Saturday, 3 November 2012

Elements of Immunity: Innate Immunity

The elements of the innate (non-specific) immune system include anatomical barriers, secretory molecules and cellular components. Among the mechanical anatomical barriers are the skin and internal epithelial layers, the movement of the intestines and the oscillation of broncho-pulmonary cilia. Associated with these protective surfaces are chemical and biological agents.

  • It is a non-specific immunity.
  • This mechanism of immunity is similar for most types of infections, hence its name.
  • Its resistance to infection is by blocking the entry of pathogens into the body or by destroying the microbes through means other then antibodies.
  • Thus no-specific defense mechanism provides innate immunity which operates through many factors.

1) First line Defense (External Defense):

The external defense comprises physical and chemical barriers to the entry of pathogens into the body.


  • Skin is the largest (9.7m2) water proof, germ proof, self-disinfecting, defensive organ
  • The skin is providing a protective cover to the body.
  • It secretes some acids like Pyruvic acid and Lactic acid by sweat glands, which creates acidic environment on the skin.
  • In this environment some pathogens does not survive on the skin and it can not penetrate through the skin.


  • It resists the penetration of parasites into tissues.
  • Mucus secreted by mucus glands traps the pathogen and immobilizes them.
  • Mucus is a clear sticky mixture of mucin (glycoprotein) and water.
  • It is present in all internal tracks.

Vaginal Bacterias:

These bacteria produce lactic acid from the Glycogen thereby producing acidic environment to vagina as female’s best natural defense against pathogens.

2) Second line Defense (Internal defense) :

Body’s internal defense involves the participation of cellular and non-cellular factors possessing anti-bacterial property which have been found in blood and tissue.

I) Biochemical Factors:

The major biochemical factors are Interferon and Complement system.


  • These are group of soluble non-toxic glycoproteins produced in small amount by all cells of the body.
  • A special defense works specially against viral infection.
  • They offer resistance by blocking viral m-RNA transcription there by disrupting the viral life-cycle.
  • Cells involved by a virus produce an antiviral protein Interferon.
  • The interferon reaches the uninfected cells; it makes them resistant to the viral infection.

Complement System:

  • This system comprising set of 11 proteins found in the serum.
  • It is activated characteristically by antigen-antibody complexes.
  • Activation of the complement increases phagocytosis and destruction of the microbial organisms that enter the body of an individual.
  • The system perform functions in different ways:
    • Complement proteins rupture the cell membrane of microbe.
    • Stimulate mast cells to produce histamine
    • Strengthens the inflammatory reaction.
    • Act as Chemokines
    • Attract the phagocytes to the infected area.

Lysozyme :

  • Lysozyme breaks down the cell wall of bacteria.

Coagulation system :

  • Depending on the severity of the tissue injury, the coagulation system may or may not be activated. Some products of the coagulation system can contribute to the non-specific defenses because of their ability to increase vascular permeability and act as chemotactic agents for phagocytic cells.

Lactoferrin and transferrin: 

  • By binding iron, an essential nutrient for bacteria, these proteins limit bacterial growth.


  • IL-1 induces fever and the production of acute phase proteins, some of which are antimicrobial because they can opsonize bacteria.


II) Cellular Factors:

The natural defense mechanism against microorganisms and other foreign particles is carried out mainly by phagocytic cells were originally discovered by Mitchnikoff (1883). The process of eating the cells is known as Phagocytosis.

These phagocytic cells are of TWO types:

a) Microphages

b) Macrophages

a) Microphages:

  • Microphages are also called “Granulocytes”.
  • Microphages are polymorphonuclear leucocytes.
  • These are of three types: Neutrophils, Basophils, Eosinophils (Acidophils)
  • These calls possess multilobed nuclei and hence called polymorphonuclear leucocytes
  • They do not divide and short lived.


b) Macrophages:

  • These are mononuclear phagocytic system which was originally called as Reticuloendothelial system. Macrophages
  • The pahgocytic cells of this system reach the site of inflammation in large number attracted by chemotactic substance and ingest the particulate materials.
  • Macrophages are phagocytic cells, derived from the blood monocyte.
  • The Macrophages are amoeboid cells having Bi-lobed nucleus & prominent cytoplasmic lysosomes.

  • Macrophages like cells serve different functions in different tissues & are named according to their tissue location.

  • Mononuclear phagocytic cell - monocyte circulating in the blood up to 8 hours, during which time they enlarge; they then migrate into the tissues & differentiate into specific tissue Macrophages




Alveolar Macrophages In the Lungs
Histocytes In Connective tissues
Kupffer cells In the Liver
Mesangial cells In the Kidney
Microglial cells In the Brain
Osteoclasts In the Bone


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