Cells involved in the immune response

The lymphocytes are a very heterogeneous group of cells, almost identical when studied using light microscopy methods and only showing slight differences by electron microscopy techniques and yet the group contains cells with many different roles. Although large numbers of lymphocytes can be detected in the circulating blood and body fluids, the majority of lymphocytes are to be found in the group of tissues known collectively as the reticulo-endothelial system. This includes such tissues as the liver, spleen, bone marrow, thymus and lymph nodes, all of which are important in the immune response. Experiments involving the removal of various tissues from experimental animals have indicated that there are two different features to the immune response. The removal of the thymus, a small gland located behind the sternum, impairs the ability of a young animal to reject skin transplants but does not affect to the same extent its ability to produce antibodies. This aspect of

>■ Cell-mediated immunity is that aspect in which cells are directly involved in the protective oioccssos. >■ T lymphocytes are cells produced in the thymus and responsible for cell-mediated immunity.

> Humoral immunity is that aspect that is mediated by antibodies.

>• B lymphocytes are cells produced in the bone marrow and responsible for antibody production.

the immune response is known as cell-mediated immunity and is due to a sub-population of lymphocytes called T lymphocytes (thymus derived). Experimental work with fowls has demonstrated that the removal of a lymphoid tissue nodule in the gut known as the bursa of Fabricius results in a reduced ability to produce antibodies but does not significantly alter the response to skin grafts. It was subsequently demonstrated that the production of antibodies, a feature known as humoral immunity, is associated with another sub-population of lymphocytes known as B lymphocytes (bursa derived). The bone marrow acts as the source of B lymphocytes in man.

The introduction of an antigen into the tissues of a susceptible animal results initially in increased proliferation of lymphocytes in the tissues of the reticulo-endothelial system particularly the lymph nodes and the spleen (Figure 7.1). An animal will show different responses if the antigen is completely new to it (a primary response) or if the antigen has been encountered

Precursor bone marrow cell

Precursor bone marrow cell

Bursa equivalent Thymus involvement involvement

Bursa equivalent Thymus involvement involvement

Memory Plasma lymphocyte cell

T lymphocytes

Memory Plasma lymphocyte cell

Antibody production

T lymphocytes

Lymphokine Cytotoxic production cells

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