Natural killer cell is a type of white blood cell or lymphocyte. Chemical substances such as cytotoxin, perforin, granzyme are secreted from these cells. These chemicals create holes in the membrane of virus-infected cells and tumor cells. Water enters the cell through these pores. This destroys the cell’s nucleus and the cell explodes.

The white blood cells that defend the body by eating microorganisms and foreign objects are called phagocytes. They destroy microbes by the process of phagocytosis. They feed on large amounts of bacteria in the diseased area, die and produce pus. The body contains phagocytic cells such as microphages (small), neutrophils, macrophages (large).

1. Phagocytes: The white blood cells that defend the body by eating microorganisms and foreign objects are called phagocytes. They destroy microbes by the process of phagocytosis. They feed on large amounts of bacteria in the diseased area, die and produce pus. The body contains phagocytic cells such as microphages (small), neutrophils, macrophages (large).
2. Natural Killer Cell, NKC : Natural killer cell is a type of white blood cell or lymphocyte. Chemical substances such as cytotoxin, perforin, granzyme are secreted from these cells. These chemicals create holes in the membrane of virus-infected cells and tumor cells. Water enters the cell through these pores. This destroys the cell’s nucleus and the cell explodes.
3. Interferon: Interferon is a high molecular weight glycoprotein. It is produced by virus-infected cells and prevents the virus from multiplying. This interferon attaches to the membranes of other healthy cells and helps those cells produce new interferons. In this way, the body builds a protective wall.
4. Complement System: Complement system is a group of 20 types of plasma proteins. These proteins remain inactive and are later converted into active proteins. Activating proteins induce specific and non-specific defense mechanisms. The complement system always helps cells such as NK cells, neutrophils, macrophages, etc. to destroy pathogens.
5. Inflammation: In the attack of bacteria, the wound of the cells becomes red, hot, swollen and painful. This is called inflammation or response. Histamine is released when a cell is attacked by bacteria. Histamine causes numerous pores in the capillary tube. Macrophages, neutrophils etc. enter the cell through these pores and consume microbes, toxins and dead cells.
6. Blood clotting: When a wound occurs in any part of the body, the molecule quickly prevents blood clotting and closes the wound. As a result, germs cannot enter the wound.
7. Cytokines: release antigens after entering the microbial cells. Cytokines are released from cells to destroy bacterial antigens. Cytokines stimulate the body’s immune system.
8. Fever: When macrophages attack viruses, bacteria and foreign objects, pyrogens are released from the cells. Pyrogens cause rise in body temperature and fever. Germs cannot multiply at high temperatures. The immune system of the body increases.

The layer of defense that prevents foreign microbes from entering the body from outside is called the first layer of defense. It is a non-specific defense layer.

1. Skin: Sweat and oil of the skin acts as poison for bacteria. Prevents the growth of sweat and acid bacteria on the skin. The skin’s lysozyme enzyme destroys germs.
2. Hair: The hair inside the nose prevents dust and germs from entering.
3. Cerumen: Cerumen is a waxy substance known as ear wax. It is secreted from ceruminous glands. It resists bacteria, fungi and insects.
4. Cilia: Trachea contains mucus and cilia. It breaks down the germs and expels them out of the body as a sneeze-cough-spit.
5. Tears and saliva: Tears and saliva contain lysozyme and histatin. These chemicals destroy bacteria.
6. Alimentary acid: Hydrochloric acid and proteolytic acid secreted from the stomach destroy food microbes.
7. Fecal and genital acid: Bacteria present in the vagina produce lactic acid. This lactic acid inhibits the growth of other bacteria.
8. Microbiome: Friendly microorganisms that live in the body help destroy disease-causing microbes.
9. Excretion and Vomiting: Numerous germs are released from the body through excrement and vomiting.

The system in which the specific germs that enter the body are destroyed by white blood cells or antibodies is called specific defense. This is called the acquired or adaptive immune system.

The mechanism by which any germs are prevented from entering the body or any germs that have entered are destroyed is called non-specific defence. This is called the innate or innate immune system.

The immune system of the human body can be divided into two phases. namely-

1. General defense or innate defense or non specific defense
2. Definite Defense or Acquired Defense or Specific Defence
3. General Defense or Non-Specific Defence: The mechanism by which any germs are prevented from entering the body or any germs that have entered are destroyed is called non-specific defence. This is called the innate or innate immune system.
4. Specific defense: The system in which the specific germs that enter the body are destroyed by white blood cells or antibodies is called specific defense. This is called the acquired or adaptive immune system.

The process by which the body is made resistant to diseases by destroying various types of germs in the body is called Immunity. The branch of biology that deals with Immunity is called Immunology.

High yielding varieties of wheat have been developed by hybridization between different cultivars of wheat. American scientist Norman Earnest Borlaug won the Nobel Prize in 1970 for the invention of high yielding wheat. Bangladesh Rice Research Institute (BRRI) in collaboration with Mexico’s research institute CIMMIT has developed 17 varieties of wheat. High yielding wheat varieties are – Balaka, Akbar, Kanchan, Barkat and Saugat. Besides, BR-27 and BR-28 wheat are more improved.

In the 1960s, scientists at the International Rice Research Institute (IRRI) in the Philippines developed IRRI rice. IRRI-8 rice was developed by crossing Peta rice from Indonesia and Dee-gee-woo-gen rice from Taiwan. The yield of this rice is 90-100 maunds per acre. Erichile rice was developed by breeding Peta rice from Indonesia, TKM-6 rice from India and Tichu-1 rice from Taiwan. The yield of this rice is 70-75 maunds per acre. IRRI-5 yield of paddy is 70-75 maunds per acre. High yielding IRRI rices are IRRI-20, IRRI-28, IRRI-29 etc.

Bangladesh Rice Research Institute-BRRI developed high yielding Biri rice. Birishail was developed by hybridizing BR-20 and BR-3. The four Ufshi rice varieties developed by the Bangladesh Rice Research Center are Chandina (BR-1), Mala (BR-2), Shahi Balam (BR-15) and Shrabani (BR-26). Earlier varieties cultivated in Bangladesh had a yield of 30-35 maunds per acre. Currently cultivated high yielding varieties yield 70-90 maunds per unit. BR-28 and BR-29 are more advanced varieties. Rice production in Asia has increased at least 4-fold in the last 40 years.

BRRI-28 and BRRI-29 are the most popular rice varieties for farmers in Boro season. Currently both these breeds have reduced immunity and are easily affected by blast disease. BRRI-68, 81, 86, 88, 96, 101, 105 and Bangabandhu 100 are cultivated as alternatives to BRRI-28. BRRI-89, 92, 97, 99, 102, Bangabandhu 100, BINA Bina-25 and BINA-10 in saline areas are cultivated as alternatives to BRRI-29.