When an immune response is triggered antibodies, or immunoglobulins, are secreted by B-lymphocytes. They consist of approximately 105 antibody molecules, bound to the membrane.
Each B-lymphocyte contains a receptor site specific to a non-self-antigen. The antibody produced is also specific to a specific antigen and together they form an antigen-antibody complex.
Antibodies have a Y-shaped structure comprised of four polypeptide chains (two light chains and two heavy charins):
- the two arms of the ‘Y’ are known as the Fab fragments and contain properties for binding to the antigen
- the stem is named the Fc fragment and works as the effector component by triggering the immune response
The stem section is also known as the constant region as it has the same amino acid sequence, and therefore the same structure, in all immunoglobulins. The arms, on the other hand, are known as the variable regions as these parts different in immunoglobulins due to different amino acid sequences.
After an immune response the B-lymphocytes divide to create memory cells and antibody-secreting plasma cells. This is known as the primary response. The secondary response refers to if the same antigen was to attack the body again the memory cells would recognise it much faster and a larger response would prevent harm from occurring.