Immunity

ImmunityThe major molecules of the immune response

AntigenAny molecule, usually a protein or large carbohydrate, that can be specifically recognised as foreign by cells of the immune system

EpitopeA localised region on the surface of an antigen that is chemically recognised by antibodies; also called antigenic determinant.

AntibodyA specific protein (immunoglobulin) that recognises and binds to specific antigens; produced by plasma cells

Major histocompatibility complex (MHC)A set of proteins found on the surface of cells that label the cell as belonging to a unique individual organism

Cytokines (lymphokines)Signalling proteins that regulate interactions between cells in the immune system. Important groups include interferons, interleukins and chemokines

Major cells of the immune system

MacrophageA large phagocytic white blood cell capable of ingesting and digesting bacteria and cellular debris. Macrophage is also antigen-presenting cell

B cell (B lymphocyte)A type of white blood cell responsible for antibody-mediated immunity. When stimulated, B cells differentiate to plasma cells that produce antibodies.

Plasma cellCell that secretes antibodies; a differentiated B lymphocyte (B cell)

T cell (T lymphocyte)The type of white blood cell responsible for a wide variety of immune functions, particularly cell-mediated immunity. T cells are processed in the thymus.

Cytotoxic T cell

(T-cytotoxic cell)T lymphocyte that destroys cancer cells and other pathogenic cells on contact. Also known as CD8 T cell and killer T cell.

Helper T cell(T-helper cell) T lymphocyte that activates B lymphocytes and can stimulate cytotoxic T cell production. Also known as CD4 T cell.

Suppressor T cell

(T-suppressor cell)T lymphocyte that suppresses the immune response

Memory cellB or T lymphocyte that is long-lived and provides future immunity against a second invasion by the same antigen.

Lymphatic system

The lymphatic system consists of widely distributed lymph capillaries which are found in all tissues of the body. These capillaries merge to form lymph vessels which possess valves and whose structure is similar to that of veins. The fluid within these vessels, the lymph, is therefore carried in one direction only, namely, away from the tissues. The lymph vessels from the right side of the head and thorax and the right arm combine to form the right lymphatic duct which drains into the right subclavian vein near the heart. The lymph vessels from the rest of the body form the thoracic duct which drains into the left subclavian vein.

Situated at intervals through the lymphatic system are lymph glands or nodes. Lymphocytes, in the course of circulation through the blood and lymph, accumulate in the lymph nodes. They produce antibodies and are an important part of the bodys immune system. Phagocytes in the nodes also remove bacteria and foreign particles from the lymph.

The movement of lymph through the lymphatic system is achieved in three ways:

1. Hydrostatic pressure The pressure of tissue fluid leaving the arterioles helps push lymph along the lymph system.

2. Muscle contraction The contraction of skeletal muscle compresses lymph vessels, exerting a pressure on the lymph within them. The valves in the vessels ensure that this pressure pushes the lymph in the direction of the heart.

3. Inspiratory movements On breathing in, pressure in the thorax is decreased. This helps to draw lymph towards the vessels in the thorax.

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