The body contains millions of different lymphocytes, each with receptors complementary to a different antigen, but only a very few of them match any one antigen. Clonal selection is the process by which an antigen 'selects' the lymphocytes with complementary receptors, which then multiply.1. Selection. When a pathogen enters the body, its antigens bind only to the B cells and T cells whose receptors have a complementary shape. All the other lymphocytes are unaffected.2. Clonal expansion. The selected lymphocytes are activated (B cells need cytokines from T helper cells) and divide repeatedly by mitosis. Because mitosis produces genetically identical cells, each selected lymphocyte gives rise to a clone of cells with exactly the same receptors.3. Differentiation. Cells of the clone specialise into effector cells (plasma cells, T helper cells and T killer cells) and memory cells.
Clonal selection explains the features of the specific response:
• specificity: only lymphocytes for that antigen multiply
• delay: in a first infection it takes time to find and expand the few matching cells
• memory: after the infection, many more memory cells remain than there were original matching lymphocytes, so a second response is faster.Worked example: a selected B cell divides by mitosis every 12 hours. After 5 days (120 hours) it has divided 10 times, producing 210 = 1024 cells.Most pathogens carry several different antigens, so several different clones are selected at once. This is called a polyclonal response.
Clonal selection explains the features of the specific response:• specificity: only lymphocytes for that antigen multiply
• delay: in a first infection it takes time to find and expand the few matching cells
• memory: after the infection, many more memory cells remain than there were original matching lymphocytes, so a second response is faster.Worked example: a selected B cell divides by mitosis every 12 hours. After 5 days (120 hours) it has divided 10 times, producing 210 = 1024 cells.Most pathogens carry several different antigens, so several different clones are selected at once. This is called a polyclonal response.