Lymph nodes, white blood cells and antibodies

Peyer’s patches are small groups of lymphoid cells in the lining of the lower part of the small intestine. They take part in the immune response against pathogens that enter the gut, and the appendix has a similar job. The lymph nodes lie along the network of lymph vessels all over the body: under the armpits, at the sides of the neck, in the upper thigh and near the internal organs. They range in size from a pinhead to a small bean. Several lymph vessels bring lymph from the tissues to each node, which cleans it of harmful matter and microbes and stores lymphocytes. Inside, a node is divided into pockets full of B cells, T cells and macrophages. White blood cells (leukocytes) fall into two groups. Granulocytes have granules in the cytoplasm that take up dyes: eosinophils, basophils and neutrophils. Agranulocytes have no such granules: lymphocytes and monocytes. Under the microscope, granulocytes are told apart by their size, the shape of the nucleus and the colour of the granules. They digest pathogens by phagocytosis, above all in bacterial infection and inflammation, and stay in the blood only from several hours to a few days. There are three kinds of lymphocyte. B cells are 10 to 15 % of the lymphocytes, are made and matured in the bone marrow, and recognize a microbe or foreign material, stick to it and produce antibodies. T cells are about 80 % and mature in the thymus gland. Helper T cells activate other T cells and stimulate B cells to make antibodies. Cytotoxic (killer) T cells attack cancer cells, transplanted organs and cells infected by viruses. Suppressor T cells limit the response once the pathogen has gone. Natural killer cells are 5 to 10 % of the lymphocytes, are made and matured in the bone marrow, and make perforin, a protein that punches holes in infected or cancer cells. Monocytes turn into macrophages when they are needed, as the next lesson shows.