THE IMMUNE SYSTEM

Immunity is defined as ‘the capacity to recognize the intrusion or material foreign to the body and to mobilize cells and cell products to help remove that particular sort of foreign material with greater speed and effectiveness.

In this section we shall be confining our attention to the immune response. This is the second line of defense after the phagocytes which, according to the above definitions also part of the immune system. The immune response is the production of antibodies in response to antigens. Each antigen is recognized by a specific antibody.

Antibodies, Antigens, B-cells and T-cells

An antibody is a molecule that is synthesized by an animal in response to the presence of foreign substances knows as antigens. Each antibody is a protein molecule called an immunoglobulin.

An antibody is a molecule which can cause antibody formation. All cells possess antigens in their cell membrane surface which act as markers, enabling cells to recognize each other. Antigens are usually proteins or glycoproteins, that is, proteins with a carbohydrate tail, although almost any complex molecule can be antigen. The body can distinguish its own antigens (self) from foreign antigens (non-self) and normally only makes antibodies against non-self antigens. Microorganisms carry antigens on their surfaces.

Two systems of immunity have been developed by mammals, a cell mediated immune response and a humoral immune response. The two systems involve the development of two types of lymphocytes, the T and B cells. Both types arise from precursor cells in the bone marrow.

Cell-mediated response: T Cells attack the (i) Cells that have become infected by a microorganism, most commonly a virus. (ii)Transplanted organs and tissues. (iii) Cancer-causing cells.

The whole cell is involved in the attack, so this type of immunity is described as cell-mediated immunity. T cells do not release antibodies.

Humoral Response: B cells release antibodies into the blood plasma, tissue fluid and lymph. As the antibodies are released into fluids and the attack on the microorganisms takes place in the fluid, this type of immunity is described as humoral (‘humor’ means fluid). The antibodies of B cells attack bacteria and some viruses.

The Immune System Has a Memory: There are two types of B cells namely memory cells and effecter cells (meaning they carry out the response). Effecter cells are also known as plasma cells. These secrete huge numbers of antibody molecules into the blood, tissue fluid and lymph. Effecter cells live for only a few days. The memory cells survive for long periods of time and enable a rapid response to be made to any future infection.

The memory cells are important if a second infection of an antigen occurs. The population of memory cells is much larger than the original population of B cells from which they came. Therefore the response to the second infection, called the secondary response, is much more rapid and is also greater than the primary response to the original infection. The primary response may not be rapid enough to prevent a person suffering from an infection, but if that person survives, they will rarely suffer from it again because of the greater secondary response. With each exposure, the response gets more efficient. This is the basis of vaccination (and booster doses).

Types of Immunity: Immunity may be described as active or passive. Both types may be acquired naturally or artificially. Providing immunity artificially is called Immunization.

Natural Active Immunity: This is the kind of immunity which is obtained as a result of an infection. The body manufactures its own antibodies when exposed to an infectious agent. Because memory cells, produced on exposure to the first infection, are able to stimulate the production of massive quantities of antibody when exposed to the same antigen again, this type o immunity is most effective and generally persists for a long time, sometimes even for life.

Artificial Active Immunity (Vaccination): This is achieved by injecting (or less commonly administering orally) small amounts of antigen, called the vaccine, into the body of an individual. The process is called vaccination. The antigen stimulates the body to manufacture antibodies against the antigen. Often a second, booster injection is given and this stimulates a much quicker production of antibody which is long lasting and which protects the individual from the disease for a considerable time. Several types of vaccine are currently in use.

(a) Toxoids: Toxins (poisons) produced by tetanus and diphtheria bacteria are detoxified with formaldehyde, their antigen properties remain. Therefore vaccination with the toxoid will stimulate antibody production without producing symptoms of the disease.

(b) Killed organisms: Some dead viruses and bacteria are able to provoke a normal antibody response and are used for immunization purposes. An example is the flu vaccine which contains dead flu viruses.

(c) Live Vaccines’ (attenuated organisms): An attenuated organism is one which has been crippled’ in some way so that it cannot cause a disease. Often it can only grow and multiply slowly. Attenuated vaccines for the bacterial disease tuberculosis (TB), and for measles, mumps, rubella (German measles) and polio are in general use.

(d) Smallpox: It is now extinct; a live virus vaccine was used.

(e) New Vaccines: New approaches to vaccine design are now possible using modern techniques of molecular biology and genetic engineering. An alternative approach is to synthesize antigen artificially from amino acids, once their amino acid sequences are known.

Passive Immunity

In passive immunity antibodies from one individual are passed into another individual. They give immediate protection, unlike active immunity which takes a few days or weeks to build up. However, it only provides protection against infection for a few weeks, for the antibodies are broken down by the body’s natural processes, so their number slowly fall and protection is lost.

Natural Passive Immunity

Passive immunity may be gained naturally. For example, antibodies from a mother can cross the placenta and enter her fetus. In this way they provide protection for the baby until its own immune system is fully functional. Passive immunity may also be provided by colostrums, the first secretion of the mammary glands. The baby absorbs the antibodies through its gut.

Artificial Passive Immunity

Here antibodies which have been formed in one individual are extracted and then injected into the blood of another individual which may or may not be of the same species. They can be used for Immediate protection if a person has been; or is likely to be, exposed to a particular disease. For example, specific antibodies used for combating tetanus and diphtheria used to be cultured in horses and injected into humans. Only antibodies of human origin are now used for humans. Antibodies against rabies and some snake venoms are also available. Antibodies against the human rhesus blood group antigen are used for some rhesus.