Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

B CELLS AND IMMUNOGLOBULINS
1315
Myasthenia gravis is an example of such an autoimmune disease. Most of the
affected individuals make antibodies against the acetylcholine receptors on their
own skeletal muscle cells; these receptors are required for the muscle to contract
normally in response to nerve stimulation, which releases acetylcholine (see Figure
11–39). The antibodies interfere with the normal functioning of the receptors
so that the patients become weak and may die because they cannot breathe. Similarly,
in juvenile (type 1) diabetes, adaptive immune reactions against insulin-secreting
β cells in the pancreas kill these cells, leading to severe insulin deficiency.
Summary
Innate immune responses triggered by pathogens at sites of infection help activate
adaptive immune responses in peripheral lymphoid organs. The adaptive immune
system is composed of many millions of B and T cell clones, with the cells in each
clone sharing a unique cell-surface receptor that enables them to bind a particular
pathogen antigen. The binding of antigen to these receptors, with the help of
co-stimulatory signals, stimulates the lymphocyte to proliferate and differentiate
into an effector cell that can help eliminate the pathogen. Effector B cells secrete
antibodies, which can act over long distances to help eliminate extracellular
pathogens and their toxins. Effector T cells, by contrast, produce cell-surface and
secreted co-stimulatory molecules, which mainly act locally to help other immune
cells eliminate the pathogen; in addition, some T cells can induce infected host cells
to kill themselves.
During a primary adaptive immune response to an antigen, lymphocytes that
recognize the antigen proliferate so that there are more of them to respond the next
time, during a secondary response to the same antigen; moreover, during a primary
response, some lymphocytes differentiate into memory cells, which can respond
faster and more efficiently the next time the same pathogen invades. These two
mechanisms are largely responsible for immunological memory. Both B and T cells
circulate continuously between one peripheral lymphoid organ and another via the
blood and lymph; only if they encounter their specific foreign antigen in a peripheral
lymphoid organ do they stop migrating, proliferate, and differentiate into effector
cells or memory cells. Lymphocytes that would react against self molecules either
alter their receptors (in the case of B cells) or are eliminated or inactivated; they can
also be suppressed by regulatory T cells. These mechanisms collectively are responsible
for immunological self-tolerance, which ensures that the adaptive immune
system normally avoids attacking the molecules and cells of the host.
B CELLS AND IMMUNOGLOBULINS
Vertebrates inevitably die of infection if they are unable to make antibodies.
Antibodies are secreted proteins that defend us against extracellular pathogens
in several ways. They bind to viruses and microbial toxins, thereby preventing
them from binding to and damaging host cells (see Figure 24–2). When bound
to an extracellular pathogen or its products, antibodies also recruit some of the
components of the innate immune system, including various types of leukocytes
and components of the complement system, which work together to inactivate or
eliminate the invaders.
Synthesized exclusively by B cells, antibodies are produced in billions of forms,
each with a different amino acid sequence. They belong to the class of proteins
called immunoglobulins (abbreviated as Igs) and are among the most abundant
protein components in the blood. In this section, we discuss the structure and
function of immunoglobulins and how they are made in so many different forms.
B Cells Make Immunoglobulins (Igs) as Both Cell-Surface Antigen
Receptors and Secreted Antibodies
The first Igs made by a newly formed B cell are not secreted but are instead
inserted into the plasma membrane, where they serve as receptors for antigen.
They are called B cell receptors (BCRs), and each B cell has approximately 10 5 of