**Abstract**

The human body has many mechanisms to resist invaders like pathogenic bacteria to avoid harm according to the living creature's law "survival for the best". On the opposite; *Salmonella* as pathogenic bacteria have many weapons that they utilize to invade the human body. The resistance mechanisms expressed by the human body are called immunity which represented by the immune system that has many different types of resistance processes, either specific (adaptive immune response) or non-specific (Innate Immune Response) against certain pathogenic invaders. As far as these processes are strong they will be enough to avoid infections occurrence, otherwise, the human body will get infected with *Salmonella*, be ill, show the disease symptoms, transmit the disease to others, and may become a carrier for the pathogen according to many circumstances. Prevention is still stood the most effective way to avoid getting infected with *Salmonella* by personal hygiene or suitable vaccination if available.

**Keywords:** immune system, *Salmonella* virulence factors, *Salmonella* infection, *Salmonella* resistance and *Salmonella* vaccination

### **1. Introduction and overview**

#### **1.1 Introduction to immunology and immune system**

Immunology is the study of our protection against foreign macromolecules or invading organisms and our responses to them. These invaders include viruses, bacteria, protozoa or even larger parasites. Any Human body is continuously exposed to pathogenic microorganisms. The immune system is composed of two major subdivisions of immune system, the innate or nonspecific immune system and the adaptive or specific immune system [1].

The innate immune system is our first line of defense against invading organisms while the adaptive immune system acts as a second line of defense and gives protection against re-exposure to the same pathogen. Each of the major subdivisions of the immune system has both cellular and humoral components by which they carry out their protective function and help each other to do these functions. Since pathogens may replicate intracellularly (viruses and some bacteria and parasites) or extracellularly (most bacteria, fungi and parasites), different components of the immune system have evolved to protect against these different types of pathogens [2].

#### **1.2 Innate or non-specific defenses**

Include first line of defense which acts before invasion of pathogenic microbes. And the second line of defense which acts after invasion. The anatomical barriers that works mainly against infections with microbial invaders. This first line of defense represented by the epithelial surfaces and skin form the physical barriers that are very impermeable to most infectious agents [1].

The shedding of skin epithelium also helps remove bacteria and other infectious agents that have adhered to the epithelial surfaces. Movement due to cilia or peristalsis helps to keep air passages and the gastrointestinal tract free from microorganisms. The trapping effect of mucus that lines the respiratory and gastrointestinal tract helps protect the lungs and digestive systems from infection [2].

Chemical barriers like Lysozyme and phospholipase found in saliva and other secretions can breakdown the cell wall of bacteria and destabilize bacterial membranes. The low pH of gastric secretions prevents the growth of bacteria [3].

The microbiota of the skin and in the gastrointestinal tract can prevent the colonization of pathogenic bacteria by secreting toxic substances or by competing with pathogenic bacteria for nutrients or attachment to cell surfaces. They represent the biological barriers of the innate immunity [2].

The anatomical barriers are very effective in preventing colonization of tissues by microorganisms. However, when there is damage to tissues the anatomical barriers are breeched and infection happens. Once infectious agents have penetrated tissues, another innate defense mechanisms comes into play, namely acute inflammation as the second line of innate immune defense. Many Humoral and cellular factors play an important role in inflammation against microbial invasion, which is characterized by edema and the activation of phagocytic cells [4].

These humoral factors are found in serum or they are formed at the site of infection. They contain Complement system, Interferons and Lysozymes. The most important humoral barrier is the Complement system, since it acts as with the phagocytic cells as a bridge between specific and non-specific immune response. Complement system represents a set of glycoproteins in blood. Once they are activated after rapid cascade events that can lead to increase vascular permeability, activation of phagocytic cells, opsonization of bacteria and lysis [5].

Complement glycoproteins are synthesized by liver cells (hepatocytes) and macrophages and many other cell (e.g. gut epithelial cells). All normal individuals have complement components in their blood. This system can be activated by [1, 2]:


The complement system takes part in both specific and non-specific resistance and generates a number of products of biological and immunological importance. The functions of the complement system are summarized in **Table 1** [3, 5]**:**

On the other side; the cellular factors are the main line of defense in the nonspecific immune system, they are listed in the **Table 2** [2, 5].

### Salmonella *and the Immune System DOI: http://dx.doi.org/10.5772/intechopen.95673*


#### **Table 1.**

*The functions of the complement system.*


#### **Table 2.**

*Cellular factors of the nonspecific immune system and their function.*

### *1.2.1 Phagocytosis and intracellular killing*

Phagocytosis is a very important process during non-specific immune response when specialized cells engulf foreign body like bacteria or molecule like toxin or virus. The phagocytosis has four steps, **Figure 1** [2]:


**Figure 1.** *Phagocytosis process steps.*

3.Phagolysosome formation and degradation of foreign substances. After attachment of the bacteria the phagocyte begins to extend pseudopods around the bacteria and surround and engulf them forming the phagosome. During phagocytosis the granules or lysosomes of the phagocytes bind or fuse with the phagosome and empty their contents. The result is the foreign bodies or bacteria engulfed in the phagolysosome which have the contents of the granules or lysosomes.

Intracellular killing and Digestion (Lysis and excretion): There are three means of killing the microorganisms inside phagocytic cells; either Oxygen dependent killing by formation of NADPH using Oxygen, then production of the toxic oxygen compounds like H2O2 and hydroxyl radical (OH•). These compounds are toxic to microbes and kill them, Oxygen independent killing by production of toxic hypochlorite (OCl-) and singlet oxygen (1O2) from H2O2 using the enzyme Myeloperoxidase that released into the phagolysosome or Nitric oxide dependent killing by Toxic nitric oxide synthesis and production (NO) when microorganism binds to the macrophage because of cytokines release (TNF-α and IFN-γ) [3].

Oxygen -dependent killing and Oxygen -independent killing both are called the Respiratory burst. After killing, the enzymatic system of the cell will digest all the phagosome components then absorb the useful materials and excrete the residues to the environment (blood) by fusing the phagolysosome with the cell membrane.

The cells that able to do phagocytosis are (monocytes, macrophage, PMNs and dendric cells). The results of phagocytosis are either a complete destruction of foreign body and excretion (PMNs). Or a complete destruction of foreign body and some parts (polypeptides) of it will be processed and presented on the surface of the phagocytic cells (monocytes, macrophage and dendric cells) then the phagocytic cell will be antigen presenting cell (APC) [2, 3].
