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Overview of the Human Immune System
The lymphatic system
White blood cells (leucocytes)
White cells are motile with specialised functions. The number of leucocytes in normal blood ranges between 4,500 and 11,000 per mm3. Fluctuations occur during the day; lower values are obtained during rest. Physical exertion may cause the count to exceed 20,000 per mm3. Most of the leucocytes are outside the circulation, and the few in the bloodstream are in transit from one site to another. There are three classes of leucocytes: granulocytes, monocytes, and lymphocytes.
Granulocytes The most numerous of the leucocytes, are important mediators of the inflammatory response. They are produced in the bone marrow; in a normal adult about 100,000,000,000 per day. There are three types of granulocytes: neutrophils, eosinophils, and basophils, according to the colour of the granules when the cells are stained for microscopic evaluation. About 50 to 80 % of the leucocytes are neutrophils, while the eosinophils and basophils together constitute 1-3 %.
Neutrophils live only a few hours in the blood, some migrate to the tissues to areas of infection or injury. Neutrophils are phagocytic; they engulf and digest bacteria and other microorganisms and microscopic particles.
Eosinophils are also motile and phagocytic, and migrate into the tissues. They are particularly important in the defence against parasites, and they participate in hypersensitivity and inflammatory (allergic) reactions.
Basophils also migrate into the tissues. When aggravated, basophils release histamine and other substances involved in allergic reactions.
Monocytes are also produced in the bone marrow; they are motile and phagocytic. They constitute up to 10 % of the blood leucocytes. However, the majority leave the blood after a few hours and migrate into almost all tissues, where they develop into macrophages.

Macrophages are matured monocytes found in the tissues. They play a key role in immunity by ingesting and processing antigens so that they can be recognised as "foreign" by lymphocytes.
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Scanning EM of a human 7d
monocyte-derived macrophage.

Kindly provided by
Prof. Reinhard Andreesen, Regensburg, Germany.
Dendritic cells are specialized antigen-presenting cells. They are found in the circulation, in peripheral tissues and in lymphoid organs. Maturing dendritic cells travel from the periphery to secondary lymphoid organs, where they present antigen to T lymphocytes.
Natural killer cells resemble macrophages, large granular lymphocytes. They are also produced in the bone marrow and constitute less than 10% of blood leucocytes. They have little phagocytic function and no specialised lymphoid functions (see below). They kill many virusinfected cells and cancer cells, particularly in connection with T lymphocytes.
Lymphocytes are also produced in the bone marrow. They constitute about 25-50 % of the blood leucocytes. Lymphocytes are not motile like granulocytes or macrophages, and they enter the circulation through lymphatic channels that drain into the venous system of the blood. Some lymphocytes leave and reenter the circulation, surviving for many years. They are found in large numbers in the lymph nodes, tonsils, thymus, spleen and lymphoid tissue of the gastrointestinal tract.
When stimulated by antigen for example on a microbe, the lymphocyte divides several times into daughter cells and eventually generates a clone of identical lymphocytes. Some of these cells remain in the circulation, others patrol the tissues of the body. The larger number of lymphocytes capable of reacting to the same microbe is responsible for the immunological memory that is manifested, if the organism encounters the same microbe later in time. There are two major classes of lymphocytes:
B lymphocytes (B), which after activation differentiate into plasma cells (P), which secrete antibodies (immunoglobulins).
T lymphocytes participate in what is called cell-mediated immune responses, for example against bacteria and parasite. T lymphocytes are involved in the defence against virus infection, in that so-called T-cytotoxic lymphocytes (Tc) directly kill virusinfected cells.

T lymphocytes also regulate the antibody-forming function of B lymphocytes and participate in rejection of transplants, so-called T-helper lymphocytes which are functionally subdivided into at least three types: Th1, Th2 and Th3.
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