are important in diabetes and low-carb diets.

• Protein catabolism produces ammonia, which is converted to urea in the liver for excretion.