All Free Biology MCQs with Answers

211. An antigen is best defined as

• A. any protein made by a white blood cell
• B. a chemical that destroys bacteria directly
• C. a cell that engulfs foreign particles
• D. any molecule the body recognises as foreign and responds to by making antibodies

Explanation: Antigens are usually proteins or polysaccharides on the surface of a pathogen, a pollen grain or a transplanted cell, and their shape is what a particular antibody or lymphocyte receptor fits. The antigen provokes the response and the antibody is the body's answer to it, so confusing the two reverses the whole logic of immunity. Self molecules are normally not treated as antigens because self reactive lymphocytes are eliminated during development.

212. Rheumatoid arthritis and type 1 diabetes are classified as autoimmune diseases because in each of them

• A. the immune system attacks the body's own tissues
• B. the body cannot make any antibody at all
• C. a virus destroys the helper T cells
• D. an allergen provokes the release of histamine

Explanation: Self tolerance breaks down and lymphocytes treat a normal body component as foreign, the synovial membrane in rheumatoid arthritis and the insulin secreting beta cells in type 1 diabetes. The damage is therefore done by the patient's own immune system, which is why treatment suppresses immunity. An inability to make antibody is immunodeficiency, and an exaggerated response to a harmless substance is allergy.

213. The sneezing, watery eyes and swelling of an allergic reaction are caused mainly by

• A. complement proteins puncturing the cell membrane
• B. histamine released from mast cells
• C. cytotoxic T cells destroying the airway lining
• D. the pathogen's own toxins

Explanation: An allergen binds IgE already attached to mast cells, which then release histamine, and histamine dilates blood vessels and makes capillaries leaky, producing swelling, redness and mucus secretion. Antihistamine drugs work precisely by blocking its receptors. Allergy is thus an immune response out of proportion to a harmless substance rather than damage done by a pathogen.

214. The class of antibody able to cross the placenta from mother to fetus is

• A. IgA
• B. IgE
• C. IgG
• D. IgM

Explanation: IgG is the smallest and most abundant immunoglobulin in plasma and the only one transported across the placenta, so it gives the fetus and the newborn several months of borrowed protection. IgA is the antibody of secretions, including breast milk, and protects mucous surfaces after birth. IgE mediates allergy and IgM is the large first antibody made in a primary response.

215. The antibody found in high concentration in colostrum, saliva and tears is

• A. IgG
• B. IgM
• C. IgE
• D. IgA

Explanation: IgA is secreted onto mucous surfaces and into milk, where it neutralises pathogens at the point of entry before they can invade, which is one strong reason for breastfeeding. It works outside the tissues, so it is not the antibody that crosses the placenta. IgM, being very large, stays in the bloodstream, and IgE is bound to mast cells rather than free in secretions.

216. Rejection of a transplanted kidney is brought about mainly by

• A. T lymphocytes recognising the foreign antigens on the graft
• B. histamine released by mast cells
• C. phagocytosis by neutrophils alone
• D. the failure of the graft's own blood supply

Explanation: The donor tissue carries different major histocompatibility antigens, and T cells attack the graft as they would a virus infected cell, which is why patients take immunosuppressive drugs for life and why donors are tissue matched first. This is a cell mediated response, so antibody plays a smaller part. A transplant from an identical twin needs no suppression, because the antigens are the same.

217. T lymphocytes are so named because they mature in the

• A. tonsils
• B. thymus gland
• C. thyroid gland
• D. tissue fluid

Explanation: Both kinds of lymphocyte originate in the bone marrow, but T cells travel to the thymus, where those that would attack the body's own antigens are eliminated and the rest acquire their receptors. B cells complete their maturation in the bone marrow itself, which supplies the other letter. The thymus is large in childhood and shrinks after puberty, once the T cell repertoire is established.

218. Lysozyme, present in tears and saliva, protects the body by

• A. producing memory cells against bacteria
• B. binding specifically to one type of antigen
• C. digesting the cell walls of bacteria
• D. raising the body temperature during infection

Explanation: Lysozyme is an enzyme that breaks the peptidoglycan of bacterial cell walls, so the cell bursts by osmosis, and it acts on any susceptible bacterium without previous exposure. That lack of specificity and of memory is what makes it a second line, non specific chemical defence rather than part of immunity proper. Fever is a separate non specific response coordinated by the hypothalamus.

219. Haemolytic disease of the newborn can occur when

• A. an Rh positive mother carries an Rh negative fetus
• B. both parents are Rh positive
• C. the mother's blood group is AB
• D. an Rh negative mother, already sensitised, carries an Rh positive fetus

Explanation: An Rh negative mother who has previously been exposed to Rh positive fetal blood makes anti D antibodies, and being IgG these cross the placenta in the next pregnancy and destroy the red cells of an Rh positive fetus. The first such pregnancy is usually safe because sensitisation happens mainly at delivery. Giving anti D immunoglobulin after each birth prevents the mother from becoming sensitised at all.

220. Mendel carried out his classical experiments on inheritance using

• A. maize
• B. the garden pea
• C. Drosophila
• D. the snapdragon

Explanation: Pisum sativum was an ideal choice because it has clearly contrasting characters, is normally self pollinating so pure lines are easy to keep, can be cross pollinated by hand, and produces a large number of offspring in one season. Drosophila became the standard organism only later, with Morgan's work on linkage and sex linkage, roughly half a century after Mendel.