Free Specific Defense Mechanism MCQs with Answers

21. B lymphocytes, the cells responsible for antibody production, mature in the

• A. bone marrow
• B. thymus
• C. tonsils
• D. spleen only

Explanation: B cells develop and mature in the bone marrow before settling in lymphoid tissue, while T cells migrate to the thymus to mature. The letters B and T simply recall these two organs of maturation.

22. Immunity brought about by antibodies circulating in the blood and lymph is called

• A. cell mediated immunity
• B. humoral immunity
• C. innate immunity only
• D. autoimmunity

Explanation: Humoral immunity works through soluble antibodies produced by B cells, while cell mediated immunity relies on T cells attacking directly. The two arms cooperate, but examiners keep the definitions strictly apart.

23. After an infection is cleared, some lymphocytes remain for years and respond rapidly on re-exposure. These cells are called

• A. phagocytes
• B. red blood cells
• C. memory cells
• D. platelets

Explanation: Memory B and T cells are the biological basis of lasting immunity and of the booster effect. On a second encounter they multiply so fast that symptoms often never develop.

24. An antibody binds only to the antigen that stimulated its production because

• A. all antibody molecules are identical
• B. antigens destroy competing antibodies
• C. the blood filters out mismatched antibodies
• D. the shape of its binding site matches a specific epitope of that antigen

Explanation: Antibody arms carry binding sites shaped for one epitope, like a lock for one key, which is why each clone makes only one specificity. Uniform antibodies would make targeted immunity impossible.

25. White blood cells such as neutrophils and macrophages that engulf and digest bacteria are called

• A. phagocytes
• B. lymphocytes
• C. platelets
• D. mast cells

Explanation: Phagocytes flow around a particle, internalise it in a vacuole and digest it with lysosomal enzymes. Lymphocytes instead run the specific response, and platelets clot blood, so function names the cell.

26. A person who recovers from measles is immune to it for life. This is an example of

• A. artificially active immunity
• B. naturally active immunity
• C. naturally passive immunity
• D. artificially passive immunity

Explanation: The person's own immune system met the real pathogen and built memory, so the immunity is active and natural. Vaccination is active but artificial, and antiserum or maternal antibodies are passive.

27. A patient bitten by a venomous snake is injected with antiserum containing ready made antibodies. This provides

• A. active immunity lasting many years
• B. a vaccination effect
• C. immediate but temporary passive immunity
• D. permanent genetic resistance

Explanation: Borrowed antibodies neutralise the venom at once but are gradually broken down over weeks, giving no memory. In an emergency that speed is worth more than durability, which is exactly why antiserum is used.

28. The part of an antibody molecule that binds the antigen is located in the

• A. constant region of the heavy chains
• B. Fc tail
• C. carbohydrate side chains
• D. variable region at the tips of the two arms

Explanation: The amino acid sequence of the variable region changes from one antibody to another, shaping the binding site, while the constant tail determines class functions such as binding phagocytes. Structure and specificity meet at the tips.

29. Booster doses of a vaccine are given in order to

• A. reduce the side effects of the first dose
• B. raise the antibody level and strengthen immunological memory
• C. replace antibodies that have expired
• D. treat an existing infection

Explanation: Each booster re-stimulates memory cells, driving antibody concentration higher and memory longer, the classic secondary response curve. Vaccines prevent rather than treat, so a booster is never therapy.

30. Histamine released by mast cells at a wound causes

• A. vasodilation and increased capillary permeability, producing inflammation
• B. clotting of the lymph
• C. a fall in body temperature
• D. immediate antibody production

Explanation: Histamine opens the local vessels so plasma and phagocytes reach the injured site, causing the redness, heat and swelling of inflammation. This chemical alarm is part of the rapid non specific response.