Chapter 1: The Immune System
Antibody structure -Figure 4
Figure abbreviations: Ag, antigen; CDR, complementary-determining region; Fab, fragment antigen-binding; Fc, fragment crystallisable.
Source: Fix D., Professor at Southern Illinois University
The final task of the lymphocytes (B cells) developed in the BM is the production of Ag-specific Igs, which function as antibodies (Abs). (Fig 1.4)
Igs are proteins secreted by or present on the surface of B cells, assembled from identical pairs of heavy (H) and light (L) chains.
The highly variable N-terminal regions are the fragment antigen-binding (Fab) portion. The constant domains interact with the fragment crystallisable (Fc) receptors on the effector cells.
Immunoglobulin classes - Figure 5
Figure abbreviations: Ig, immunoglobulin.
There are five classes of Igs: M, G, A, E and D, distinguished by different H chains. B cells can change the class of Ig produced: class switching. (Fig 1.5)
Before being capable of producing Ag-specific Ig, B cells must undergo a number of transformations, first in the BM and subsequently in the LNs.
In the rest of the cells in the body (not B cells), the genes encoding the H and L chains of the Ig are distributed in many segments, thus they cannot be expressed.
B-cell development - Figure 6
Figure abbreviations: D, diversity; H, heavy; L, light; J, joining; Rag, recombinase activating gene; V, variable.
These gene segments must be rearranged within the chromosome in the B cells so the final gene structure allows the expression of a functional protein.
The first stages of B-cell development occur in the BM, where pro-B cells first rearrange the Ig H chain gene to become pre-B cells. (Fig 1.6)
Pre-B cells continue this somatic recombination process by rearranging the L chain to become immature B cells, expressing IgM on their surface.
Revision Questions
- What are the Fab and the Fc portions of an Ig?
- What distinguishes a pre-B from a pro-B from an immature B cell?
- What is meant by the term 'somatic recombination'?
