Chapter 1: The Immune System
Source: Fix D., Professor at Southern Illinois University
The lymphocytes developed in the BM (B-cells) have as their final task the production of Ag-specific immunoglobulins (Ig), which function as antibodies (Ab).
Ig are proteins secreted by or present on the surface of B-cells, assembled from identical couples of heavy (H) and light (L) chains.
The highly variable N terminal regions are the Ag-binding portion (Fab fragment). The constant domains interact with the Fc receptors on the effector cells.
There are 5 classes of Ig: M, G, A, E, and D, distinguished by different heavy chains. B-cells can change the class of Ig produced: class switch.
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 for the H and L chains of the Ig are distributed in many segments so that they cannot be expressed.
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 a pre-B-cell.
Pre-B-cells continue this somatic recombination process by rearranging the L chain to become an immature B-cell, expressing IgM on their surface.
Revision Questions
- What are the Fab and the Fc portions of an immunoglobulin?
- What distinguishes a pre-B from a pro-B from an immature B-cell?
- What is meant with the term “somatic recombination”?
