**1. Immune system: a brief introduction**

The immune system is a complex and highly developed system, yet its mission is simple: to seek and kill intruders. It is the body's defense system against infectious organisms and other invaders. The purpose of the immune system is to keep infectious microorganisms, such as bacteria, viruses, and fungi, out of the body, and to destroy any infectious microorganisms that do invade the body. Through a series of steps called the immune response, the immune system protects us against invading organisms. It is a network of cells, tissues, and organs working together to protect the body. The most important cell types involved in immune response are white blood cells, which come in two basic types that combine to seek out and destroy disease-causing organisms. Leukocytes are produced or stored in different locations in the body, like the thymus, spleen, bone marrow lymph nodes, and special deposits of lymphoid tissue (as in the gastrointestinal


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nutrition.

**Table 1.**

*Major cell types of adaptive immunity.*

*Immune Cell Activation: Stimulation, Costimulation, and Regulation of Cellular Activation*

Dendritic cells Originate in the bone marrow and form another class

Neutrophils These cells, which ingest, kill, and digest pathogens,

cytochrome b558.

cationic proteins.

their biological effects.

biologic functions.

Eosinophils Defend against parasites and participate in hypersensitivity

Basophils Along with their tissue counterparts, mast cells produce

Monocytes/macrophages These are involved in phagocytosis and intracellular

of APCs. These are found in the lymphoid organs such as the thymus, lymph nodes, and spleen along with the bloodstream and other tissues. They function to capture and process antigens in lymphoid organs at the time of

are the most highly adherent and motile, phagocytic leukocytes; the first cell types to be recruited to acute inflammatory sites. Their functions are dependent on adherence molecule CD11b/CD18, or biochemical pathways, such as the respiratory burst associated with

reactions. Their cytotoxicity is mediated through cytoplasmic granules containing eosinophilic basic and

cytokines required for defense against parasites and allergic inflammation. They display surface membrane receptors for IgE antibodies and possess cytoplasmic granules containing heparin and histamine. When cell-bound IgE antibodies are cross linked by antigens, the eosinophils degranulate releasing low-molecular weight vasoactive mediators (e.g., histamine), which mediate

killing of microorganisms. Macrophages are differentiated monocytes, residing in the

reticuloendothelial systems and act as antigen-presenting cells presenting processed peptides to T cells. They are recruited to inflammatory sites and further activated by exposure to certain cytokines, which potentiate their

initiation of an immune response.

tract). These cells circulate through the body between the organs and nodes via lymphatic and blood vessels, and work in a coordinated manner to monitor the body for foreign invasions. A potent and active immune system is vital for staying healthy. The immune system differentiates between invaders and the body's own cells—when immune system is not able to differentiate between self and nonself, a reaction against "self " cells and molecules causes autoimmune disease. The immune system will remain active by getting enough sleep, exercise, and good

Immune response leads to inflammation. The goal of inflammation is to get rid of the stimulus—both the disease-causing pathogens and/or neoplastic tissue. Significant steps involved in inflammation include recruitment of immune cells, interactions of these cells in the affected tissue and activation pattern of the interacting cells. The immune system continuously looks for pathogens, xenobiotics, and other nonself signals. Thus, the cell types of the immune system are incredibly dynamic and capable of upregulating processes required for handling these insults

*DOI: http://dx.doi.org/10.5772/intechopen.81568*

**Cell type Location and function**


*Immune Cell Activation: Stimulation, Costimulation, and Regulation of Cellular Activation DOI: http://dx.doi.org/10.5772/intechopen.81568*

#### **Table 1.**

*Immune Response Activation and Immunomodulation*

**Cell type Location and function**

Leukocytes Derived from myeloid or lymphoid lineage, these are

infections [1] B cells In mammals, B cells mature in the bone marrow where

T cells Originate in the bone marrow and mature in the thymus

T helper cells Subset of T cells, found throughout the body, with

T cytotoxic cells Subset of T cells, found throughout the body, with

immune response.

secretions from TH cells.

Natural Killer cells NK cells are similar to Tc cells. They directly kill certain

Macrophages These are phagocytic cells and function as antigen-

phagocytosis.

the main cells in immune system, which provide either innate or specific adaptive immunity. Myeloid cells include phagocytic, motile neutrophils, monocytes, and macrophages, providing the first-line defense against pathogens. Other myeloid cells involved in defense against parasites & in genesis of allergic reactions include eosinophils, basophils, & mast cells. Lymphocytes regulate the action of other leukocytes & generate specific responses to prevent chronic/recurrent

as in birds, B cells mature in the bursa of Fabricius, a lymphoid organ first discovered by Chang and Glick [2]. They develop into antibody-secreting plasma cells. B cells express B cell receptors (BCRs) on their cell membrane that allows them to bind to specific antigen, against which

giving rise to helper, regulatory, cytotoxic T cells, or memory T cells. From the thymus, they migrate to peripheral tissues, blood, & lymphatic system. On stimulation, they secrete chemical messengers called cytokines, which stimulate the differentiation of B cells into antibody-producing cells also called plasma cells. Cytotoxic T cells in the presence of various cytokines bind

especially high titers in lymphoid organs (lymph nodes and spleen), as well as the liver, lung, blood, and the intestinal tract. T helper TH or CD4+ T cells coordinate and regulate immunological responses. TH cells mediate responses by secreting lymphokines that act on other cell types involved

especially high titers in lymphoid organs (lymph nodes & spleen), as well as the liver, lung, blood, and the intestinal tract. T cytotoxic Tc or CD8+ T cells are involved in directly killing virus-infected cells, transplanted cells, and sometimes, eukaryotic parasites and tumor cells. CD8+ T cells have been shown to play a role in downregulating the

tumors such as melanomas, lymphomas and virus-infected cells- and clear herpes and cytomegalovirus-infected cells. In contrast to Tc cells, NK cells kill their target cells more effectively without the need for recognition of antigen in association with MHC molecules and are activated by

presenting cells (APCs) as they ingest foreign materials and present these to other members of the immune system such as T cells and B cells. Besides being the initiators of an immune response, they also act as immune modulators by secreting cytokines and can also be stimulated by lymphokines, to exhibit increased levels of

it initiates a specific antibody response.

to and kill infected and/or cancer cells.

in mounting an immune response.

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*Major cell types of adaptive immunity.*

tract). These cells circulate through the body between the organs and nodes via lymphatic and blood vessels, and work in a coordinated manner to monitor the body for foreign invasions. A potent and active immune system is vital for staying healthy. The immune system differentiates between invaders and the body's own cells—when immune system is not able to differentiate between self and nonself, a reaction against "self " cells and molecules causes autoimmune disease. The immune system will remain active by getting enough sleep, exercise, and good nutrition.

Immune response leads to inflammation. The goal of inflammation is to get rid of the stimulus—both the disease-causing pathogens and/or neoplastic tissue. Significant steps involved in inflammation include recruitment of immune cells, interactions of these cells in the affected tissue and activation pattern of the interacting cells. The immune system continuously looks for pathogens, xenobiotics, and other nonself signals. Thus, the cell types of the immune system are incredibly dynamic and capable of upregulating processes required for handling these insults

on a time scale of minutes to hours. Several cell types in this system are capable of activation to secrete cytokines, rapidly proliferate, or otherwise communicate to surrounding cells that there is a pathogen to consider. Upon clearance of the pathogen, the cell population must contract in a controlled manner. Furthermore, in some cell populations (e.g., T cells), a subset of cells is retained as long-lived memory cells to protect and prime the system for future insults. Researchers are increasingly focused on early events in immune cell activation, where the response to an inflammatory signal can be tuned to impact overall cell function. In areas such as immuno-oncology, increased activation is connected to improve cell expansion whereas in the field of immunosuppression, the converse is desired. Clinical successes in targeting the immune system for treating cancer have generated a resurgence of effort to harness the immune system more routinely for therapeutic intervention (**Table 1**).
