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Food Lipids: Chemistry, Nutrition, and Biotechnology

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or complement, or a result of the function of natural killer (NK) cells or phagocytes,<br />

or barriers such as mucosal surfaces. The natural immune system is able to attack a<br />

substance to which it has had no previous exposure; subsequent exposure to that<br />

stimulus does not increase the response. On the other h<strong>and</strong>, the specific immune<br />

response recognizes a specific feature (antigen) of the foreign substance or cell <strong>and</strong><br />

generates an immunologic memory, which amplifies responses during subsequent<br />

exposures. The specific immune system can recruit portions of the innate system to<br />

function simultaneously. Lymphocytes are the primary effectors of the specific immune<br />

response. These cells ‘‘direct’’ much of the immunologic activity by the production<br />

of cytokines of many types.<br />

Cytokines are soluble protein molecules that function as messengers to other<br />

leukocytes. They are produced <strong>and</strong> secreted primarily by monocytes, <strong>and</strong> their main<br />

targets are lymphocytes <strong>and</strong> other monocytes. Some of the important cytokines include<br />

interleukins, interferons, <strong>and</strong> tumor necrosis factors (TNFs). There are at least<br />

15 distinct forms of interleukins, 3 inteferons, <strong>and</strong> 2 tumor necrosis factors.<br />

A. Humoral Immunity<br />

Humoral immunity is a result of the circulation in the plasma of antibodies <strong>and</strong> the<br />

activity of B lymphocytes. The B st<strong>and</strong>s for bursa of Fabricius in birds, which is the<br />

immunological equivalent of the bone marrow in mammals. Humoral immunity characterizes<br />

the production of antibodies by the interaction of B lymphocytes with<br />

plasma cells. Circulating antibodies are the products of these cells, but they cannot<br />

be produced without interaction with T, or thymus-derived, cells. In addition to<br />

antibody production, the complement system is part of the immune response. Complement<br />

is a series of proteins in the plasma that participate in antigen-antibody<br />

reactions, displaying a wide range of actions ranging from aiding in phagocytosis by<br />

leukocytes to killing of tumor cells. Complement serves to stimulate various functions<br />

of the leukocytes.<br />

B. Cell-Mediated Immunity<br />

Cell-mediated immunity is based on the interactions of T lymphocytes <strong>and</strong> monocytic<br />

cells. T cells have been divided into many subclasses based primarily on cell surface<br />

receptors, which vary in relation to the functions these cells perform. The generally<br />

accepted classification of T cells is the CD (cluster designation or cluster of differentiation)<br />

scheme, which depends diagnostically on antibodies to distinguish these<br />

cellular sets of lymphocytes. Therefore, T cells can be classified as CD4 � <strong>and</strong> CD8 � ,<br />

but these cells are often referred to by their effector functions as helper <strong>and</strong> suppressor<br />

cells, respectively. In addition, there are NK cells, LAK (lymphokine-activated<br />

killer) cells, <strong>and</strong> lymphocytes responsible for delayed-type hypersensitivity.<br />

The monocytes <strong>and</strong> their derived macrophages are cells that protect against<br />

certain bacteria <strong>and</strong> tumor cells. While monocytes circulate in the blood, once they<br />

have migrated into tissues they usually become fixed in place <strong>and</strong> are called macrophages.<br />

There are macrophages normally lining venules in the liver (Kupffer cells),<br />

spleen, <strong>and</strong> lymph nodes; these permanently fixed cells are termed histiocytes. The<br />

relative proportions of leukocytes in the blood differ across species. While the predominant<br />

circulating cells in rodents are lymphocytes, granulocytes are the most<br />

common in humans. This difference is relevant in comparing the experimental work<br />

Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.

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