Internal Immunity is the second line of defence.
This internal defence of the body carried on by white blood corpuscles (WBC or Leucocytes), macrophages, inflammatory reactions, fever and antimicrobial substances.
1. Phagocytic Barriers (By WBC and Macrophages):
The normal number of WBCs or Leucocytes per cubic millimeter of blood varies from 5000 to 10,000. This number increases in response to infection and is called as Leukocytosis.
(a) Neutrophils (a granular leucocyte) content of the blood cells is about 67% to 70%. These cells move throughout the body by amoeboid movement and engulf the microbes by a process of phagocytosis.
(b) The granules of neutrophils secrete antibacterial secretions like phagocytin and lactoferrin.
(c) An anticoagulant is secreted by basophils.
(d) The toxic materials produced by foreign microbes, is destroyed by eosinophil’s.
(e) The monocytes, which comprises of about 1.5% of the WBC, are the largest and motile. They are phagocytic in nature.
(f) The lymphocytes (25% to 28% of the WBC) give rise to B-cells which secrete antibodies and T-cells that destroy the microbes.
Macrophages:
These are modified form of monocytes. They are two types wandering and fixed macrophages, together they form reticuloendothelial system (RES). They are phagocytic and can engulf 100 bacteria before it dies. The macrophages, of the blood can be activated by various stimulants, including microbes and their products, antigen-antibody complexes, inflammation and injury. The activated macrophages contain increased number of lysosomes and kill the microbes by intracellular digestion. The macrophages and neutrophils feed on the invading pathogens and dead tissue cells. So they are popularly known as the soldiers and scavengers of the body.
2. Inflammatory barriers:
Injuries in the form of cut, bite or burn of an animal make way for the foreign pathogens to enter the tissues which produce toxic substances. These toxic substances break the mast cells of connective tissue. The histamine, released from the broken mast cells, dilate the capillaries and small blood vessels which surround the injury.
Thus it increases the permeability of the capillary wall, allowing more blood flow to that area. For that it looks red and feels warm. The fluid, released from damaged cells, causes swelling. This reaction of the body is known as inflammatory response. The neutrophils gather around the injury and kill and engulf microbes.
3. Fever:
It is an inflammatory response caused by the entry of foreign germs. The rise of body temperature is due to increase in number of WBC counts, causing fever. The fever occurs due to the release of endogenous pyrogen (a chemical produced in the body, causing fever) or interleukin-I by the macrophages. The interleukin-I stimulates hypothalamus of the brain that regulates the thermoregulatory centre maintaining higher temperature or fever. Higher temperature (or fever) activates the phagocytes and also inhibits the growth of microbes.
4. Antimicrobial Substances:
The antimicrobial substances are biochemical tissue constituents, interferon’s, complemental proteins and natural killer cells.
(a) Biochemical tissue constituents:
Some animal tissues have basic polypeptides with antimicrobial properties against specific microbes like Bacillus anthracis.
(b) Interferon’s:
Interferon’s are antiviral proteins released by the host cells, when attacked by a virus. The interferon’s released by dead, infected host cells, do not inactivate the virus, but enter the nearby uninfected, healthy cells to prevent the viral multiplication in them. So the interferon’s do not act directly on the virus, but induces the healthy cells to prepare its own antiviral proteins that check viral multiplication.
The interferon’s (IFN), produced by leucocytes, fibroblasts and lymphocytes are not virus-specific (i.e. different viruses produce one type of interferon). But it is host-specific (i.e. interferon of one host will not work in another host). Interferon’s have proved to be effected in the treatment of influenza and hepatis but its role in the treatment of cancer is doubtful.
(c) Complemental Proteins:
The antigen-antibody complexes or chemical markers, present on the surface of the microbes activate twenty numbers of proteins present in normal human.
They assist or complement other defence mechanism of the body in following ways:
(a) Some complemental proteins rupture the cell membrane of microbes or tumour cells (cytolysis). With the help of a protein, called perforin, a hole can be caused for the release of the cell contents.
(b) Some complemental proteins stimulate the mast cells to release histamines that develop inflammation through vasodilation and enhance vascular permeability.
(c) Some complemental proteins attract the phagocytes to the infected area (chemotaxis).
(d) Peculiarly, a few complemental proteins develop a coat around the invading pathogens to make it suitable for phagocytosis by macrophages. This process is known as opsonization.
(e) Some complemental proteins bind to antigen-antibody complexes and remove them from the serum. This process is called complement fixation (CF).
The complement proteins are synthesized in the liver. Its deficiency increases the chances of infections. The complement system participates in both innate and acquired immunities of the body.
(d) Natural killer cells in killer:
Besides phagocytes, there are natural killer cells which destroy virus, infected cells and some tumour cells making holes on the surfaces of the target cells by perforin. The body fluid enters through the holes into the target cells and swells to burst.