Immunity is the state of protection from infectious disease. It has two components, namely specific and non-specific. Innate or non specific immunity refers to the basic resistance to disease that an individual is born with. Acquired or specific immunity requires activity of the functional immune system, involving lymphocytes and their products. Innate defence mechanisms provide the first line of defence against invading pathogens until an acquired immune response develops.
The innate immunity comprises of four types of defensive barriers, anatomic, physiologic, endocytic and phagocytic and inflammatory.
The physical and anatomic barriers are the first line of defence against infection.This includes the skin and the surface of mucous membranes.
The skin consists of two layers, the outer thin epidermis and the inner thicker layer called dermis. The epidermis consists of layers of tightly packed epithelial cells. The outer epidermal layer consists of dead cells and is filled with a waterproofing protein called keratin. Older epidermal cells are removed and replaced by new cells derived from cells lying next to the dermis every 15-30 days. The epidermal cells do not contain blood vessels and are nourished by nutrients diffusing from the underlying dermis. The dermis is composed of connective tissue, contains blood vessels, hair follicles, sebaceous glands and sweat glands. The sebaceous glands are associated with the hair follicles and produce an oily secretion called sebum. The sebum consists of lactic and fatty acids which maintain the pH of skin between 3 and 5, which is inhibitory to most microbes. Breaks in skin resulting from wounds or abrasions or from biting insects are obvious routes of infection.
The conjunctivae and the alimentary, respiratory and urinogenital tracts are lined with mucous membranes. These membranes consist of an outer epithelial layer and an underlying connective tissue layer. The viscous fluid called the mucus, which is secreted by the epithelial cells of mucous membranes, entraps foreign microorganisms. In the lower respiratory tract and the gastrointestinal tract, the mucous membrane is covered with hair like projections called cilia. The synchronous movement of the cilia propel mucus entrapped microorganisms from these tracts. In addition, non pathogenic organisms tend to colonize the epithelial surface of mucous membranes and they outcompete pathogens for attachment sites and nutrients.
The physiologic barriers that contribute to innate immunity include temperature, pH,oxygen tension and various soluble proteins like lysozyme, interferon and complement.
Lysozyme is a hydrolytic enzyme found in mucous secretions. It can cleave the peptidoglycan layer of bacterial cell wall. Interferon comprises of a group of proteins produced by virus infected cells. Aming the many functions of interferons is the ability to bind to nearby cells and induce a generalized anti viral state. Complement is a group of serum proteins that circulate in an inactive proenzyme state. These can be activated by a variety of specific and non-specific immunologic mechanisms.
Endocytic and Phagocytic Barriers
Another important defense mechanism is the ingestion of extracellular macromolecules via endocytosis and of particulate material via phagocytosis.
In endocytosis, the macromolecules within the extra cellular tissue fluid are internalized by cells by the invagination and pinching off of small regions of the plasma membrane. Endocytosis occurs through one of the two processes: pinocytosis or receptor mediated endocytosis. In pinocytosis, non-specific membrane invagination occurs in proportion to the extracellular concentration of macromolecules. In receptor mediated endocytosis, selective internalization of macromolecules occurs after binding to specific membrane receptors.
The endocytic vesicles formed either way fuse with each other and are delivered to endosomes, which are intracellular acidic compartments. The acidic interior of endosomes facilitate dissociation of macromolecules from their receptors and the latter are recycled back to the cell surface. Free macromolecules in the endosomes fuse with lysosomes which contain large number of degenerative enzymes and are digested into small breakdown products which are eventually eliminated from the cell.
In phagocytosis, particulate material, including whole pathogenic microorganisms are ingested by the expansion of the plasma membrane around it. The phagosomes thus formed undergo the same process as the endosomes. Specialized cells capable of phagocytosis include monocytes, neutrophils and macrophages.
Tissue damage caused by a wound or by invasion of by a pathogenic organism induces a complex sequence of events collectively known as the inflammatory response. The cardinal signs of inflammation are described as rubor (redness), tumor (swelling), calor (heat), dolor (pain) and functio laesa (loss of function). These signs reflect the three major events that occur during an inflammatory response.
Vasodilation: An increase in diameter of blood vessels as the vessels that carry blood away from the affected area constrict, resulting in engorgement of the capillary network. This is responsible for tissue redness (erythema) and increase in tissue temperature.
An increase in capillary permeability facilitates an influx of fluid and cells from the engorged capillaries to the tissue. This fluid that accumulates causes tissue swelling (edema)
There is an influx of phagocytes from the capillaries. As the phagocytes accumulate at the site and begin to phagocytose bacteria, they release lytic enzymes which can damage nearby cells. The accumulation of dead cells, digested material and fluid forms a substance called pus.
Kuby, Janis. (1997). Immunology. W.H. Freeman &Co.