The immune system has several functions such as defense against foreign organisms, homeostasis, and the destruction of damaged cells and surveillance.24, 25
There are two types of immune responses: innate or non-specific immunity and adaptive or specific immunity. Cytokines are naturally occurring proteins produced by cells of the immune system (such as lymphocytes and macrophages) that coordinate and initiate effector defense functions.26
Cytokines include the interleukins, interferons, colony stimulating factors and tumour necrosis factor. Cytokines can be defined by the following properties:26
- they mediate and regulate the immune defense functions by acting as messengers between the various immune cells
- they usually function over short distances and their half-life is brief
- they are produced by a variety of cells types, and can act on diverse cell targets within the immune system and on organs such as the liver
- their actions are both overlapping and contradictory in that they can both stimulate and inhibit growth. They can act directly or indirectly on a cell causing a cytokine cascade.
Immune system response
An important function in the defense against cancer is surveillance and identification of foreign or 'non-self' substances.24 Foreign antigens may be exogenous microbes or endogenous altered or virally transformed cells.26
The immune system, which recognises foreign micro-organisms as 'non-self' and mounts a response to destroy these disease-causing agents, plays a similar role in protecting the body from malignancy. The damaged DNA in cancer cells frequently directs the mutated cell to produce abnormal proteins known as tumour antigens. These abnormal tumour proteins mark cancer cells as 'non-self'. The immune system likely encounters and eliminates cancer cells on a daily basis. However, it is apparent that cancer cells possess mechanisms that allow them to escape the immune responses that ordinarily prevent the development of malignant tumours.4, 7, 11, 27, 28
When the immune system loses its function of surveillance, tumour cells have the ability to form a tumour. Tumour cells that evade detection can be explained by the following proposed mechanisms:29
- down regulation of major histocompatibility class (MHC) I expression - allowing antigen to go unrecognised
- lack of co-stimulatory signals needed for antigen presentation - loss or alteration of the MHC molecule
- tumour secretion of immunosuppressive products inhibiting the body's immune response
- tumour being immunogenic by expression of one or more antigens
- antigen modulation - where the antigen either enters the cell or leaves it completely, limiting the ability of the immune system to recognise the tumour cell as 'non-self'
- tumours do not give off inflammatory warning signals.
Access a current text and identify the role of the following cells in the immune response to cancer:
- Natural killer (NK) cells.
Describe the ways in which cancer cells evade the immune response.
Explain the differences between innate immunity and adaptive immunity.
Outline the role of cytokines in the immune system.
Outline your evidence based response to a man's question of how his acute leukaemia developed.