There are two main components of the immune system: the humoral and the cell-mediated responses. They work both together and separately to protect the body from disease.
The humoral response is associated with the serum (the non-cellular part of the blood) and involves the action of antibodies secreted by B lymphocytes (plasma cells). Antibodies are found in extracellular fluids such as lymph, plasma and mucus. The humoral response protects against circulating viruses, and bacteria and their toxins.
The cell-mediated response is associated with the production of T lymphocytes. It is most effective against bacteria and viruses located within host cells, as well as parasitic protozoa, fungi and worms. It is also an important defence against cancer, and is responsible for the rejection of implanted tissue.
Both T and B lymphocytes develop from stem cells in the bone marrow of adults and the liver of foetuses. However T cells mature in the thymus, whereas B cells mature in the bone marrow.
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