CIE A Level Biology Topic 11: Immunity Exam Questions

About CIE A Level Biology Topic 11: Immunity

Immunity in the Cambridge 9700 syllabus explains how the body defends itself once a pathogen gets past the external barriers. You need to describe how phagocytes (neutrophils and macrophages) engulf pathogens, how antigens trigger the specific response, and the roles of B lymphocytes, plasma cells, T helper cells, T killer cells and memory cells. The structure of antibodies and how they act is a regular exam favourite. You also need to distinguish active and passive immunity, natural and artificial, and explain how vaccination works. The topic builds on the infectious diseases studied in Topic 10 and on cell signalling and mitosis from earlier topics.

What you need to know

• Phagocytes Neutrophils and macrophages are made in the bone marrow. They engulf pathogens by phagocytosis into a phagosome, which fuses with lysosomes whose enzymes digest the pathogen. Macrophages present antigens to lymphocytes.
• Antigens and lymphocytes An antigen is a molecule that triggers an immune response. B lymphocytes mature in the bone marrow and T lymphocytes in the thymus. Each lymphocyte has receptors for one specific antigen.
• B cells When a B cell meets its antigen and is stimulated by T helper cells, it divides by mitosis (clonal expansion) into plasma cells, which secrete antibodies, and memory cells.
• T cells T helper cells release cytokines that stimulate B cells and phagocytes. T killer cells attach to infected cells displaying the antigen and destroy them.
• Antibodies Antibodies are glycoproteins with two heavy and two light polypeptide chains joined by disulfide bonds, a constant region and two identical variable regions that form antigen-binding sites. They cause agglutination, neutralise toxins and mark pathogens for phagocytosis.
• Immunity types and vaccination Active immunity follows exposure to an antigen and produces memory cells; passive immunity is the transfer of antibodies and is short-lived. Vaccines contain antigens that stimulate a primary response so a later infection triggers a faster, larger secondary response.

Common mistakes in Topic 11: Immunity

• Saying antibodies kill pathogens directly; antibodies bind antigens and cause agglutination or mark pathogens for phagocytes.
• Confusing antigens with antibodies; antigens are on the pathogen, antibodies are produced by plasma cells.
• Writing that memory cells produce antibodies; they divide quickly into plasma cells on second exposure.
• Describing passive immunity as long-lasting; no memory cells are made so protection fades as antibodies are broken down.
• Stating that the variable region is the same on all antibodies; it differs between antibodies, giving each a specific binding site.

Try three Topic 11: Immunity exam-style questions