Themes > Science > Life Sciences > General Biology > Immunology > The Immune System & Its Effector Mechanisms > The Immune System: Organs and Cells > Cells and tissues of the mammalian Immune system


Immune cells

A: LYMPHOCYTES. These cells have receptors for antigen and confer specificity on an immune response. Lymphocytes express receptors with varying affinity for the antigen in question. The cell with the highest affinity for the most abundant antigen will have growth advantage and will preferentially generate progeny of itself. This process is called clonal expansion and is antigen driven.

B lymphocytes produce antibodies and some soluble mediators called cytokines. They arise in the bone marrow in adult mammals.

T lymphocytes arise in bone marrow but mature in the thymus. They do not produce antibody molecules but have surface receptors structurally related to Ig. T cells see antigen in a different way to B cells. They recognise peptide fragments of antigen complexed with cell surface MHC glycoproteins on neighbouring cells. The cell surface glycoproteins encoded by genes in the Major Histocompatibility Complex(MHC) bind fragments of antigen after it has been subjected to antigen processing.

There are two sub-types of T cell defined on the basis of function, accessory molecule expression and the type of MHC protein presenting antigen to them. This can be summarised as follows:

*Different cells of the immune system express complex arrays of cell surface proteins which can be distinguished with monoclonal antibodies. These surface markers have been given standardised names CD1 etc (up to 247 at present; you can find out more about these via the following links - for CD 1-166 see ImmunologyLink; the latest International workshop designated a further 81 CD# which are shown here).

Natural killer (NK) cells are large granular lymphocytes that are cytotoxic in the absence of prior stimulation. NK cells represent a first line of defence to infections, tumour growth and other pathogenic alterations of tissue homoeostasis. NK cells do not express antibodies or T cell receptors at their cell surface. They produce cytokines and express receptors for immunoglobulin. They also possess other receptor molecules which allow them to detect some infected host cells, including tumour cells, virus, or intracellular bacteria-infected cells.

B: MONONUCLEAR PHAGOCYTES

If you inject "vital" dyes into experimental animals they will be taken up by various cell types including macrophages (mf), microglial cells in the CNS, endothelial cells of vascular sinusoids and reticular cells of lymphoid organs. These are the cells of the Reticulo-Endothelial System (RES). These cells all take up dye by pinocytosis. Only cells of the monocyte-macrophage lineage take up large particulate antigens, pieces of tissue, senescent cells, bacteria etc. by phagocytosis.

These cells have important properties:

  1. they express a myeloid receptor (CD14) which serves as a recognition molecule for a wide variety of bacterial envelope molecules, such as LPS from Gram -ve organismsand components of Mycobacterial and Gram +ve cell walls. Ligation of this receptor leads to macrophage activation.
  2. they can act as antigen presenting cells (APC) for T cells.
  3. they are activated by T cell derived cytokines leading to increased phagocytosis and microbicidal activity (increased activity of degradative enzymes, nitrogen and oxygen free radical production and prostaglandins etc.).
  4. they express receptors for antibody and complement which means that they bind immune complexes, especially if the antibody involved has complement components bound to it (if the antibody has fixed complement), and endocytose/phagocytose these rapidly.
  5. they act as scavengers for cell debris and senescent cells (Kupffer cells in the liver bind "old" erythrocytes).
NOTE: T cell derived cytokines increase the antigen presenting activity of macrophages which, in turn, are able to present antigen to T cells. This cycle will continue as a positive feedback loop until the antigen is eliminated.

C: DENDRITIC CELLS

There are two cell types with similar names but different functions. Cells of the dendritic cell (DC) lineage are bone marrow derived. In the skin they are known as Langerhans Cells (LC). These cells efficiently process antigen but cannot present it to T cells. LC have been shown to pick up antigen in skin and carry it via afferent lymphatic vessels to lymph nodes. Dendritic cells in lymph are known as "veiled" cells. In lymph nodes the cells, now known as tissue dendritic

Follicular dendritic cells (FDC) are found in lymphoid follicles. They are called dendritic because of their morphology rather than any lineage relationship with DC. In fact, there is considerable uncertainty about their developmental origin [some evidence suggests they are long-lived bone marrow derived cells, other data that they are of epithelial origin]. FDC have receptors for immunoglobulin and complement and are able to trap antigen at their cell surface, in the form of antigen/antibody/C3d complexes, for long periods of time. They cannot present antigen to T cells but are important in developing responses by B cells.

D: GRANULOCYTES

There are three types of granulocyte distinguished according to their histological staining patterns.

  • Neutrophils, also known as polymophonuclear leukocytes, express receptors for immunoglobulin and complement and are involved in the acute inflammatory response.
  • Eosinophils carry receptors for IgE, are involved in the destruction of IgE coated parasites, such as helminths, and contribute to the response to allergens.
  • Basophils are the circulating counterpart of tissue mast cells. They express high affinity receptors for IgE and are stimulated to secrete the chemicals responsible for immediate hypersensitivity following antigen induced aggregation of these receptors.

Lymphoid Tissue

Lymphoid tissue is conveniently divided into the central or primary and peripheral or secondary organs. Central organs include the bone marrow and thymus. Lymphocytes, monocytes and granulocytes derive from precursor stem cells in the bone marrow. B lymphocytes migrate directly from marrow to the peripheral lymphoid tissue whereas T lymphocytes undergo further maturation in the thymus. The bone marrow and thymus are involved in generating precursor lymphocytes rather than immune responses.

Once released from the bone marrow and thymus lymphocytes begin a life of patrol and respond. Some 2.5 x 1010 lymphocytes pass through a lymph node per day with 1-2% of the lymphoid pool traversing the whole lymphoid system every hour. This degree of patrol allows rapid response to infectious agents.

The lymph nodes ( see diagram below) and spleen are designed to optimise interaction between APC and T and B lymphocytes.
The lymphatic system is a series of vessels which drain and filter the tissue fluids . Lymph fluid enters the node via afferent lymphatics, passes through the sinuses lined with macrophages and leaves via the efferent lymphatic (ultimately all drain into the portal vein).
Lymphocytes enter the node primarily from the blood via specialised endothelia also within the T areas and leave via the efferent lymphatics. Dendritic cells migrating from the tissues enter the node into the T cell areas. B cells entering nodes from the blood must cross the T rich area in transit to the B cell rich areas thus optimising the chance of T-B co-operation.
The B cell rich areas contain mature, resting B cells organised into structures around follicular dendritic cells (primary follicles).

node5.gif - 7Kb The spleen is fed by a single artery and does not receive afferent lymphatic drainage. Small splenic arterioles are surrounded by periarteriolar lymphoid sheaths mainly consisting of CD4 with some CD8 +ve T cells. The sheaths are associated with lymphoid follicles of similar organisation to the lymph node.

The largest concentration of lymphoid tissue is, however, in the gut and other mucosal areas. Here aggregates of lymphoid tissue, similar to lymph nodes in organisation, constitute Peyer's Patches in the lamina propria of the small intestine, tonsils in the pharynx, and submucosal lymphoid follicles in the appendix and throughout the upper airways.


Information provided by: http://www-immuno.path.cam.ac.uk