biology4alevel600

1 Phagocytes and lymphocytes are the cells of the immune system. 2 The diagram shows how to recognise phagocytes and lymphocytes in microscope slides and photomicrographs of blood. 3 Phagocytes originate in the bone marrow and are produced there throughout life. There are two types: neutrophils that circulate in the blood and enter infected tissues, and macrophages that remain inside tissues. They both destroy bacteria and viruses by phagocytosis. 4 Antigens are ‘foreign’ (non-self) macromolecules that stimulate the immune system. 5 Lymphocytes also originate in bone marrow. There are two types: B-lymphocytes (B cells) and T-lymphocytes (T cells). As they mature, B cells and T cells gain glycoprotein receptors that are specific to each cell. Each cell divides to form a small clone of cells that spreads throughout the body in the blood and in the lymphoid tissue (e.g. lymph nodes and spleen). 6 B cells mature in bone marrow. T cells mature in the thymus gland. During maturation, many T ...

A person is immune to a disease if the pathogen that causes the disease is unable to reproduce in the body and cause illness. This happens when the body already contains, or is able rapidly to make, large quantities of antibodies against the antigens associated with the pathogen. Immunity Active immunity   occurs when the person's own lymphocytes make the antibody. This could be  natural,  as a result of the person having previously had the disease and forming B or T memory cells. It could also be  artificial , as a result of  vaccination . This involves introducing weakened pathogens into the body. The lymphocytes react to the antigens on the pathogens by producing antibodies and memory cells. Passive immunity  occurs when antibodies from elsewhere are introduced into the body. In a young baby this can be  natural , as the baby acquires antibodies from its mother in breast milk. It can also be  artificial , as the result of an injection of antibo...