Chapter 30 Inflammation and the immune system
The Function of the Immune System
The construction of the immune system allows it to adapt so as to mount a more rapid and vigorous attack each time it encounters a particular pathogen.
Components of the Immune System
Leucocytes
• Neutrophils
Other Components
How Does the Immune System Work?
Pathogens that invade the body stimulate the immune system, which has three main stages of response:
2. To fragment the pathogen by phagocytosis: the fragments are presented on the surface of antigen-presenting cells (APCs) in the lymph tissue and provide a marker that enables the cells of the immune system to recognize that type of pathogen. The APCs also release a cytokine (interleukin-1; see below), which encourages the proliferation of B and T cells.
3. To secrete interleukins: the above stimulates the cells of the immune system to secrete interleukins, which further stimulate the immune system. The B and T lymphocytes are cloned in the lymph nodes. Some cells produce a pool of memory cells (M) specific to a particular antigen, which will enable the body to respond much faster with another exposure. This takes place in the lymph node and is known as the induction phase. The other cells go on to the effector phase.
The effector phase is one of the following:
• Humoral-mediated phase: involves antibodies, which act in the fluid of the blood and tissues to involve the B cells. There is also some added stimulation from interaction between the B cells and T helper cells.
• Cell-mediated phase: this works where the antibodies cannot reach, deals with pathogens in cells and involves the T cells.
Humoral Response
A humoral-mediated response (Figure 30.1):
• Is an immediate reaction. Hence the speed of anaphylactic shock, hay fever, asthma and some food allergies. It is a type 1 reaction.
• The T helper cells not only activate B cells but also release interleukins (signalling molecules and part of a group classed as cytokines). These interleukins stimulate eosinophils, mast cell production and the generation of IgE, which are the mediators in chronic inflammation.
• IgE has a strong tendency to attach to mast cells and basophils, changing the composition of their cell membranes so that they release their contents. People with atopic allergies have high levels of IgE in their blood.
• Some of the components are histamine, protease, platelet-activating factors, etc., which open-up the local blood vessels, attracting the eosinophils and neutrophils to the site. The eosinophils and neutrophils attack and destroy bacteria. The site is also damaged by the contents such as the proteases. There is loss of fluid into the tissues and contraction of the local smooth muscles.
Antibodies or immunoglobulins (hence the shorthand Ig) have two main functions:
1. To recognize and interact specifically with particular antigens (Figure 30.2): this can provide some damage limitation.
2. To activate the immune system: the tails act as a tag to activate the complement sequence (see below).
There are five classes of antibody: IgA, IgD, IgE, IgG and IgM.
