Defense Mechanism

Category : 12th Class

Immune response : Nature has provided certain ways in the body to defend ourselves from the invention of pathogens and therefore, from the disease. The ability of a host’s body to prevent or overcome the effects caused due to the invention by pathogenic organisms and its toxins is known as resistance and immunity. Resistance is considered as an inherent factor and those acquired during life to overcome the disease, while the immunity is accepted to be due to the acquired factors that help in resistance. The host body has two lines of defence that must be overcome by a pathogen before establishing an infection.

External defence mechanism : This defence mechanism involves mechanical and chemical factors e.g. skin, mucous membrane, mucous secretion, peristalsis, coughing, sneezing, shedding tears, etc. Chemicals are lysozymes present in the body.

Internal defence mechanism : This mechanism of defence has two lines of defence against pathogen :

(1) Non-specific Defence Mechanism : It is further of two types : external defence or first line of defence and internal defence or second line of defence.

(i) External Defence : It includes physical and chemical barriers.

(a) Physical Barriers

  • Skin : The skin is physical barrier of body. Its outer tough layer, the stratum corneum prevents the entry of bacteria and viruses.
  • Mucous Membrane : 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.

(b) Chemical barriers : Oil secreted by the oil glands and sweat secreted by sweat glands make the surface of the skin acidic \[(ph\,\,3-5).\] This does not allow the microorganisms to establish on the skin. Some friendly bacteria also occur on the skin which releases acids and other metabolic wastes that check the growth of pathogens. The sweat also contains an enzyme named lysozyme that destroys the cell walls of many bacteria.

The mesh of fine hair in our nostrils filters out particles which may carry pathogens. Nasal secretions also destroy the harmful foreign germs with their lysozyme.

Certain bacteria normally live in vagina. These bacteria produce lactic acid. Lactic acid kills the foreign bacteria.

Thus physical and chemical barriers form the first line of defence.

(ii) Internal Defence : The internal defence is carried on by white blood corpuscles, macrophages, inflammatory reaction, fever and interferons.

(a) White blood corpuscles (Leucocytes) : The leucocytes in general and lymphocytes in particular are capable of squeezing out through the wall of the blood capillaries into the extra-vascular regions. This phenomenon is called diapedesis. The leucocytes protect in different ways.

  • Lymphocytes : Lymphocytes can produce plasma cells which secrete antibodies to provide immunity.
  • Monocytes : They are phagocytic in action.
  • Eosinophils : Eosinophils can attach themselves to parasitic forms and cause their destruction by liberating lysosomal enzymes on their surface.
  • Neutrophils : They eat harmful germs and are, therefore phagocytic in nature.

(b) Macrophages : The macrophages are formed by enlargement of monocytes. They are large cells which are phagocytic in nature.

(c) Inflammatory Response : When the microorganisms like bacteria, viruses, etc. enter the body tissue through some injury, these produce some toxic substances which kill more cells. These broken cells also release some material which attract the mast cells. The mast cells release histamine. Histamine causes dilation of capillaries and small blood vessels surrounding the injury and increases the permeability of the capillary walls. The more blood flows to area making it red and warm. The fluid (plasma) leaks out into the tissue spaces, causing its swelling. This reaction of the body is known as inflammatory response. The plasma that accumulates at the injured site dilutes the toxins secreted by bacteria and decreases their effect.

(d) Fever : The inflammatory response may be in the region of the wound (localized), or it may spread all over the body (systemic). In systemic inflammatory response, the number of WBC increases generally, the fever is caused by the toxins released by the pathogens or by compounds called pyrogens (fever producing substances; Gr. Pre = fire). These compounds are released by W.B.C. in order to regulate temperature of the body. Moderate fever stimulates the phagocytes and inhibits growth of microorganisms. However, a very high fever is dangerous.

(e) Interferons : These are the proteins released by the cells in response to a viral infection which they help to combat. These interferons do not inactivate the virus, but they make the unattacked cells less susceptible so they are prevented from the attack of virus. They also prevent the viruses from taking over the cellular machinery. Interferon proteins have proved to be effective in, treating influenza and hepatitis, but their role in cancer treatment is doubtful. Thus the leucocytes. Macrophages, inflammatory response, fever and interferons from second line of defence.


Differences between Antibodies and Interferons



These act inside the cells.

They are slow acting.

They act against bacteria and viruses.

Their action is long lasting

These act outside the cells.

They are quick acting.

They act only against viruses.

Their action is temporary.


(2) Specific Defence Mechanism (The Immune System) : Immune system forms third line of defence. There are two components of immune system in the body : Humoral immune system and cell-mediated immune system. One of the most important characteristics of the immune system is that it can recognize body’s own cells and macromolecules (self) from those which are foreign invaders (nonself).

(i) Humoral Immune System or Antibody-mediated Immune system (AMIS) (Humoral  : Pertaining to body fluids): Humoral immune system results in production of antibodies. These antibodies circulate as soluble proteins in the plasma of blood and lymph which were earlier called humors. The humoral system protects the body against bacteria and viruses that enter the blood and lymph of the body. Antibodies are of many kinds.

(ii) Cell-mediated Immune System (CMIS) : In this system, highly specialized cells carry out defensive activities. These circulate in the blood and tissue. It protects the body against pathogens including the protists and fungi which have entered the host’s cells. This system also reacts against tissue transplants and perhaps also against the body’s own cells if they become cancerous. Two kinds of cells (T and B cells) are responsible for these responses.

The antigens are foreign ‘molecules’ that invade the body of an organism. The word ‘antigen’ is a shortened form of ‘antibody generating’ because they stimulate the production of antibodies in response to infection. Antigens are generally large molecules. The majority of them are made of proteins or polysaccharides found on the cell walls of bacteria and other cells or on the coats of viruses. All antigens are not the parts of microorganisms. Other structures like pollen grains, white of an egg, shell fish, certain fruits and vegetables, chicken, feathers of birds, blood cells from other persons or animals, drugs, chemicals, etc. can also induce the immune system to produce antibodies.


Types of Antibodies






Main antibody type in circulation; attacks microorganisms and their toxins.



Main antibody type in secretions, such as saliva and milk; attacks microorganisms and their toxins.



Antibody responsible for allergic reactions.



Antibody type found in circulation; largest antibody, with 5 subunits,



Antibody type found primarily as a membrane bound immunoglobulin.


Cells of the Immune System : Lymphocytes (a type of WBCS) are the main cells of immune system of the body. Lymphocytes, meant for immune system, are of two types : T-cells and B-cells. Both types of cells develop from the stem cells found in the liver of the foetus and in the bone marrow cells of the adult. Those lymphocytes that migrate to the thymus and differentiate under its influence are called ‘T-cells’, while those cells that continue to be in the bone marrow for differentiation are known as ‘B-cells’. The final maturation of young lymphocytes occur in lymphoid tissues like lymph nodes, spleen and tonsils. T-cells are responsible for cellular immunity, however, B-cells produce the antibodies-about 20 trillions per day that take part in the humoral immunity. Both T-cells and B-cells require antigens to trigger them into action but they respond differently.

B-lymphocytes are independent of the thymus and in man probably complete their early maturation within the bone marrow. They are called B-cells because they mature within the Bursa of Fabricius in birds


Cells of Immune System


Cell Type



Helper T Cell

Assists the immune process by helping other cells in the immune system to achieve an efficient immune response.


Cytotoxic T Cell

Detects and kills infected body cells recruited by helper T cells.


Suppressor T Cell

Guards against the overproduction of antibodies and overactivity of cytotoxic T cells.


Memory cell

?Remembers? the original stimulation by the immune system and remains in the lymphoid tissue.


Natural killer cell (NK)

The lymphocyte without receptor site and help to attack and neutralize virus-infected and tumor cells.


B Cell

Precursor of plasma cell, specialized to recognize a specific foreign antigen.


Plasma cell

Biochemical factory devoted to the production of antibodies directed against a specific antigen.


Mast cell

Initiator of the inflammatory response which aids the arrival of leucocytes at a site of infection, secretes histamine and is important in allergic response.



Precursor of macrophage.



The body?s first cellular line of defence; also serves as antigen presenting cell to B and T cells and engulfs antibody covered cells.


(1) Mode of Action of B-Cells to Antigens : When antigens enter a tissue fluid, B-cells are stimulated to produce antibodies. The body has thousands of antigen-specific B-cells. The membrane of each B-cell type would have been sensitized by the previous contact with the antigen. If this does not happen, the B-cells are destroyed. However, the new B-cells will keep on producing. Once an antigen-specific B-cell is activated by the antigen it multiplies very fast to form a clone of plasma cells. These plasma cells produce antibodies at a rate of about 2,000 molecules per second. This ‘capacity’ of the B-cells to produce specific antibodies is acquired during its process of development and maturation even before it was exposed to an antigen. However, an antigen is necessary to stimulate the production of antibodies.

(2) Mode of Action of T-cells to Antigens : Like B-cells, T-cells also respond to antigens by producing a clone (a group) of T-cells. T-cells live for 4-5 years or even longer. There are separate T-cells for each type of antigen that invades the body. T-cells of a clone that are produced in response to an antigen are similar morphologically but they perform different functions. According to their functions, they are of three types.

(i) Killer T-cells : These cells attack directly and destroy antigens. In the process, these cells move to the site of invasion and produce chemicals that attract phagocytes and stimulate them so that they can feed more vigorously on antigens. They also produce substances that attract other T-cells.

(ii) Helper T-cells : These cells stimulate B-cells to produce more of antibodies.

(iii) Suppressor T-cells : These cells suppress the entire immune system keeping it away from attacking the own body cells. Some of these cells also become memory cells.


Distribution of B- and T-Cells in Man

S.No. Tissue B-Cells % T-Cells %










Bone marrow




Thoracic duct




Lymph nodes




Thymus gland




Differences between B-Lymphocytes (B-Cells) and T-Lymphocytes (T-Cells)

S.No. Feature

B-Lymphocytes (B-cells)

T-Lymphocytes (T-cells)


Origin and site of differentiation

Bone marrow Bursa of Fabricus (in fowl), gut-associated lymphoid tissue (Peyer?s patches)

Bone marrow Thymus


Immune System

B-cells form humoral or antibody-mediated immune system (AMIS).

T-cells form cell-mediated immune system (CMIS).



They defend against viruses and bacteria that enter the blood and lymph.

They defend against pathogens including protists and fungi that enter the cells.



They are formed by the division of plasma cells.

They are formed by the division of lymphoblasts of three types : killer, helper and suppressor cells.



Plasma cells do not move to the site of infection.

Lymphoblasts move to the site of infection.


Reaction against Transplants and cancer cells

Plasma cells do not react against transplants and cancer cells.

Killer cells react against transplants and cancer cells.


Effect on Immune System

Plasma cells have no inhibitory effect on immune system.

Suppressor cells inhibit immune system.

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