All Free Biology MCQs with Answers

691. Antiserum differs from a vaccine in that antiserum

• A. contains weakened pathogens
• B. contains ready made antibodies and gives only temporary protection
• C. produces lifelong immunity
• D. stimulates the formation of memory cells

Explanation: Antiserum lends antibodies for immediate, short lived defence, while a vaccine teaches the body to make its own, lasting protection. Passive versus active is the distinction behind every option here.

692. Vaccination, in which a weakened or killed pathogen is introduced to provoke immunity, is an example of

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

Explanation: The body actively responds and makes memory, but the exposure is deliberate and medical rather than natural infection. All four immunity types differ only in whether exposure is natural or artificial and whether the body or a donor supplies the response.

693. The complement system helps destroy bacteria by

• A. carrying oxygen to the wound
• B. digesting food particles
• C. producing antigens
• D. forming pores in the bacterial membrane that cause lysis

Explanation: Complement proteins assemble into a membrane attack complex that punches holes in pathogen membranes, and also coat microbes for phagocytosis. It is a plasma protein cascade, not a cell type.

694. According to the clonal selection theory, an antigen

• A. creates antibodies from scratch
• B. converts T cells into B cells
• C. selects and stimulates the lymphocyte clone bearing the matching receptor
• D. suppresses every lymphocyte equally

Explanation: The body already holds millions of lymphocyte clones, each with one receptor; the antigen simply picks the matching one and drives its expansion. Selection, not instruction, is the theory's key word.

695. Skin, mucus secretions and lysozyme in tears belong to the

• A. first line of defence
• B. second line of defence
• C. third line of defence
• D. adaptive immune system

Explanation: These physical and chemical barriers stop most pathogens from entering at all, before any cellular response is needed. Phagocytosis and inflammation form the second line, and specific immunity the third.

696. The second line of defence against pathogens includes

• A. antibody production by plasma cells
• B. phagocytosis, inflammation and fever
• C. the action of memory cells
• D. vaccination

Explanation: These responses act rapidly and without targeting a specific antigen, attacking anything foreign. Antibodies and memory belong to the specific third line, which is slower but precise.

697. When a large proportion of a population is vaccinated, even unvaccinated individuals are protected because the pathogen cannot spread easily. This is called

• A. cross immunity
• B. passive immunity
• C. herd immunity
• D. autoimmunity

Explanation: Vaccinated people act as dead ends for transmission, shielding those too young or too ill to be vaccinated. Herd immunity is the public health reason vaccination coverage matters beyond the individual.

698. Which of the following correctly describes what happens after a vaccine is administered?

• A. memory cells are destroyed to reset the system
• B. the antigens multiply and cause the full disease
• C. the vaccine replaces the plasma proteins
• D. antigens stimulate lymphocytes, producing plasma cells and memory cells

Explanation: The weakened or dead antigens cannot cause disease but are presented to lymphocytes, which respond exactly as in infection, leaving memory behind. This controlled rehearsal is the entire principle of vaccination.

699. Mendel chose the garden pea for his experiments partly because it

• A. has many easily distinguished contrasting characters
• B. has an unusually long life cycle
• C. cannot self pollinate
• D. blends its characters in every generation

Explanation: Peas offer clear alternatives such as tall versus dwarf, self pollinate to give pure lines and produce many offspring quickly. These practical features let Mendel count ratios, which is where his laws came from.

700. The observable characteristics of an organism, produced by its genes and the environment, are called its

• A. genotype
• B. phenotype
• C. karyotype
• D. pedigree

Explanation: Phenotype is what can be seen or measured, while genotype is the allele combination behind it. Two plants can look identical yet differ in genotype, which is exactly why a test cross is needed.