Immunity
Immunity is the ability of the body to protect against all types of foreign bodies like bacteria, virus, toxic substances,
etc. which enter the body. Immunity is also called disease resistance. The lack of immunity is known as susceptibility.
The science dealing with the various phenomena of immunity, induced sensitivity and allergy is called immunology.
Types of Immunity: There are two major types of immunity: innate or natural or nonspecific and acquired or adaptive.
(A) Innate or Natural or Nonspecific Immunity: Innate immunity is inherited by the organism from the parents and
protects it from birth throughout life. As its name nonspecific suggests that it lacks specific responses to specific
invaders. Innate immunity or nonspecific immunity is well done by providing different barriers to the entry of the
foreign agents into our body. Innate immunity consists of four types of barriers— physical, physiological, cellular and
cytokine barriers.
1. Physical Barriers: (a) Skin: The skin is physical barrier of body. Its outer tough layer, the stratum corneum
prevents the entry of bacteria and viruses, (b) Mucous Membranes: Mucus secreted by mucous membrane traps the
microorganisms and immobilises them. Microorganisms and dust particles can enter the respiratory tract with air
during breathing which are trapped in the mucus. The cilia sweep the mucus loaded with microorganisms and dust
particles into the pharynx (throat). From the pharynx it is thrown out or swallowed for elimination with the faeces.
2. Physiological Barriers: The skin and mucous membranes secrete certain chemicals which dispose off the
pathogens from the body. Body temperature, pH of the body fluids and various body secretions prevent growth of
many disease causing microorganisms. Some of the important examples of physiological barriers are as follows: (a)
Acid of the stomach kills most ingested microorganisms, (b) Bile does not allow growth of microorganisms, (c)
Cerumen (ear wax) traps dust particles, kills bacteria and repels insects, (d) Lysozyme is present in tissue fluids and
in almost all secretions except in cerebrospinal fluid, sweat and urine. Lysozyme is in good quantity in tears from
eyes. Lysozyme attacks bacteria and dissolves their cell walls. Lysoenzyme is also found in saliva, (e) Nasal Hair.
They filter out microbes and dust in nose, (f) Urine. It washes microbes from urethra, (g) Vaginal Secretions. It is
slightly acidic which discourages bacterial growth and flush microbes out of vagina, (h) Sebum (sweat). It forms a
protective acid film over the skin surface that inhibits growth of many microbes.
3. Cellular Barriers: These are certain white blood corpuscles (leucocytes), macrophages, natural killer cells,
complement system, inflammation, fever, antimicrobial substances, etc.
(i) Certain Leucocytes: Neutrophils and monocytes are major phagocytic leucocytes.
(ii) Macrophages: Monocytes circulate in the bloodstream for about 8 hours, during which time they enlarge and
then migrate into the tissues and differentiate into specific tissue macrophages. Macrophages are long lived and are
highly motile phagocytic. Macrophages contain more cell organelles especially lysosomes.
(iii) Natural Killer Cells (NK Cells): Besides the phagocytes, there are natural killer cells in the body which are a
type of lymphocytes and are present in the spleen, lymph nodes and red bone marrow. NK cells do not have antigen
receptors like T cells and В cells. NK cells cause cellular destruction in at least two ways: (a) NK cells produce
perforins which are chemicals that when inserted into the plasma membrane of a microbe make so weak that
cytolysis (breakdown of cells particularly their outer membrane) occurs and creates pores in the plasma membrane of
the target cells. These pores allow entry of water into the target cells,
which then swell and burst. Cellular remains are eaten by
phagocytes. (b) Another function of NK cells is apoptosis which
means natural cell death. It occurs naturally as part of the normal
development, maintenance and renewal of cells, tissues and organs.
Thus functions of NK cells are to destroy target cells by cytolysis and
apoptosis. NK cells constitute 5%-10% of the peripheral blood
lymphocytes in humans.
(iv) Complement: Complement is a group of 20 proteins, many of
which are enzyme precursors and are produced by the liver. These
proteins are present in the serum of the blood (the fluid portion of the
blood excluding cells and clotting factors) and on plasma mem-
branes. They are found circulating in the blood plasma and within
tissues throughout the body. They were named complement by
Ehrlich because they complement the actions of other components of
the immune system (e.g., action of antibody on antigen) in the fight
against infection. Jules Bordet is the discoverer of complement.
Complement proteins create pores in the plasma membrane of the
microbes. Water enters the microbes. The latter burst and die. The
proteins of complement system destroy microbes by (i) cytolysis (ii)
inflammation and (iii) phagocytosis. These proteins also prevent
excessive damage of the host tissues.
Immunity is the ability of the body to protect against all types of foreign bodies like bacteria, virus, toxic substances,
etc. which enter the body. Immunity is also called disease resistance. The lack of immunity is known as susceptibility.
The science dealing with the various phenomena of immunity, induced sensitivity and allergy is called immunology.
Types of Immunity: There are two major types of immunity: innate or natural or nonspecific and acquired or adaptive.
(A) Innate or Natural or Nonspecific Immunity: Innate immunity is inherited by the organism from the parents and
protects it from birth throughout life. As its name nonspecific suggests that it lacks specific responses to specific
invaders. Innate immunity or nonspecific immunity is well done by providing different barriers to the entry of the
foreign agents into our body. Innate immunity consists of four types of barriers— physical, physiological, cellular and
cytokine barriers.
1. Physical Barriers: (a) Skin: The skin is physical barrier of body. Its outer tough layer, the stratum corneum
prevents the entry of bacteria and viruses, (b) Mucous Membranes: Mucus secreted by mucous membrane traps the
microorganisms and immobilises them. Microorganisms and dust particles can enter the respiratory tract with air
during breathing which are trapped in the mucus. The cilia sweep the mucus loaded with microorganisms and dust
particles into the pharynx (throat). From the pharynx it is thrown out or swallowed for elimination with the faeces.
2. Physiological Barriers: The skin and mucous membranes secrete certain chemicals which dispose off the
pathogens from the body. Body temperature, pH of the body fluids and various body secretions prevent growth of
many disease causing microorganisms. Some of the important examples of physiological barriers are as follows: (a)
Acid of the stomach kills most ingested microorganisms, (b) Bile does not allow growth of microorganisms, (c)
Cerumen (ear wax) traps dust particles, kills bacteria and repels insects, (d) Lysozyme is present in tissue fluids and
in almost all secretions except in cerebrospinal fluid, sweat and urine. Lysozyme is in good quantity in tears from
eyes. Lysozyme attacks bacteria and dissolves their cell walls. Lysoenzyme is also found in saliva, (e) Nasal Hair.
They filter out microbes and dust in nose, (f) Urine. It washes microbes from urethra, (g) Vaginal Secretions. It is
slightly acidic which discourages bacterial growth and flush microbes out of vagina, (h) Sebum (sweat). It forms a
protective acid film over the skin surface that inhibits growth of many microbes.
3. Cellular Barriers: These are certain white blood corpuscles (leucocytes), macrophages, natural killer cells,
complement system, inflammation, fever, antimicrobial substances, etc.
(i) Certain Leucocytes: Neutrophils and monocytes are major phagocytic leucocytes.
(ii) Macrophages: Monocytes circulate in the bloodstream for about 8 hours, during which time they enlarge and
then migrate into the tissues and differentiate into specific tissue macrophages. Macrophages are long lived and are
highly motile phagocytic. Macrophages contain more cell organelles especially lysosomes.
(iii) Natural Killer Cells (NK Cells): Besides the phagocytes, there are natural killer cells in the body which are a
type of lymphocytes and are present in the spleen, lymph nodes and red bone marrow. NK cells do not have antigen
receptors like T cells and В cells. NK cells cause cellular destruction in at least two ways: (a) NK cells produce
perforins which are chemicals that when inserted into the plasma membrane of a microbe make so weak that
cytolysis (breakdown of cells particularly their outer membrane) occurs and creates pores in the plasma membrane of
the target cells. These pores allow entry of water into the target cells,
which then swell and burst. Cellular remains are eaten by
phagocytes. (b) Another function of NK cells is apoptosis which
means natural cell death. It occurs naturally as part of the normal
development, maintenance and renewal of cells, tissues and organs.
Thus functions of NK cells are to destroy target cells by cytolysis and
apoptosis. NK cells constitute 5%-10% of the peripheral blood
lymphocytes in humans.
(iv) Complement: Complement is a group of 20 proteins, many of
which are enzyme precursors and are produced by the liver. These
proteins are present in the serum of the blood (the fluid portion of the
blood excluding cells and clotting factors) and on plasma mem-
branes. They are found circulating in the blood plasma and within
tissues throughout the body. They were named complement by
Ehrlich because they complement the actions of other components of
the immune system (e.g., action of antibody on antigen) in the fight
against infection. Jules Bordet is the discoverer of complement.
Complement proteins create pores in the plasma membrane of the
microbes. Water enters the microbes. The latter burst and die. The
proteins of complement system destroy microbes by (i) cytolysis (ii)
inflammation and (iii) phagocytosis. These proteins also prevent
excessive damage of the host tissues.
