Immunity (Ch. 5)

Back to neuro-revision list or Tutorial 7 notes or BBB Home Page

The immune system:

A vast, diffuse lymphoid system, including bone marrow, spleen, lymph nodes (glands), cells of the reticulo-endothelial system (macrophages) and circulating white blood cells (leukocytes).
Function: defence. Identification and destruction/inactivation of 'foreign' (non-self) material, e.g. pathogens - bacteria, viruses, toxins.


The immune system identifies 'foreign' material by its chemical nature (antigen)

On first exposure to an antigen, the immune response is slow, taking about one week to mobilise an effective defence. Thus, many pathogens (e.g. viruses) cause overt disease the first time they invade.

However, 'memory cells' (see below) in the system 'remember' this antigen and the next time it is encountered, the response is very rapid. 'Immunity' has been established, and the disease does not recur. The immune 'memory' is longlasting.

Mechanisms of defence:

Phagocytosis - ("cell-eating") some white cells (polymorph neutrophils, macrophages) can engulf and destroy pathogens.

Cytotoxicity - killing or immobilising pathogens with 'poisons' secreted from the white cells.

Inflammation - tissue and vascular reaction that isolates the pathogen. and allows access of defence cells to site of invasion or damage.

Humoral immunitv - antibodies. Antibodies are specific chemicals produced in response to 'foreign' antigens. They do not actually 'kill' pathogens carrying the antigens, but they 'label' them as foreign, identifying them for attack by defence cells.

(In some diseases - auto-immune diseases - antibodies are produced against the body's own cells, which are then attacked by the host's own immune system.)


Small lymphocytes are unique in that they are the only type of white cell that has an 'immune memory'. After exposure to a particular antigen, some of the small lymphocytes involved in the response convert into 'memory cells', capable of responding rapidly to this antigen, should it be encountered again (even after many years). There are different types of small lymphocyte: There are many similarities between lymphokines and neurotransmitters.
Small lymphocytes have receptors that can bind a wide range of neurotransmitters, e.g. opioid peptides, hormones, catecholamines. This raises the possibility of interactions between neurons and immune cells.

Some neurons secrete lymphokines (It is not certain if these have an immune function, or if it merely reflects the common embryonic origins of these cells.)

Neural control of the immune system

Lymphoid organs are innervated by autonomic (sympathetic) nerves. White cells in lymphoid organs carry adrenoreceptors. Thus in theory, autonomic nerves could control the function of these white cells. There is some evidence that blocking these adrenoreceptors can depress lymphokine release by the white cells.

Does the nervous system influence the immune system?

Classical conditioning of the immune response

Rats become ill when given an injection of cyclophosphamide (CPA). CPA is a cytotoxic drug used for treating some tumours, and which kills rapidly dividing cells, including some responsible for the immune response; thus CPA depresses the immune response. If the CPA injection (UCS) is accompanied by saccharin-flavoured water (CS), the rats associate the illness with the saccharin flavour - and they will avoid this flavour in the future. (A classically conditioned avoidance response). If the CPA-treated rats are later given saccharin-flavoured water, but without further doses of CPA ('extinction conditions'), they still display a long-standing aversion to the saccharin (as expected), but some rats continue to show a profound depression of the immune response. This was not simply due to prolonged action of the CPA. In the animals given an injection of CPA paired with saccharin-flavoured water there is a greater suppression of the immune response if they are given saccharin-flavoured water (CSr) afterwards than in other rats that drink only plain water afterwards (CS0).

Levels of antibodies in the blood of rats at various times after immunization with foreign cells. P is the placebo group, given a sham injection (i.e. no CPA) and plain drinking water. CS0 animals were given one dose of CPA paired with saccharin, and drank plain water thereafter. CSr animals received one dose of CPA paired with saccharin, but were re-exposed to saccharin-flavoured water daily thereafter. (Book 2, Fig 5.8)

Thus, the immune response can be suppressed by classical conditioning. This raises the possibility that such immunosuppressive conditioning could be used instead of immunosuppressant drugs for reducing the rejection of tissue grafts and even for the treatment of autoimmune diseases.

The functioning of the immune system can also be impaired by the effects of STRESS.

Back to neuro-revision list or Tutorial 7 notes or BBB Home Page