Specific defense mechanism is the ability of the body to develop immunity against specific pathogens, toxins or foreign things. This is possible by a special immune system that produces antibodies and/or activated lymphocytes that attack and destroy specific invading organisms or toxins. Specific defense mechanisms are also referred to as adaptive or acquired immunity (Table 1).
Organisms that possess an adaptive immunity also possess innate immunity and many of the mechanisms between the systems are common, so it is not always possible to draw a hard and fast boundary between the individual components involved in each, despite the clear difference in operation. Higher vertebrates and all mammals have both an innate and an adaptive immune system.
The characteristic features of the adaptive immunity are the following:
a. Specificity:
The immune system has the ability to work against specific pathogens.
b. Diversity:
It also has the ability to recognise a large number of foreign molecules.
c. Discrimination between ‘Self’ and ‘Non-Self’:
The immune system does not normally work against the molecules present in the body. It functions against molecules that are foreign, i.e. ‘non-self’.
d. Memory:
The immune system generates an immune response when the invader or pathogen attacks the body for the first time. This is retained in the memory and if the system encounters the same pathogen for the second time, it is eliminated rapidly offering protection against that particular pathogen.
The Organs of the Immune System:
The immune system is made of the primary lymphoid and the secondary lymphoid organs. The primary lymphoid includes the bone marrow and the thymus, while the others such as the spleen, Peyer’s patches of small intestine and the lymph nodes are included in the second category.
a. Bone Marrow:
The bone marrow produces B-cells, natural killer cells, granulocytes and immature thymocytes, in addition to red blood cells and platelets. The process of formation of the cells of the immune system is called hematopoiesis. During hematopoiesis, cells differentiate into either mature cells of the immune system or into precursors of cells that migrate out of the bone marrow to continue their maturation elsewhere.
b. Thymus:
The function of the thymus is to produce mature T-cells. Immature thymocytese or prothymocytes, leave the bone marrow and migrate into the thymus. The prothymocytes undergo a maturation process, where the cells beneficial to the body are spared, while the cells detrimental to the body are eliminated. The mature T- cells are then released into the blood stream.
c. Spleen:
The spleen is the ‘immunological filter’ of the blood. It is a bean-shaped organ. It is made up of B-cells, T-cells, macrophages, dendritic cells, natural killer cells and red blood cells. Old red blood cells are destroyed in the spleen.
d. Lymph Nodes:
The lymph nodes as small solid structures located at different points along the Lymphatic system. They are the immunologic filters of the lymph. Lymph nodes are located throughout the body. It is composed mostly of T-cells, B-cells, dendritic cells and macrophages. It traps microorganisms and other antigens that enter the lymph or tissue fluid. The trapped antigens are responsible for activation by lymphocytes and initiating immune response.
e. Mucosal Associated Lymphoid Tissue (MALT):
These are lymphoid tissue located along the lining of the respiratory, digestive and urinogenital tracts. It constitutes about 50% of the lymphoid tissue in the human body.
Important Cells of the Immune System:
a. Lymphocytes:
The lymphocytes are of two types that are functionally and phenotypically different from each other. They are the T lymphocytes and the B lymphocytes.
b. Natural Killer Cells or NK Cells:
NK cells attack cells that have been infected by pathogens, including viruses and cancer cells.
c. Macrophages:
Macrophages are important in the regulation of immune responses. They are often referred to as scavengers or antigen-presenting cells (APC). They pick up and ingest foreign materials and present these antigens to other cells of the immune system such as T-cells and B-cells. This is one of the important first steps in the initiation of an immune response.
d. Dendritic Cells:
Dendritic cells, which also originate in the bone marrow, function as antigen presenting cells (APC).
Types of Specific Defense Mechanisms:
There are two main types of specific defense mechanisms involved in the immune system:
a. The Humoral Immune System:
This system acts against bacteria and viruses in the humour or body liquids such as blood and lymph. The body produces antibody molecules or immunoglobulins in the plasma that can attack the invading agent.
b. The Cellular Immune System or Cell-mediated Immune System:
This system takes care of cells that are infected by viruses or cells that are cancerous.