Man suffers from various diseases by different pathogenic organisms such as viruses, bacteria, parasitic protozoans, helminthes etc. Also many harmful organisms live in our body as symbionts, but sometimes they invade certain of our body tissues when the resistance of our body to the action of these organisms is very low. Our body broadly has two lines of defense to fight against these infections. They are as follows:

1)    Non-specific defence mechanism: The skin of our body acts as the physical barrier and it forms the first line of defence against the pathogens. The outer firm covering layer of the skin does not allow the bacteria and virus to enter into our body. The oil and sweat secreted by respective glands prevent the growth of microorganism on the surface of the skin. Further, the lysozyme contained in the sweat helps in destroying the cell wall of the bacteria. Therefore the entry of the pathogens into the body does not become possible. So the pathogens may enter into the body through some injuries, if they occur on the surface of the body. Now the body’s second line of defence starts working. The capillaries of the blood vessels surrounding injured tissue become dilated and their wall becomes more permeable to the phagocytic WBCs. Therefore the WBCs come out and invade the foreign germs.

2)    Specific Defence mechanism or the immune system: Immune system was discovered by Edward Genner and he also discovered the small pox vaccination. The immune system has two components, such as:

a)    Humoral immune system: This system consists of varieties of proteins called antibodies present in the blood plasma. When any foreign germ invades the body, the antibodies inactivate the same and then these foreign germs are invaded by the phagocytic action of the WBCs. The different kinds of antibodies form the third line of defence.

b)    Cell-mediated immune system: This system protects the body from different pathogens including fungi and protists that enter into the body from time to time. The system also reacts against tissue transplants and even against some cells of the body, which become cancerous. One of the important characteristics features of this immune system in our body is that it is able to differentiate the body’s own cell and the cells entering into the body from out side. Antibodies are produced by the immune system in response to the entry of foreign antigens into the body.

Cells of immune system

Lymphocytes are the most important type of cells of the immune system. These are produced in the bone marrow. Some of these lymphocytes migrate to the thymus gland and are called T-cells, whereas others continue to develop in the bone marrow and are called B-cells. Out of the B-cells and T-cells, the former produce the antibodies and the latter impart cellular immunity.

B-cells and T-cells are set into action when a foreign antigen enters into the body. The B-cells are stimulated to produce antibodies. Once an antigen-specific B-cell is activated, it continues to multiply at a faster rate to produce a clone of B-cells. These are called as the plama cells.

In a similar way, when a T-cell responds to any specific antigen, it multiplies rapidly to form a clone of T-cells which are differentiated to be of following types.

1) Killer T-cells: These cells attack the antigens directly and destroy them.

2) Helper T-cells: these cells stimulate the B-cells to produce more antibodies.

3) Suppressor T-cells: These cells suppress the entire immune system so that the latter does not attack the body’s own cells.

4) Memory cells: These cells undergo only limited differentiation after a first exposure to an antigen. They are stimulated into activity on subsequent entry of the same antigen into the body.

Interferon: Interferon is a protein produced and released in small quantities when the cells are invaded by virus and it is effective in inhibiting the viral growth. It is effective against a wide range of viruses.

Immunity

Immunity refers to the body’s ability to resist infection, afforded by the presence of circulating antibodies and white blood cells. A person may develop immunity in the following ways:

1.    Immunity through diseases: When a person develops certain disease due to the entry of a pathogen for the first time, then the pathogen multiplies in his body at a faster rate and gradually spread to the different parts. Then the antibody formation starts slowly and by that time the person suffers from the disease. But as the rate of antibody production increases, the person gets cured gradually. This is because of the fact that the antibody formed in response to the foreign antigen, neutralizes the action of the latter. But the same person does not suffer from the disease due to subsequent infection by the same pathogen. For example, If a person suffers from measles due to the entrance of measles virus into the body, then corresponding antibody production will start in order to neutrslise the action of the virus. After some days the person gets cured, but the antibody remains in his blood fro long time. When the virus enters into the body again, because of the presence of the antibody the person does not suffer from the disease again.

2.    Vaccination: It is the process of producing immunity to a disease by the vaccine. In this process the weaken or dead pathogens are injected into the body, so that immunity develops against the pathogen in him but the disease is not caused. Vaccination can be injected or can be given orally.

Types of Immunity

Immunity may be natural or acquired. Natural immunity is also called inborn immunity which means resistance to a disease is there in the body since birth. But acquired immunity develops in the body after the entry of disease causing pathogens.

Autoimmunity: This is a disorder in the defence mechanism of the body, because the antibody produced in this process may act against the products of the own body tissue, treating them as foreign materials.

Immuno deficiency: It means a deficiency in the immune response by the body due to decreased number of less active lymphoid cells, or even in some, where there is complete absence of these cells. Individuals with such deficiency are highly susceptible to even minor infections.