The main function of the immune system in humans and animals is to detect and then neutralize or destroy invading pathogens, such as viruses, bacteria, fungi, and parasites. In addition, it is responsible for eliminating worn-out and abnormal self-cells. For these purposes, two types of immune responses have evolved, classified as innate and adaptive. Innate immune responses are also called nonspecific because they can be triggered by certain molecular patterns common to whole classes of pathogens and do not vary in subsequent encounters with the same antigen. The major mechanisms of innate immunity include phagocytosis, inflammation, complement activation, and induction of cell death. The main classes of phagocytes, that is, cells able to engulf entire bacteria or particulate matter, are neutrophils and macrophages. The chemical messengers they and some other cell types produce play an important role in the initiation of an inflammatory response. The killing of virus-infected cells and cancer cells through induction of programmed cell death or apoptosis is accomplished by natural killer (NK) cells.

Adaptive immune responses are highly specific for a particular antigen and become stronger and faster in subsequent encounters. They are the responsibility of B cells and T cells. The main function of B cells is to produce antibodies, which neutralize pathogens or stimulate their elimination by other cell types. There are five major classes (isotypes) of antibodies: immunoglobulin (Ig)A, IgD, IgE, IgG, and IgM, with IgA and IgG having 2 and 4 subclasses, respectively. Resting B cells express IgM and IgD on their cell surface as antigen receptors. Upon activation, B cells start to secrete IgM. It is only later in the immune response that they undergo a process called isotype switching and begin to produce other types of Ig. T cells are classified into helper T cells and cytotoxic T cells. One subclass of helper T cells provides help to macrophages in killing pathogenic microorganisms they have engulfed. The other subclass plays a vital role in inducing B cell antibody production. Cytotoxic T cells directly eliminate infected cells by initiating apoptosis.

All immune cells are ultimately derived from the same precursor or progenitor cells in the bone marrow, which eventually differentiate into myeloid (monocytes/macrophages, neutrophils, basophils, eosinophils, and mast cells) and lymphoid cells (B and T cells). Differentiation occurs under the influence of a variety of growth factors and cytokines. The differentiation of lymphoid progenitors into lymphocytes takes place in the central lymphoid organs, that is, bone marrow in the case of B cells and thymus in the case of T cells. After their maturation in these primary lymphoid organs, both types of lymphocytes migrate to the peripheral lymphoid tissues, that is, the lymph nodes, spleen, and lymphoid tissues associated with mucosa. It is in these peripheral lymphoid tissues that the reaction of B and T lymphocytes with foreign antigens takes place.

During immune responses, immune cells themselves and several other cell types start to produce a variety of messenger molecules, including chemokines and cytokines. Chemokines attract specific immune cells or subsets of them to the affected tissue(s). Cytokines are vital in shaping the exact nature of the response and in coordinating the functions of the various cell types involved. They do so in part by inducing the production of numerous proteins and the activity of certain enzymes, some of which produce further chemical messengers.

Spirulina is the designation commonly, but incorrectly, used for several types of cyanobacteria (blue-green algae) belonging to the Oscillatoriaceae family. The algae used for dietary supplements and animal feed belong to the genus Arthrospira. In order to avoid confusion, we will follow the common practice of calling them Spirulina rather than Arthrospira. Three species of these algae have been used in the experiments described here: S. platensis, S. fusiformis, and S. maxima.

Spirulina contains approximately 60% protein, a variety of polysaccharides, essential fatty acids, vitamins and minerals, and phenolic compounds. One of the major proteins in Spirulina (15%-20% of algal dry weight) is C-phycocyanin, which consists of the apoprotein and covalently attached phycocyanobilin chromophores, which are responsible for the blue coloring of these cyanobacteria. In addition to whole Spirulina, some of its polysaccharide fractions and C-phycocyanin have been investigated for their ability to influence immune functions.

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