**2. Methodology**

Google Scholar (https://scholar.google.com) and PubMed (https://pubmed.ncbi. nlm.nih.gov) scientific databases were used to search for articles published between the years 2000 and 2022 containing the keywords, "immune response" AND "coccidiosis" in combination with "broiler chickens," "avian immunity," "intestinal immunity," "Coccidiosis Vaccines," "*Eimeria* Vaccines." Only manuscripts and book chapters in English or Spanish were included. Data from other animal species were also omitted except for the general overview of immune system. Data obtained in broiler chickens were grouped in tables including, the overview of avian immune response, peculiarities, intestinal immune response against coccidiosis and vaccines, type of *Eimeria* spp. infected, age at infection, among others.

## **3. A brief overview of the avian immune system**

The immune system (IS) may be compared with a symphony orchestra in which a variety of molecules, cells, and tissues are finely organized to maintain the ideal state of homeostasis. In a nutshell, the IS may be defined as "A set of cells and molecules that defend the host against external (infections, trauma, among others) and internal aggressions (internal infections, autoimmunity, allergy as well as cancerous tumors)" [13].

### *From Understanding the Immune Response against Coccidiosis to the Use of Coccidia Vaccines DOI: http://dx.doi.org/10.5772/intechopen.110611*

The IS works as a passive system, meaning that it requires a threat to trigger an immune response (**Figure 1**). Once the IS is activated after the first contact with a foreign microorganism through the recognition of pathogen associated molecular patterns (PAMPs) and binding it with a variety of pattern recognition receptors (PRRs) the immune response is triggered. If innate immunity fails to eliminate the pathogen, adaptative immunity goes into action and activates more specific mechanisms to eliminate, obtain memory, and restore homeostasis [13].

Adaptative immunity comprises antigen presenting cells, lymphocytes (lym) including B and T cells as well as cytokines. There are fundamental properties of adaptative immune responses called cardinal features. Some include specificity, diversity, memory, nonreactivity to self (self-tolerance), and systemic localization (because of the ability of lym and other immune cells to circulate among tissues) [14]. There are two types of adaptative immunity: humoral and cell-mediated immunity which are mediated by different types of lym and work to kill different types of microbes [14]. Humoral immunity is conducted by molecules in the blood and mucosal secretions and is termed the secretory system [15].

T lym orchestrate cell-mediated immunity. Many pathogens can survive and replicate within the cells of the host. They are inaccessible to humoral response secretory molecules in these locations. As a result, cell-mediated immunity plays a role in the defense against this internal microorganism [14].

Protective immunity against a pathogen may be provided either by the host response (active immunity) or by transfer of secretory molecules that defend against the microbe. An important example of this form of immunity is the transfer of maternal antibodies by the bird to its offspring through the egg yolk, when the antibody is absorbed and enters the circulatory system, thus preventing or reducing clinical outcomes [16].

Among avian species, immune response in chickens is currently most studied followed by turkeys [17]. In theory, the avian immune response works similarly to the mammalian system. There are far more immunology studies conducted in mice compared with chickens. The use of pathogen infection models in mice has led to a greater advance of immunology understanding in mammals. Extrapolations from mammals to birds must be cautiously performed. A quote by the famous chicken evolutionist and immunologist Jim Kaufmann "chickens are not mice with feathers"

supports that the study of the avian IS is worthwhile [18]. Avian IS seems to be simpler than mammals. Although both do the same actions, different pathways are sometimes used [19, 20].

The most known difference is that Avian B lym are developed in the Bursa of Fabricius (BF), a unique bird organ, and not in bone marrow as in mammals [21]. Other important differences include the major histocompatibility complex (MHC), tumor necrosis factor (TNF) and its receptor (TNFR) superfamilies, chemokines as well as the interleukin (IL) 1 superfamily, where the chicken repertoire is smaller. There are other cases with the opposite relationship such as the immunoglobulin-like receptor family where the chicken repertoire is greater than that of mammals [22]. The full descriptions and details about the avian immune system are found elsewhere and are beyond the scope of this review [17, 19, 23].

## **4. Intestinal immunity in birds**

The gastrointestinal tract (GIT) is a complex environment because it is responsible for the digestion and absorption of nutrients, is constantly exposed to pathogens, and harbors beneficial microbiota of the host [24]. In addition, the GIT is the largest immune and nervous system, which is constantly challenged with immunogens from different sources including food, foodborne, and infectious pathogens as well as microbiota [25]. These actions may sound like a biological paradox which can be explained as follows: the poultry host must simultaneously maintain homeostasis (or the absence of disease) with nutrient absorption, intestinal integrity, exclusion of harmful microbes, tolerance of beneficial microbiota, and shaping mucosa immune response [26, 27].

The structure of the GIT varies throughout the length of the gut. In a nutshell, the intestine is a pipe with a tubular structure surrounded by a linear layer of epithelial cells embedded in a basement membrane (**Figure 2**). It is also composed of columnar absorptive cells (enterocytes), enteroendocrine, goblet cells, as well as immune intestinal cells. Tight junctions are an intercellular complex protein system that connects epithelial cells. These compartments are organized in protruding villus structures to increase the surface area of absorption. These structures are composed of an epithelial layer, a core of underlying lamina propria (containing the microvasculature), and a thin layer of smooth muscle (muscularis mucosae). In the intestine, each villus is an absorptive unit [28]. There are also structures, known as crypts, which are defined as the site of stem cells with proliferating abilities for self-renewal and differentiation, thus maintaining homeostasis in the intestinal epithelium [29]. These crypts are interspersed in indentations. The villus crypt blocks may vary in their maturation stage in distinct locations along the intestine. There is a zone known as "proliferative" within the crypt where stem cells are located and divide to form daughter cells that migrate from crypt to villus and survive between 48 to 96 hours, after which they are sloughed into the lumen and die by apoptosis in the tip [30]. The time depends on the length of the villus and age of the chicken. During this migration process, the enterocytes acquire differentiated functions in terms of digestion, absorption, and mucin secretion [31, 32]. The intestinal mucosa is covered by mucus, a complex hydrated gel that protects epithelial cells from chemical, enzymatic, microbial, and mechanical damage. The epithelium and its mucus layer permit the selective movement of ions, nutrients,and water, but restrict the translocation of microbes and toxins from the lumen [33].

*From Understanding the Immune Response against Coccidiosis to the Use of Coccidia Vaccines DOI: http://dx.doi.org/10.5772/intechopen.110611*

**Figure 2.** *Schematic diagram of the architecture of intestinal immune cells.*

The structures between the small and large intestine of the birds are quite different. While villus/crypt units are present throughout the whole small intestine, the large intestine has villus-like outgrowth structures, with a ruffled structure, known as folds. Hyperactive crypts are found within each folded unit [29].

Gut mucosa is exposed to food immunogens as well as microbiota antigens that are required for the processing of nutrients and the education of the local immune system early after hatching. As a result, there are organized structures which function as key organized elements of cells and molecules to defend the host against intestinal threats. These structures are known as Gut Associated Lymphoid Tissue (GALT). GALT is the largest compartment of the immune system and is comprised of lymphoid cells residing in the epithelial lining and distributed in the underlining in the lamina propria. In addition, there are specialized lymphoid structures. GALT's main role is to limit progression of systemic infection by detecting and destroying infectious agents in their early stages. In poultry, GALT encompasses esophageal tonsils, pyloric tonsils, Meckel's diverticulum, Peyer's patches, and two caecal tonsils (this is the most GALT important organ) [34, 35]. GALT is comprised of more immune cells than any other host tissue including different cell subsets and including most major cell populations found at other sites. These include heterophils, macrophages, DC, natural killer (NK) cells, as well as B and T lym (although the proportions of each cell type differ according to locality, microbial status, and age) [29].

The entire GIT is covered by a protective mucus consisting of Mucins family proteins which are produced by Goblet cells. Lysozyme, native microbiota, gastric juices, bile salts, as well as cationic peptides and other substances which act as a nonspecific defense are also important participants in the process [36]. Thus, GALT detects not only harmful pathogens as a potential threat of the intestine but also normal gut

microbiota and self-antigens that can elicit autoimmune responses. Therefore, a comprehensive study of the avian GALT is crucial to develop oral vaccines which can be alternatives to replace antibiotic growth promoters and immunomodulatory molecules that maintain intestinal homeostasis with the best performance [36].
