(i) HCI of stomach belongs to the physiological barrier.
(ii) Macrophages come under cellular barrier.

(a) Contact inhibition is the property shown by normal cells,by the virtue of which contact with other cells inhibits their uncontrolled proliferation and growth.
(b) All normal cells have cellular oncogenes (c-onc) or proto-oncogenes;when activated under certain conditions,such genes could lead to oncogenic transformation of cells,i.e.they become cancerous.

(i) Cancer cells grow rapidly,invade and damage the surrounding normal cells.
(ii) They starve the normal cells by competing for vital nutrients.
(iii) Some cells get sloughed off and reach distant sites through body fluids and start a new (secondary) tumour there,i.e. metastasis.

1. Tetanus antitoxin,i.e. preformed antibodies for tetanus are injected,as the child infected with deadly microbe (tetanus bacteria) to which quick immunisation response is required.
2. If tetanus vaccine is injected,it will take some time for the body to develop antibodies and this can lead to a fatal end.

1. Active immunity is produced by injecting the microbes deliberately during immunisation.
2. Antibodies are produced by the B-cells of our body in response to the antigens;they neutralise the pathogenic agents during actual infection.
3. The vaccines also generate memory B-cells and T-cells,which recognise the same pathogen on subsequent exposure and destroy them by a massive production of antibodies.