Inflammation: Causes and Types

• Inflammation is a local response of living vascularized tissues to irritants
• This response involves local vascular and cellular changes

Aim of Inflammation

• Elimination of the irritant.
• Destruction of invading organisms.
• Inactivation of toxins.
• Preparation of tissue for healing and repair.

Causes of Inflammation

• Irritants can be physical agents like electricity, excess heat or cold, and radiation
• Irritants can be chemical agents like concentrated acids and alkalies
• Infectious agents such as viruses, bacteria, fungi, and parasites can cause inflammation
• Mechanical causes of inflammation include crushing injuries, fractures, and foreign bodies
• Necrotic tissue is also a cause of inflammation

Types of Inflammation

• Acute inflammation is characterized by rapid onset, short duration, and a high dose of irritant
• Acute inflammation involves cells such as neutrophils and macrophages
• Chronic inflammation is characterized by gradual onset, long duration, and a low dose of irritant
• Chronic inflammation involves lymphocytes, macrophages, giant cells, and eosinophils
• Subacute inflammation grades between acute and chronic inflammation

Nomenclature of Inflammation

• The nomenclature of inflammation typically uses the Greek, Latin, or English name of the organ with the suffix "itis"
• Gastritis refers to inflammation of the stomach
• Orchitis refers to inflammation of the testis
• Glossitis refers to inflammation of the tongue
• Hepatitis refers to inflammation of the liver
• Appendicitis refers to inflammation of the appendix
• Exceptions to this naming convention include: Pneumonia (inflammation of the lung) and Pleurisy (inflammation of the pleura)

Acute Inflammation Definition

• There is an immediate response of living vascularized tissue to an irritant
• The aim is to deliver leukocytes and mediators from the blood to the site of inflammation

Pathogenesis of Acute Inflammation

• Involves local tissue damage
• Involves a vascular response
• Involves cellular responses

Vascular Response

• Transient vasoconstriction occurs
• Permanent vasodilation occurs
• Increased vascular permeability occurs leading to the formation of inflammatory fluid exudate
• Slowing of blood flow is due to vasodilation, increased capillary permeability, increased blood viscosity, resistance to blood flow, and opening of new capillary buds
• Dilatation of lymphatics occurs when lymph flow increases leading to drainage of excess fluid back to blood

Inflammatory Fluid Exudate

• Increased capillary hydrostatic pressure
• Increased capillary permeability
• Increased osmotic pressure of interstitial tissue due to the escape of plasma protein to the tissue

Composition of Inflammatory Fluid Exudate

• High protein content (4-8gm%)
• High specific gravity (above 1018)
• Turbid and yellowish appearance due to the presence of leukocytes and plasma protein which is mainly fibrin

Functions of Inflammatory Fluid Exudate

• Dilutes toxins to minimize their effects on cells
• Brings fibrinogen to form a fibrin network that helps with the movement of acute inflammatory cells to the irritant site
• Surrounds the area of inflammation to localize infection, and also acts as a framework for repair cells
• Transports antibodies of different types to the site of inflammation

Cellular Responses

• Involves Exudation of leucocytes
• Involves Phagocytosis

Exudation of Leucocytes

• Margination: leukocytes leave the axial zone and adhere to the endothelial wall due to stasis
• Rolling: leukocytes transiently stick along the endothelial cells
• Adhesion: leukocytes firmly stick to endothelial cells with the help of certain adhesion molecules
• Emigration: white blood cells pass through widened inter-endothelial gaps and outside the capillaries by ameboid movement, neutrophils predominate for 24-48 hours, then macrophages predominate
• Chemotaxis: attraction of leukocytes towards the site of irritant along a concentration gradient of chemotactic substances like bacterial products and cytokines

Phagocytosis Definition

• A process of recognition, engulfment, and destruction of irritants by phagocytic cells involving 3 steps

Phagocytosis

• Recognition and Attachment: leukocytes have cell surface receptors to recognize and attach bacteria with the help of opsonins
• Engulfment: phagocytic cells form cytoplasmic pseudopods around the bacteria, enveloping it in a phagocytic vacuole (phagosome), the membrane of the phagosome then fuses with the membrane of the lysosomes resulting in release of lysosomal granules (phagolysosome)
• Killing and Destruction: lysosomal enzymes or oxygen derived free radicals are released