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

1338 Chapter 24: The Innate and Adaptive Immune Systems
(A) DENDRITIC CELL ACTIVATES T CELL (B) HELPER T CELL ACTIVATES B CELL
NAÏVE
HELPER
T CELL
cytokines
CD28
B7
T H
TCR
ACTIVATED DENDRITIC CELL
foreign
peptide
class II
MHC
protein
cytokines
B cell antigenic
determinant
Many Cell-Surface Proteins Belong to the Ig Superfamily
Most of the proteins that mediate antigen recognition and cell–cell recognition
in the immune system contain one or more Ig or Ig‐like domains, suggesting that
the proteins have a common evolutionary history. Included in this very large Ig
superfamily are antibodies, TCRs, MHC proteins, the CD4, CD8, and CD28 co-receptors,
the B7 co-stimulatory proteins, and most of the invariant polypeptide
chains associated with TCRs MBoC6 and m25.73/24.49
BCRs, as well as the various Fc receptors on lymphocytes
and other leukocytes. Many of these proteins are dimers or higher oligomers,
in which Ig or Ig‐like domains of one chain interact with those in another
(Figure 24–48).
= disulfide bond
BCR
CD40
B
CD40
ligand
T H
EFFECTOR HELPER T CELL
latent T cell antigenic
determinant (peptide)
folded foreign
protein antigen
B CELL
T cell antigenic
determinant
(foreign peptide)
Figure 24–47 Comparison of the costimulatory
proteins required to activate
a helper T cell and a B cell in response
to the same foreign protein. (A) A naïve
helper T cell is activated by a peptide
fragment of a foreign protein bound to a
class II MHC protein on the surface of an
activated dendritic cell. The co-stimulatory
protein on the dendritic cell (a B7 protein—
either CD80 or CD86) binds to the CD28
co-receptor on the T cell, providing a
necessary co-stimulatory signal to the
T cell; in addition, cytokines secreted by the
dendritic cell (or other nearby cells) influence
what subtype of effector helper cell the
T cell becomes (see Figure 24–44). (B) Once
activated to become an effector cell, the
helper T cell can help activate
B cells that have the same peptide–MHC
protein complexes on their surface as
the dendritic cell that activated the T cell.
These B cells have BCRs that bind an
antigenic determinant on the surface of
a folded foreign protein and endocytose
the protein (red arrow); the protein is then
cleaved into peptides, which are carried to
the B cell surface by class II MHC proteins,
where some of them can be recognized by
the TCRs on the helper T cell (see Figure
24–39). Note that the BCRs and TCRs
recognize different antigenic determinants of
the protein. As indicated, the co-stimulatory
protein used by the effector helper T cell
is CD40 ligand, which binds to the CD40
co-receptor on the B cell; the T cell also
secretes cytokines such as IL4 to help
stimulate the B cell to undergo somatic
hypermutation and class switching (not
shown). The CD4 co-receptor on T H cells is
omitted in both (A) and (B) for simplicity.
plasma
membrane
EXTRACELLULAR
SPACE
Fc receptor
class I MHC
protein
class II MHC
protein
TCR
BCR
generated by gene-segment
rearrangement
ε
Igα Igβ
γ δ ε CD4 CD8 CD28 B7
CD3 complex
CYTOSOL
Figure 24–48 Some of the cell-surface
proteins discussed in this chapter that
belong to the Ig superfamily. The Ig
and Ig‐like domains are shaded in gray,
except for the antigen-binding domains
(not all of which are Ig domains—the
class I and class II MHC proteins are the
exception), which are shaded in blue.
The Ig superfamily also includes many
cell-surface proteins involved in cell–cell
interactions outside the immune system,
such as the neural cell adhesion molecule
(N-CAM) discussed in Chapter 19 and the
receptors for various protein growth factors
discussed in Chapter 15 (not shown). There
are more than 750 members of the Ig
superfamily in humans.