Describe the role of blood in defending the body against disease.
Firstly, the blood contains cells called B-lymphocytes which produce antibodies. These antibodies recognize antigens, causing agglutination of disease-causing bacteria and thus ensure the digestion of these bacteria. Secondly, the blood contains many cells such as neutrophils, basophils and eosinophils which are involved in the inflammatory response, defending the body against bacteria, parasites and other foreign particles whenever there is a cut in the skin. Lastly, the blood contains platelets and clotting factors which cause blood clotting at the site of an open wound. This physically covers the wound and thus prevents the bacteria from having free and easy access to the body. The white blood cells (leucucytes) has phagocytes and lymphocytes. The phagocytes engulf disease causing bacteria in the blood at sites of infection and lymph nodes while the lymphocytes produces antibodies(proteins)against antigens (foreign proteins)made by disease causing bacteria. Antibodies cause bacteria to disintegrate or clump together so they cannot produce and eventually die. When the skin is cut, damaged tissue and platelets on exposure to air produce a chemical called thromboplastin.Thromboplastin and calcium ions start a series of reactions which finally turn the soluble plasma protein fibrinogen into insoluble firbin.Fibrin forms a network of fibres over the ct which trap blood cells forming a clot. The clot prevents further blood loss and entry of disease causing organisms. When dry the clot forms a scab in which a new skin forms below the scab which drops off.

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