Cellmediated Immunity

Figure 7.1 Cellular processes of the immune response. A simplified summary of the sequence of events in both the humoral and cell-mediated response to an antigen.

> A primary response describes the type of reaction when an animal encounters an antigen for the first time.

► A secondary response describes the type of reaction when an animal encounters an antigen more than once.

> Plasma cells are B lymphocytes that are in the process of synthesizing antibody protein.

> Complement are nonimmune plasma proteins that act in conjunction with antibodies.

> Lymphokines are substances produced by leucocytes which are involved in signalling between cells of the immune system.

> Clonal selection is the process by which only a limited number of lymphocytes are stimulated to replicate by an antigen.

Figure 7.2 The kinetics of the immune response. The injection into an animal of a second dose of an antigen several weeks after the first injection will result in a response that is more rapid and more intense than the first.

on a previous occasion (a secondary response) (Figure 7.2). The secondary response shows a reduced lag period and a considerably increased rate of antibody synthesis compared with the primary response and the antibody persists for a longer period. The kinetics of the response vary depending upon the antigen and the animal but the relationship between the primary and secondary responses is characteristic.

There are three major types of lymphocyte which, when stimulated, are directly responsible for the effects of an immune reaction. B lymphocytes during proliferation develop into plasma cells, which are the antibody-producing cells, the process referred to earlier as humoral immunity. An antibody protects in a variety of ways. It can render the free antigen more susceptible to elimination by normal cellular processes such as phagocytosis or it may block the effects of a toxic substance. It can also function in conjunction with a series of plasma proteins known collectively as complement to cause lytic damage to the membrane of a target antigenic cell. There are two types of T lymphocyte involved: the cytotoxic cells (Tc cells) when stimulated are capable of binding to the antigen cell and causing irreversible lytic damage to the cell membrane; other T lymphocytes (Td cells) release soluble substances known as lymphokines, which damage the invading antigen cells and stimulate other aspects of the host immune system. The effects of Tc and Td lymphocyte activity are features of cell-mediated immunity.

Immunologically competent cells, whether they are T or B lymphocytes, have membrane receptors that are specific for an antigen. It is basically the binding of the antigen to the specific receptor on the appropriate lymphocyte which initiates the whole process, stimulating the cell to proliferate and producing a clone of identical cells, a process known as 'clonal selection'. The nature of the secondary response is due in the main to this large number of cells now available.

The processes of antigen recognition and immune stimulation are

> Antigen-presenting cells are cells that take up antigen and present it, in recognizable form, to the lymphocytes.

extremely complex and involve a sequence of steps. Upon entering the tissues, the antigen is taken up by macrophages or other 'antigen-presenting cells' (APCs) and, after modification, is exhibited on the outer membrane of the cells. This membrane-bound antigen can be recognised by lymphocytes via the antigen-specific receptors carried on their membranes. A key group of cells in the process are the T helper cells (Th cells). These cells recognize the antigen presented by the macrophage and when stimulated by this recognition process they are capable of stimulating the specific B, Tc or Td cells (Figure 7.3).

Figure 7.3 Process of antigen recognition and lymphocyte stimulation.

Antigen

Macrophage

Interleukins

Antigen

Macrophage

Figure 7.3 Process of antigen recognition and lymphocyte stimulation.

Interleukins

Antigen receptor

Tr cell

Th cell

Interleukins

Antigen receptor

Tr cell

Th cell

Interleukins

>■ Interleukins are a group of lymphokines that act on other leucocytes to control the immune process.

In all cellular interactions in the immune process, as well as antigen recognition being a major feature, there are complex stimuli resulting from the secretion of a range of cytokines by the different cells involved. In the immune process, because the interactions are between leucocytes, these cytokines are known as interleukins. They are specific for the type of cell on which they can act and are necessary, often in conjunction with antigen recognition, to stimulate the target lymphocyte to act in the required manner. This action may involve protein synthesis, replication or differentiation and is an essential step in mounting and controlling a full immune response. An individual lymphocyte is capable of being stimulated by only a limited number of antigens (probably only one but at the most only two or three) and in order to provide a large number of different antibodies it is essential that there are an equivalent number of different lymphocytes in the tissues.

> Human serum proteins - see Figure 11.13.

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