question_answer

(i) What are lymphoid organs? Write name of any four lymphoid organs and their functions.

(ii) On what basis primary lymphoid organs and secondary lymphoid organs are categorised?

Or

(i) What is cancer? How do normal cells get transformed into cancerous neoplastic cells? Mention the difference between viral oncogenes and cellular oncogenes.

(ii) Write the major properties of cancer cells.

Answer:

(i) Lymphoid organs are those organs where the maturation and proliferation of lymphocytes take place. The four lymphoid organs and their functions are as follows
(a) Bone marrow It is a major lymphoid organ where all blood cells, including lymphocytes are formed. It is site of B-lymphocytes maturation.
(b) Thymus It is a lobed organ located near the heart and beneath the breast bone, T - lymphocytes mature inthymus and are responsible for both cellular and humoral immune response.
(c) Spleen It is a large bean - shaped organ comprising of single mass of lymphoid tissues. In foetal stage, it produces all type of blood cells, but only lymphocytes in adult stage. It filters the blood by trapping blood borne microorganism.
(d) Lymph nodes These are small solid structures composed of lymphoid tissue. They produce lymphocytes and plasma cells and also act as filters to trap microorganisms or other antigens that enter the lymph and tissue fluid.
(ii) In the primary lymphoid organs both B and T - lymphocytes mature and acquire their antigenic specificity, e.g., bone marrow and thymus. However, in the secondary lymphoid organs, lymphocytes undergo proliferation and differentiation. These are the sites of acquired immune response to antigens and formation of effector cells, e.g. spleen and lymph nodes.
Or
(i) Cancer is an abnormal and uncontrolled division of cells, known as cancer cells, that invade and destroy the surrounding tissues. Cancer is defined as uncontrolled proliferation of cells without any differentiation.
Differentiation of normal cells into cancer cells Normal cells show a property called contact inhibition. Due to this property they contact with other cells and inhibit their uncontrolled growth. Cancer cells seem to have lost this property. The cancer cells do not respond to normal growth control mechanism. These cells proliferate in an unregulated manner and form a mass of cells having uncontrolled growth called tumour or neoplasm.
Difference between viral oncogenes and cellular oncogenes is as follows
Viral oncogenes	Cellular oncogenes
Genes of viruses which cause cancer.	Genes present in normal cells, which become activated under certain conditions and cause oncogenic transformation of the cell.
(ii) Properties of cancer cells are as follows
I. Uncontrolled proliferative ability.
II. Extracellular growth factors are not required.
III. Overgrowth and ability to invade new sites (metastasis).
IV. Cancer cells resist induction of cell death which promotes development of tumours.