Biomedical Engineering Reference
In-Depth Information
22.2.1.1 B Cells
When BCRs bind soluble antigens, the bound antigen molecules are engulfed by the B cell by recep-
tor-mediated endocytosis. The antigen is digested into fragments, which are then displayed at the
cell surface inside a histocompatibility molecule (MHC). Helper T cells specii c for this structure
(with a complementary TCR) bind to the B cell and secrete lymphokines. Lymphokines are mes-
senger proteins secreted by lymphocytes that effect their own activity and/or that of other immune
cells. Examples of lymphokines are interleukines and cytokines. Lymphokines stimulate the B cell
to enter the cell cycle and develop into a clone of cells with identical BCRs. These cells switch from
synthesizing their BCRs as integral membrane proteins to a soluble version called antibodies (Abs)
and to differentiate into plasma cells that secrete these antibodies (Figure 22.1).
B cell receptors
(BCR)
Antibodies
Secretion
B cell
Mitosis and
diferentiation
Plasma
cell
Helper
T cell
Lymphokines
FIGURE 22.1
Schematic representation of an antigen displayed at the cell surface of a B cell inside a histo-
compatibility molecule and Helper T cells specii c for this structure (with a complementary TCR) binding the
B cell and secreting lymphokines. Differentiation of the B cell into an antibody secreting plasma cell.
22.2.1.2 T Cells
The TCR binds a bimolecular complex displayed at the surface of an APC. This complex consists
of a fragment of an antigen lying within the groove of a histocompatibility molecule constituted by
two
-helixes exposed on the surface (Figure 22.2). Most of the T cells in the body belong to one
of two subsets. These are distinguished by the presence, on their surface, of one or the other of two
glycoproteins, namely, CD4 and CD8. Which of these molecules is present determines what types
of cells the T cell can bind to.
α
+
T cells bind epitopes that are part of class I histocompatibility molecules (MHC
class I). Almost all the cells of the body express class I molecules.
CD4
•
CD8
+
T cells bind epitopes that are part of class II histocompatibility molecules (MHC
class II). Only specialized APCs express class II molecules (DCs, phagocytic cells like
macrophages and B cells).
•
