Info

of IL1

I LI acts on "T helper" cells to promote differentiation and release of IL2

Specific Defense Mechanisms

There are two types of specific defense mechanism. Humoral immunity defense involves the production of specific immunoglobulins (Ig), which are globulin proteins that have antibody activity. The cellular immunity involves the stimulation of certain white blood cells to destroy specific antigens. The organs involved include the anterior kidney, thymus and spleen. The head kidney and the spleen have antibody-producing cells as well as phagocytes and macrophages. The spleen produces lymphocytes. Fishes lack bone marrow, and the above tissues act, in place, to supply the necessary elements for a functional immune system. Fish immunoglobins are tetramer molecules as opposed to the pentamers in humans and there is only one class of immunoglobins in fish compared to the two to five classes in mammals. These proteins are found in lymph, blood, mucus, and interstitial fluids.

Humoral Immunity. In this type of specific immune response, antigens are selectively removed from the circulation by the use of antibody-producing white blood cells, which are specific to particular antigen. There are two types of cell involved in humoral immunity. Some of the cells produce the antibody and some act as memory cells. Once exposed to a particular antigen, the memory cells can rapidly grow and multiply when they encounter that antigen again.

Cellular Immunity. In this type of specific immunity, the antigen causes certain groups of primed white blood cells to act specifically against the invading antigen. These WBCs can ingest or destroy the antigen as well as proliferate chemical messengers signaling the presence of non-self material. It seems that immersion-vaccination results in eliciting a particularly strong cellular immunity. This type of immunity is also referred to as cell mediated immunity. The following are various cell types involved in cell mediated immunity.

Neutrophils are the first line of defense after antigen invasion. These cells are phagocytic in humans and some fish such as catfish, but they are not in salmonids. They may release a toxic substance in the vicinity of the antigen. They are abundant in the WBC picture. Macrophages have a phagocytic function and also play a significant role as a secretory immune cell. They are more like the human neutrophil in their phagocytic and bactericidal action. Special processing macrophages process and present the antigen to the lymphocytes. The antigen is taken up by these macrophages and processed internally. The antigen is then brought to the surface for lymphocytes to destroy. T-cell lymphocytes are antigen-specific lymphocytes that send chemical messengers that enhance phagocytic and bactericidal activity of macrophages. T-cells do not engulf pathogens, although they do have receptors for antigens. They also enhance macrophage activity. The activities of T cells are restricted by the fact that the antigen must be first processed by a macrophage and presented to this lymphocyte. They are unable to respond directly to free antigen. Their response to an antigen presented to them is to differentiate into effector T-cells and memory T-cells. The co ordinated action of the various components of the specific immune system is demonstrated in Figure 12.

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