An activated B cell proliferates after stimulation by cytokines released by helper T cells. The B cell's clone enlarges. Some cells of the clone give rise to antibody-secreting plasma cells and others to dormant memory cells.

Secondary response

Primary response j_I_I

Days after exposure to antigen

Figure 16.23

A primary immune response produces a lesser concentration of antibodies than does a secondary immune response.

Sometimes a person who has been exposed to infection requires protection against a disease-causing microorganism but lacks the time necessary to develop active immunity. This happens with hepatitis, a viral infection of the liver. In such a case, it may be possible to inject the person with antiserum (ready-made antibodies). These antibodies may be obtained from gamma globulin separated from the blood plasma of persons who have already developed immunity against the disease.

An injection of antibodies (gamma globulin) or antitoxin provides artificially acquired passive immunity. This type of immunity is called passive because the recipient's cells do not produce the antibodies. Such immunity is short-term, seldom lasting more than a few weeks. Furthermore, because the recipient's lymphocytes might not have time to react to the pathogens for which protection was needed, susceptibility may persist.

tll[U|] Practical Classification

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