OxfordAQA International A Level Biology 3.2.5 The Defensive Functions of Mammalian Blood Exam Questions

About OxfordAQA International A Level Biology 3.2.5 The Defensive Functions of Mammalian Blood

This Unit 2 topic covers how mammalian blood defends the body. You need to explain how antigens allow the immune system to identify pathogens, toxins, abnormal cells and cells from other individuals, describe phagocytosis, and explain the roles of T cells in cell-mediated immunity and B cells in humoral immunity. You also need to describe the structure of antibodies, the primary and secondary responses, vaccination, herd immunity and active versus passive immunity. It links to HIV, protein structure and data handling. Examiners often ask you to explain why a secondary response is faster and to evaluate data from vaccine trials.

What you need to know

• Antigens Antigens are molecules, usually proteins, on cell surfaces that generate an immune response. Antigen variability, where pathogens change their antigens, means immunity and vaccines may not protect for long.
• Phagocytosis A phagocyte engulfs a pathogen into a phagosome. Lysosomes fuse with it and release lysozymes that hydrolyse the pathogen, and the phagocyte can present the antigens on its surface.
• T cells T cells are involved in cell-mediated immunity. Helper T cells with complementary receptors bind to antigen-presenting cells, divide by mitosis and stimulate phagocytes, B cells and cytotoxic T cells.
• B cells and antibodies A B cell with a complementary antibody binds the antigen and, with help from T cells, divides to form plasma cells and memory cells. Antibodies are proteins with two heavy and two light chains, a constant region and variable regions forming two antigen-binding sites. Antigen-antibody complexes cause agglutination of bacteria, making phagocytosis easier.
• Primary and secondary responses In the primary response antibody production is slow. Memory cells remain, so on second exposure the secondary response is faster and produces more antibody.
• Vaccination and immunity Vaccines contain antigens that trigger a primary response and memory cells. Herd immunity occurs when enough people are vaccinated to reduce spread. Active immunity involves making your own antibodies; passive immunity involves receiving antibodies, which is fast but short-lived.

Common mistakes in 3.2.5 The Defensive Functions of Mammalian Blood

• Saying antibodies kill pathogens directly. Antibodies bind to antigens and cause agglutination, which helps phagocytes destroy the pathogens.
• Saying memory cells produce antibodies. Memory cells divide rapidly to form plasma cells, which secrete antibodies.
• Describing the antibody as having the same shape as the antigen. Its binding site is complementary to the antigen.
• Saying passive immunity gives long-term protection. No memory cells are made, so the antibodies are broken down and protection is short-term.

Try three 3.2.5 The Defensive Functions of Mammalian Blood exam-style questions