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Dear Student

(a)
Antibodies are naturally produced in the body by the immune system in response to an invading germ or pathogen. Antibiotics are chemically purified substances obtained from plants or microorganisms which produce them naturally to target infections.
Antibodies have two large and two small chain of proteins which are bonded to give a Y shaped structure. Antibiotics can have varied chemical structures.

(d) Leukocytes are white blood cells. They are larger than RBCs and do not have haemoglobin. They fight against germs that enter the body. Thus, they protect the body from diseases.
The main function of erythrocytes is to transport oxygen to the tissues of the body and carbon dioxide from the tissues to the lungs. Erythrocytes also help in maintaining the pH of blood by regulating the carbon dioxide concentration in it.  

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