Free Immunity MCQs with Answers

1. Which of the following belongs to the first line of defence against infection?

• A. Production of antibodies by plasma cells
• B. Intact skin and the mucous membranes of the body openings
• C. Activation of cytotoxic T lymphocytes
• D. Formation of memory cells after an infection

Explanation: The first line is a physical and chemical barrier that stops pathogens entering at all: unbroken skin, mucus that traps microbes, cilia that sweep them out and acidic secretions. It is non specific, acting the same way against every organism, and it needs no previous exposure. Antibodies, T cells and memory cells all belong to the third line, the specific immune response.

2. Antibodies are secreted by

• A. plasma cells derived from B lymphocytes
• B. cytotoxic T lymphocytes
• C. macrophages in the lymph nodes
• D. neutrophils in the blood

Explanation: When a B lymphocyte meets its matching antigen it divides, and most of the daughter cells become plasma cells packed with rough endoplasmic reticulum for mass producing one type of antibody. T cells act directly on infected cells and secrete cytokines but never antibody, which is the usual confusion. Macrophages present antigen to lymphocytes rather than making antibody themselves.

3. An antibody molecule consists of

• A. a single polypeptide chain
• B. two identical polypeptide chains
• C. four polypeptide chains, two heavy and two light, arranged in a Y shape
• D. a chain of nucleotides folded into a Y shape

Explanation: The two arms of the Y carry the variable regions that give each antibody its specific antigen binding sites, while the constant stem determines the class and how the antibody is disposed of. Antibodies are immunoglobulins, that is proteins, so a nucleotide structure is impossible. The two binding sites are what allow an antibody to clump antigens together in agglutination.

4. Cell mediated immunity is carried out mainly by

• A. antibodies dissolved in the plasma
• B. B lymphocytes
• C. complement proteins
• D. T lymphocytes

Explanation: T cells act on cells rather than on free antigen, destroying virus infected cells, cancer cells and transplanted tissue by direct contact, which is why this branch is called cell mediated. Humoral immunity is the other branch, in which B cells give rise to plasma cells whose antibodies circulate in body fluids. The two branches cooperate, since helper T cells are needed to activate most B cells.

5. A cell infected by a virus is destroyed by

• A. helper T cells
• B. cytotoxic T cells
• C. plasma cells
• D. free antibody in the plasma

Explanation: Cytotoxic T cells recognise viral antigen displayed on the surface of an infected cell and release perforin, which punches holes in its membrane so that the cell dies before the virus finishes replicating. Antibody cannot reach a virus that is already inside a cell, which is exactly why this branch of immunity exists. Helper T cells coordinate the response but do not kill.

6. Helper T cells are central to immunity because they

• A. activate B cells and cytotoxic T cells by secreting cytokines
• B. engulf and digest bacteria
• C. secrete large amounts of antibody
• D. produce histamine during an allergic reaction

Explanation: Helper T cells sit at the centre of the response, recognising antigen presented by macrophages and then releasing chemical signals that switch on both the antibody producing and the cell killing arms. Their loss is why HIV, which destroys them, disables immunity as a whole rather than only one part of it. Engulfing bacteria is the job of phagocytes and histamine comes from mast cells.

7. The secondary immune response is faster and much stronger than the primary response because

• A. the antigen is weaker the second time
• B. the skin has become impermeable to that pathogen
• C. memory cells produced in the first exposure are already present in large numbers
• D. antibodies from the first exposure are still circulating in the same amount

Explanation: A first exposure leaves behind long lived memory B and T cells specific to that antigen, so a second exposure finds a large clone ready to divide and antibody rises within a day or two instead of a week. Antibody from the first response falls to a low level over time, so persistence of antibody is not the main explanation. Vaccination and booster doses work by deliberately creating this memory.

8. Vaccination against measles gives

• A. natural passive immunity
• B. artificial passive immunity
• C. natural active immunity
• D. artificial active immunity

Explanation: It is artificial because the antigen is given deliberately rather than met by chance, and active because the person's own lymphocytes respond and form memory cells, so protection is slow to appear but lasts for years. Passive immunity means receiving ready made antibodies, which acts at once but fades within weeks and leaves no memory. Recovering from an actual attack of measles gives natural active immunity.

9. The antibodies a newborn baby receives across the placenta and in its mother's milk provide

• A. natural passive immunity
• B. natural active immunity
• C. artificial passive immunity
• D. no immunity at all

Explanation: The baby is given ready made antibody rather than making its own, so the protection is passive, and because it comes through a natural route rather than an injection it is natural. It covers the first few months while the infant's own immune system is still immature, then disappears, which is why vaccination begins early. An injection of antitetanus serum is the artificial version of the same thing.

10. A vaccine usually contains

• A. ready made antibodies against the pathogen
• B. killed or weakened pathogens, or antigens taken from them
• C. antibiotics that kill the pathogen
• D. living virulent pathogens in a small dose

Explanation: The point of a vaccine is to present the antigen without causing the disease, so the organism is killed, attenuated, or replaced by one of its surface proteins or an mRNA coding for it. The immune system responds as though to a real infection and leaves memory cells behind. A preparation of ready made antibodies is an antiserum, which treats rather than prevents and gives no lasting protection.