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Antibodies

| Some develop into plasma cells (effector cells) that secrete the same antibody as the parent cell.

Plasma cells

Antibodies

Memory cells

| Some develop into plasma cells (effector cells) that secrete the same antibody as the parent cell.

| A few develop into memory cells that divide at a low rate, perpetuating the clone.

18.7 Clonal Selection in B Cells The binding of an antigenic determinant to a specific antibody on the surface of a B cell stimulates the cell to divide, rapidly producing a clone of cells to fight the invader.

Immunity and immunological memory result from clonal selection

An activated lymphocyte produces two types of daughter cells, effector cells and memory cells.

► Effector cells carry out the attack on the antigen. Effector B cells, called plasma cells, produce antibodies. Effector T cells release cytokines, which initiate reactions that destroy nonself or altered cells. Effector cells live only a few days.

► Memory cells are long-lived cells that retain the ability to start dividing on short notice to produce more effector and more memory cells. Memory B and possibly T cells may survive in the body for decades, dividing at a low rate.

When the body first encounters a particular antigen, a primary immune response is activated, in which the lymphocytes that recognize that antigen produce clones of effector and memory cells. The effector cells destroy the invaders at hand and then die, but one or more clones of memory cells have now been added to the immune system and provide im-munological memory.

After the body's first immune response to a particular antigen, subsequent encounters with the same antigen will trigger a much more powerful attack. The huge army of plasma and T cells launched by the memory cells at this time is called the secondary immune response. The first time a vertebrate animal is exposed to a particular antigen, there is a time lag (usually several days) before the number of antibody molecules and T cells slowly increases (Figure 18.8). But for years afterward—sometimes for life—the immune system "remembers" that particular antigen. The secondary immune response is characterized by a shorter lag time, a greater rate of antibody production, and a larger production of total antibody or T cells than the primary response.

Vaccines are an application of immunological memory

Thanks to immunological memory, recovery from many diseases, such as chicken pox, provides a natural immunity to those diseases. However, it is possible to provide artificial immunity against many life-threatening diseases by inoculation—the introduction of antigenic determinants into the body. Immunization is inoculation with antigenic proteins, pathogen fragments, or other molecular antigens. Vaccination is inoculation with whole pathogens that have been modified so that they cannot cause disease.

Immunization or vaccination initiates a primary immune response, generating memory cells without making the person ill. Later, if the same or very similar pathogens attack, specific memory cells already exist. They recognize the antigen and quickly overwhelm the invaders with a massive production of lymphocytes and antibodies.

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Essentials of Human Physiology

Essentials of Human Physiology

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