International A Level Biology / Immunity

The specific response: B cells and antibodies

Edexcel International A Level Biology (YBI11), chapter 18: Immunity, specification point 6.9, 6.10. Free lesson.

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Clonal selection produces the plasma cells that secrete antibodies and the memory cells that remain in the body. These memory cells make the response to the same antigen faster, higher and longer lasting.

How clonal selection makes a clone

Your body has millions of different B cells, each with a differently shaped receptor on its surface. An antigen selects the few B cells whose receptors are complementary to it. The antigen does not change a B cell; it selects one that already fits. The selected B cell must then be activated by a T helper cell before it can divide. A T helper cell activated by a macrophage presenting the same antigen binds to the B cell and releases cytokines that activate it. The activated B cell divides repeatedly by mitosis. This clonal expansion makes genetically identical cells with the…

What the two types of B cell do

The clone divides into two cell types with different jobs. Most become B effector cells, also called plasma cells. They make and secrete antibodies complementary to the antigen. The rest become B memory cells, which stay in the body and wait for the same antigen to return. Plasma cells secrete antibodies during the current response. They are short lived, so antibody levels fall after the infection is cleared. Memory cells do not secrete antibodies. They are long lived, sometimes for years. If the same antigen returns, memory cells divide rapidly and form new plasma cells.

Which cell secretes antibodies

Use the cell names in the correct order: B cells differentiate into plasma cells, and plasma cells make and secrete antibodies. An antibody is a protein, not a cell, so a plasma cell cannot differentiate into one.

How the primary and secondary responses differ

The primary response follows the first exposure to an antigen. Antibody production has a lag of about a week because only a few B cells fit at first, and they need to be activated, divide and differentiate into plasma cells. The secondary response follows exposure to the same antigen again. Memory cells quickly form plasma cells, so antibody levels rise faster, reach a much higher peak and stay high for longer. The pathogen is usually destroyed before symptoms develop. In the graph, the first exposure is at day 0 and the second is at day 40. The primary peak is about 10 units, around…

Why antibodies take days to appear at first

Only a few B cells have receptors complementary to a particular antigen. Before antibodies appear in the blood, these cells must be activated by T helper cells, multiply by mitosis and differentiate into plasma cells. These stages take several days.

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