The correct answer is option 3. When an ill person is exposed to an antigen, their body responds by increasing the production of anti-serum against bacteria.
Option 1 is incorrect because antigens do not directly increase the production of white blood cells (WBC). WBC production may increase as a result of the immune response to the antigen, but this is not the direct effect of the antigen itself.
Option 2 is incorrect because antigens do not increase the production of antibiotics. Antibiotics are medications that are used to treat bacterial infections and are not produced by the body in response to antigens.
Option 4 is incorrect because antigens do not prevent the growth of bacteria. The immune response to antigens, which includes the production of anti-serum against bacteria, helps to fight off bacterial infections, but does not directly prevent the growth of bacteria.
In summary, the effect of antigens in an ill person is to increase the production of anti-serum against bacteria.