Characteristics of the Secondary Response

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Memory B cells are responsible for the swift and effective reaction of the secondary response, eliminating identical repeat invaders before they cause noticeable harm. Thus, once a person has recovered from a particular disease, he or she generally has long-lasting immunity to that disease. Vaccination exploits this naturally occurring phenomenon.

Memory B cells that bind antigen can promptly become activated provided they receive necessary signals from effector T-helper cells. When compared to the few naive B cells that initiated the primary response, the memory B cells are markedly faster and more effective. For one thing, there are more cells that are able to respond to a specific antigen. In addition, the B cells are able to scavenge antigen even when it occurs at low concentration because their receptors have been fine-tuned through affinity maturation to bind antigen more avidly. Likewise, the antibodies coded for by these cells more effectively bind antigen.

Some of the memory B cells that become activated will differentiate to form plasma cells, resulting in the rapid production of antibodies. Because of class switching, most of the circulating antibodies produced are IgG (see figure 16.11). Other activated memory cells begin proliferating, once again undergoing affinity maturation to further enhance the effectiveness of the antibodies they encode. Subsequent exposures to antigen lead to an even stronger response.

Igg Igm First And Second Exposure

Figure 16.11 The Primary and Secondary Responses to Antigen The first exposure to antigen elicits relatively low amounts of first IgM, followed by IgG in the blood.The second exposure, which characterizes the memory of the adaptive immune system, elicits rapid production of relatively large quantities of IgG.

Figure 16.11 The Primary and Secondary Responses to Antigen The first exposure to antigen elicits relatively low amounts of first IgM, followed by IgG in the blood.The second exposure, which characterizes the memory of the adaptive immune system, elicits rapid production of relatively large quantities of IgG.

406 Chapter 16 The Adaptive Immune Response

406 Chapter 16 The Adaptive Immune Response

Figure 16.12 Affinity Maturation B cells that bind antigen most tightly for the longest duration are the most likely to proliferate.The plus signs indicate the relative quality of binding of the antibody to the antigen; those in green indicate the most "fit" to continue proliferating.

The Response to T-Independent Antigens

T-independent antigens can stimulate an antibody response by activating B cells without the aid of effector T cells. Relatively few antigens are T-independent, but they can be very important medically.

Molecules such as polysaccharides that have numerous identical evenly spaced epitopes characterize one type of T-inde-pendent antigen. Because of the arrangement of epitopes on the antigen, clusters of B-cell receptors bind the antigen simultaneously, which leads to B-cell activation without the involvement of T-helper cells (figure 16.14). These antigens are particularly significant because the immune systems of young children respond poorly to them. Children less than 2 years of age are more susceptible to diseases caused by organisms that cloak themselves in polysaccharide capsules, such as Streptococcus pneumoniae and Haemophilus influenzae; antibodies against the capsules would be protective, but children do not effectively make antibodies against them. Vaccines made from purified capsules are available, but, likewise, they do not elicit a protective response in young children. Fortunately, newer vaccines designed to evoke a T-dependent response have been developed. They will be discussed later, when the role of effector T-helper cells in the antibody response is described in more detail.

Figure 16.13 Class Switching B cells are initially programmed to produce IgM antibodies.With the direction of effector T cells, the B cells can switch to encode a different class.The plasma cells descended from circulating B cells that have undergone class switching most commonly produce IgG. Plasma cells that descend from B cells residing in the mucosal-associated lymphoid tissues most commonly produce IgA. Note that class switching does not alter the antigen specificity.

B-cells receptors

Polysaccharide antigen with multiple repeating subunits

Polysaccharide antigen with multiple repeating subunits

B-cells receptors

Independent Antigen
Figure 16.14 T-Independent Antigens Antigens such as some polysaccharides have multiple repeating epitopes. Because of the arrangement of epitopes, clusters of B-cell receptors bind to the antigen simultaneously, leading to B-cell activation without the involvement of T-helper cells.

Another type of T-independent antigen is lipopolysac-charide (LPS), a component of the outer membrane of Gramnegative bacteria. The constant presence of antibodies against LPS, evoked without the need for T-cell help, is thought to provide an early defense against Gram-negative bacteria that breach the body's barriers.

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  • eetu
    What are characteristics of secondary adaptive response?
    10 months ago
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    What are the characteristics of secondary immune response?
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