WJEC Biology for A2: Student Bk

Examtip There is a separate population of memory B lymphocytes and of memory T lymphocytes for each antigen. Key Term Antibody : An immunoglobulin produced by the body's immune system in response to antigens. The adaptive response There are two components to this specific immune response. 1 The humoral response The humoral response results in the production of antibodies . B lymphocytes mature in the spleen and lymph nodes. The receptors on their cell membranes respond to a foreign protein in the blood stream and they divide, making: ■ Plasma cells, which release antibodies. ■ Memory cells, which remain dormant in the circulation and then divide to form more B lymphocytes if the same antigen is encountered in the future. Antibodies are Y-shaped glycoprotein molecules, called immunoglobulins. They have quaternary structure as each molecule is made of four polypeptides, held together by disulphide bonds. The variable portions are specific to each antigen. An antigen molecule binds to an antibody and each antibody molecule can bind to two antigen molecules. Microbes with antigens on their surfaces are clumped together, i.e. they agglutinate. KEY Polypeptide chain description Heavy Constant Light Constant Heavy Variable Light Variable ––– Disulphide bond An antibody molecule 2 The cell-mediated response The cell-mediated response refers to the activation of phagocytic cells, B lymphocytes and T lymphocytes. T lymphocytes are activated in the thymus gland. The receptors on their cell membranes respond to antigens and they divide making: ■ T memory cells, which remain dormant in the circulation and then divide to form more T lymphocytes if the same antigen is encountered in the future. ■ T killer or cytotoxic T cells, which kill pathogenic cells with the antigens, by lysing them. ■ T helper cells, which release chemicals including cytokines. Cytokines stimulate: ■ Phagocytic cells, including macrophages, monocytes and neutrophils, to engulf pathogens and digest them. antigen binding site antigen binding site Microbes agglutinated by antibodies are engulfed more efficiently by phagoctic cells WJEC A2 Biology: Chapter 14 272