1.7 T cell help for antibody production

In order for B cells to become active, they normally require help from helper T cells – TH2 cells. Figure 6 shows how B cells interact with TH2 cells by presenting antigen on MHC molecules. Notice, however, that it is slightly different from the antigen presentation shown in Figures 1 and 3; the B cell uses MHC class 2 molecules to present antigenic peptides to the TH2 cell, whereas the cytotoxic T cells are presented with antigenic peptides on MHC class I cells.

Figure 6 Antigen presentation by B cells

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B cells bind antigen (Ag) via their B cell receptor, internalise it, process it and present antigen peptides on MHC class 2 molecules (MHC-II), which are then available for recognition by any TH2 cells that recognise the antigen on that MHC molecule.

Figure 6 Antigen presentation by B cells

In effect, the antigen is recognised twice: once by the BCR and then by the TCR on the TH2 cell. Both cells must recognise the antigen before the B cell receives an activation signal. The activation signal consists of a combination of direct cell-cell signals and cytokines released by the T cell, which promote B cell division and differentiation. This process is described in Video 3 below.

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Transcript: Video 1 Immune defence

DAVID MALE:

In this video, I will outline how T cells and B cells cooperate in antibody production. Here we have a population of mature B cells expressing surface antibody and T cells with T cell antigen receptors on their surface. At any one time in a population of lymphocytes, there will be millions of different clones of cells each carrying a different receptor, and therefore, only able to recognize a particular type of antigen. This means that in a normal immune system, there are always a small population of cells, which are able to recognize any specific antigen. If this were not the case, we would be completely unable to protect ourselves from pathogens.

By circulating around the body, lymphocytes have the opportunity of encountering the antigen that they can actually recognise. One consequence of this is that most lymphocytes are never called upon to fight infection. They are simply there just in case they are needed to combat a new strain of pathogen that might enter the host. We’re going to look at how B cells and T cells differentiate and the mechanisms which bring this about.

Here we have a virgin lymphocyte pool. Only one of these many different B cells is able to bind to that antigen successfully. This process known as clonal selection initiates a primary immune response, which causes the cell to undergo a series of irreversible changes.

The antigen is taken up by binding to the cell’s surface immunoglobulin antigen receptor. It is internalised and processed. Antigen fragments are subsequently re expressed on the B cell surface associated with MHC class II molecules for presentation to Th2 cells.

Receptors for cytokines including interleukin 4 are also induced on the B cell. If a Th2 cell recognises the antigenic peptide on the MHC molecule, it releases cytokines, which together with co stimulatory signals activate the B cell and trigger its division.

Other cytokines released from the Th2 cell bind to the appropriate receptors on the B cell causing it to differentiate.

Some of the B cells become plasma cells, which produce a secreted form of the original antibody. Consequently, they are also known as AFCs or antibody forming cells.

Plasma cells are a different shape from B cells, and they have lost all their original cell surface antibody. They will go on dividing several times but within days or weeks, they will die. Other cells become memory cells.

There are now more of them, and they have undergone a series of changes making them more efficient at reacting to the same antigen if they ever encounter it again. This means that there is always a supply of cells with the appropriate specificity to fight infection. Memory cells can live for many years, and their increased efficiency at fighting infection underlies the enhanced secondary immune response. Memory cells confer lasting immunity against a pathogen, which is the basic principle behind vaccination.

End transcript: Video 1 Immune defence

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Video 1 Immune defence