Immunity is the defensive mechanism of our body to resist against infection. Immunology deals with the defense mechanisms against parasites, so that these animals are normally capable of resisting the infection by most pathogens.

     The pathogens are always recognized by the host cells through chemical interactions; a specific antigen or immunogen, present in the parasite’s body, elicits in the host the production of a specific glycoprotein complex an antibody molecule called immunoglobulins.

     It is also possible that a foreign protein, not necessarily belonging to a pathogen, may act as an antigen so that when injected, it may induce antibody formation. Certain low molecular weight molecules called haptens, ray though bind to antibodies, do not individually stimulate antibody production. Basic immunity is provided by the leukocytes.

The two basic types of leukocytes are:

     1. Phagocytes, cells that chew up invading organisms

     2. Lymphocytes, cells that allow the body to remember and recognize previous invaders and help the body destroy them

A number of different cells are considered phagocytes. The most common type is the neutrophil, which primarily fights bacteria. There are two kinds of lymphocytes: the B-lymphocytes and the T lymphocytes. Lymphocytes start out in the bone marrow and either stays there and matures into B cells, or they leave to the thymus gland, where they mature into T cells. B-lymphocytes and T lymphocytes have separate jobs to do:

T cells can be
     (i) Cytotoxic T lymphocytes (CTLs), which destroy infected cells of the host;

     (ii) Helper T cells that help B cells and the cytotoxic T cells to mature and perform their immune response related function

     (iii) Suppressor T cells, which block B and T cell responses, whenever required.

B-lymphocytes are like the body’s military intelligence system, seeking out their targets and sending defenses to lock onto them. T cells are like the soldiers, estroying the invaders that the intelligence system has identified.

Immune response
     An injection of an antigen may initiate several important changes in the living. The first response is to eliminate the antigen through extensive phagocytosis. So that nearly 90% of the antigen is removed from the circulation in about 10 to 20 minutes of its introduction. A number of different cells are considered phagocytes. The most common type is the neutrophil, which primarily fights bacteria. Besides these interferons and antiviral proteins are secreted by the leukocytes when infected with virus. If an antigen gets passed through the external barriers, it is attacked and destroyed by other parts of the immune system.

Cell mediated and Humoral response
     In the second phase, this lasts for about 4 to 7 days. Activation of immune response typically begins when a pathogen enters the body. Macrophages that encounter the pathogen ingest process and display the antigen fragments on the cell surfaces. Macrophages with Antigen fragments displayed on the cell surface are called “Antigen presenting cells”. An Antigen presenting macrophage interacts with T helper cells that recognize the same antigen. During the interaction, the macrophage releases the chemical alarm signal called Interleukin-1, which stimulates the T helper cells to secrete Interleukin-II. The interleukin-II stimulated the Cyto toxic Lymphocytes and B cells. The immune response from this point follows two paths, one using T cells and one using B cells.

     Normal cells of the body that become infected can also digest the same antigen on their cellular surfaces. The body makes millions of Cytotoxic T cells. Each type of CTL will be able to recognize a particular antigen. The Cytotoxic T cells release chemicals and kill the infected cell, which also causes the destruction of pathogens.

     B cell also recognizes the particular antigen. When B cells are activated by the T helper cells, they differentiate into plasma cells. The plasma cells are the Antibody producing factories, flooding the blood stream with antibodies that bind the particular antigen involved in the infection.

     Antibodies bind to the antigens present on the surface of the pathogens, marking them for destruction by the macrophages. Some of the B cells do not turn into antibody factories, but instead become Memory B cells that may survive for several decades. Because of the memory B cells the secondary immune response” to the future infection by the same pathogen is swifter and stronger. This powerful secondary immune response is what the” Immunity” to some diseases. This concept is prime part of vaccination now days.