Innate Immunity

Innate immunity is the evolutionary defensive reflex against foreign materials owned by birth. The response is nonspecific in nature. It serves as the first line of defense to present infection.

5.2.1 Anatomical Barriers

The skin and mucosal membrane restrict the entry of pathogens. The epidermis of the skin contains tightly packed epithelial cells inside and dead cells and keratin in the out­side. The keratin is bacteriostatic due to the presence of esterified and nonesterified fatty acids like myristic acid, palmitoleic acid, and linoleic acid. It also contains cationic proteins that make alterations in the cell wall of pathogens making them more prone to osmotic damage. Keratinocytes in the skin also express pattern recognition receptors (PRRs) to recognize pathogens and produce cytokines and antimicro­bial peptides. The sebaceous glands present in the dermis layer contain lactic acid and fatty acids and maintain the skin pH acidic that also restricts the growth of many pathogens. The tight junctions present in the epithelial surface of the skin, lung, guts, and urogenital tracts also provide physical barrier against the pathogens. The mucous layer present at the interior of the epithelial surfaces also provides protective covering against invading pathogens. Mucin and other glycoproteins secreted in the mucous layer prevent the adher­ence of pathogen to the epithelium and are subsequently cleared by the cilia. The antimicrobial peptides defensins present in the mucosal layer also kill the pathogens.

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Influenza virus has a unique ability to bind tightly with the mucous membrane of the respiratory tract due to the presence of a surface molecule, which enables them to escape the ciliary action of the epithelial cells.

5.2.2 Physiological Barriers

The physiological barriers of pathogens include body tem­perature, pH, and several other soluble factors. An increase of core body temperature to the tune of 1-4 °C was proved to be detrimental to many pathogens. Some animals have inherent capabilities to resist infections due to their high body temper­ature (e.g., chicken is naturally resistant to anthrax). The pyrogenic response helps to induce certain cytokines (IL-6) that help in lymphocyte trafficking. “Gastric bactericidal barrier” comprising gastric HCl has the ability to inactivate microorganisms entered during ingestion. Lysozyme of saliva and tears has the ability to cleave the cell wall of bacteria. Virus-infected cells produce interferons, a group of signaling proteins that improve the antiviral defense of neigh­boring cells.

5.2.3 Immune Effector Cells

These are phagocytic cells (granulocytes, monocytes/ macrophages, natural killer cells, and dendritic cells), endo­thelial cells, epithelial cells, lymphoid cells, and platelets. The phagocytes engulf pathogens and kill them by oxygen­dependent and oxygen-independent mechanism.

Granulocytes: Neutrophils, eosinophils, basophils, and mast cells are collectively known as granulocytes due to their granular cytoplasm. These cells are involved in pathogen recognition, engulfment, and phagocytosis. They possess a variety of microbicidal enzymes like lysozyme, collage­nase, and elastase.

Mononuclear Phagocyte System (MPS): MPS consists of circulating monocytes and tissue macrophages. Monocytes after maturation migrate to the tissue and differentiate into tissue macrophages, which have more intracellular organelles and increased phagocytic capabilities with higher hydrolytic enzymes compared to monocytes. There are several tissue macrophages named according to their tissue locations such as histiocytes in skin, alveolar macrophages in lungs, Kupffer cells in liver, mesangial cells in kidney, microglial cells in CNS, and osteoclasts in bone.

Type 1 activated macrophages are concerned with Th 1 immune response and destroy pathogens by nitric oxide (NO) and oxygen-dependent phagocytosis.

The alternatively activated macrophages are unable to produce NO and hence lack phagocytic properties. They produce extracellular matrix proteins and are mainly involved in tissue remodeling.

Type 2 activated macrophages are stimulated in response to IgG and secret IL-10, IL-4, TNF-α, and IL-6.

Dendritic cells (DCs): These are the antigen-presenting cells that reside in the skin and mucosal surfaces. They take the antigen by means of endocytosis, phagocytosis, pinocyto- sis, and macropinocytosis; carry the antigen from periph­eral lymph nodes; and present it to primary lymph nodes. The antigen processing and presentation by dendritic cells are achieved through major histocompatibility complex II. Other important functions of dendritic cells include regulation of cell-mediated immune response and induc­tion of immune tolerance at peripheral lymph nodes. Immature DCs (imDCs) and precursor dendritic cells (pre-DCs) are the two subsets of dendritic cells. imDCs are seen in bone marrow as their precursors are hematopoietic stem cells. A portion of the imDCs then migrate to the epidermis of the skin and become Langerhans cells, while other portions migrate to the dermis and other tissues and differentiate into interstitial imDCs. The mature dendritic cells are potent T-cell activators and inform T cells about the information of pathogens; thus, dendritic cells act as a bridge between innate and adaptive immune response.

Innate lymphoid cells (ILCs): These cells are involved in inflammation.

Natural killer (NK) cells: NK cells are responsible for cell- mediated immune response due to their cytotoxic activity. They possess a unique property called “negative recogni­tion.” The surface receptors of NK cells are inhibitory receptors. These receptors suppress the cytotoxic activity of NK cells in the presence of MHC antigens, and when the infected or malignant cells have decreased expression of MHC antigen, they are recognized by NK cells and undergo cell lysis by perforins secreted from NK cells.

Epithelial and endothelial cells: They express PRRs that recognize pathogen-associated molecular patterns (PAMPs) of pathogens. In addition, they also secrete cytokines like IL-1, IL-6, and IL-8 and antimicrobial peptides.

Platelets: Platelets are the components of blood coagulation mechanism but they also express PRR on their surface and secrete cytokines and chemokines to recruit leukocytes at the inflammatory sites. Platelets can interact with endothe­lium and leukocytes by P-selectin, an adhesion molecule, and initiate pro-inflammatory events.

5.2.4 Pattern Recognition Receptors

The PRRs are able to sense the pathogen-associated molecu­lar pattern (PAMP), conserved molecular pattern of a patho­gen.

Toll-like receptors (TLRs) are expressed on all immune effector cells. They are able to recognize external patho­gen-associated molecular patterns (PAMPs) and internal damage-associated molecular patterns (DAMPs). Till date, around ten TLRs have been identified. Some of them are expressed on cell surface (TLR-1, 2, 4, 5, and 6), and some are intracellular and localized in endoplasmic reticulum (ER), endosomes, and lysosomes (TLR-3, 7, 8, 9, and 10). The intracellular TLRs are also called nucleic acid sensors due to their ability to sense dsRNA and ssRNA of the pathogens.

C-type lectin receptors (CLRs) are mainly recognized bacte­rial sugar moieties but are able to identify molecules associated with dead cells. They are of two types, mem­brane CLRs like Dectin-1 and -2 and soluble CLRs like collectins. The ligands of CLRs are β-glucans, mannose, oligosaccharides, and other microbial carbohydrates.

The nucleotide-binding oligomerization domain (NOD) receptors (NLRs) are intracellular PRRs that recognize peptidoglycans and DAMPs and induce synthesis and secretion of cytokines.

Retinoic acid inducible gen-I (RIG)-like receptors (RLRs) are also intracellular PRRs mainly responsible for antiviral immune response. They have the ability to sense viral dsRNA.

5.2.5 Inflammatory Serum Proteins/Acute-Phase Proteins (APPs)

There are several proteins that act as the mediators or inhibitors of inflammatory process. They are also called acute-phase proteins (APPs). They are mainly synthesized in the liver and their concentrations are increased (or decreased) at the rate of 25% or more at the time of inflammation. They therefore act as a suitable biomarker of inflammation. There are two classes of APPs, viz. positive APPs and negative APPs. The concentrations of positive APPs are increased during inflammation (within 1-2 days). Based on the degree of increment, positive APPs can be categorized as major (usually present in very low concentra­tion but may increase up to 100-1000-fold within 24-48 h and rapidly decline thereafter), moderate (increase five- to tenfold within 48-72 h and decrease at a slower rate than major APPs), or minor (increase only 50-100% above basal levels at a gradual rate).

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Hp and SAA can be used as markers of early detection of subclinical mastitis in cows.

5.2.6 Antimicrobial Peptides (AMPs)

They are used by many organisms as the first line of defense against pathogens. They are multifunctional peptides with bacteriostatic, bactericidal, and cytolytic properties. They are promptly synthesized after infection and kill a wide range of pathogens. Various AMPs along with their functions have been presented in Table 5.4.

Table 5.3 Species variations in terms of major and minor APPs along with their functions

Table 5.4 Antimicrobial peptides and their functions

5.2.7 The Complement System

The complement system is one of the major components of the innate immune response composed of several interlinked proteins that serves a wide array of functions like pathogen recognition, regulation inflammatory processes, killing of the pathogen, and removal of damaged cells. Another major function of complement system is the regulation of adaptive immune responses. Thus, complement system acts as a bridge between innate and adaptive immune responses.

5.2.7.1 Components of Complement System

The complement system consists of several proteins synthesized primarily from liver, macrophages, and neutro­philic granules. In 1963, when the complement system was first discovered, it consisted of only nine proteins labeled by the letter “C” followed by the numbers and their activated forms were designated by added symbol “a” (C1a is an activated form of C1). Till then, a variety of proteins have been identified under complement system. As per the last nomenclature recommended by the International Complement Society (ICS), Complement Nomenclature Committee, and European Complement Network (ECN) boards, there were 50 different proteins and protein complexes (for derailed review, refer Kemper et al. 2014). The complement proteins comprise 5-10% of total plasma proteins. The sizes of complement proteins vary from 24 (D) to 460 kDa (C1q).

5.2.7.2 Mechanism of Action of Complement System

The complement system remains inactive in an uninfected animal. They can be activated either by PAMP or through antigen-bound antibodies. The activation leads to a series of reaction cascades, which ultimately produce a key protein named C3b.

5.2.7.2.1 Activation

There are three different mechanisms by which complement can be activated.

Alternative pathway: The main regulatory protein of alter­nate pathway is C3, which is synthesized in liver and macrophages. It has the highest abundance in serum among the complement components. The alternate path­way operates through three major steps, initiation, ampli­fication, and regulation. In the initiation process, C3 proteins undergo autoactivation by a process called “tickover.” The “tickover” of C3 facilitates the conforma­tional changes in C3 and generates C3(H₂O), which in turn binds with another factor B. The C3(H₂O)∙B complex undergoes cleavage by another serine protease named factor D. Factor D cleaves factor B into Ba and Bb. Bb itself acts as another serine protease that cleaves C3 into its active form C3b. Once C3b is generated, it again associates with factor B to generate more C3 and the proteolytic cycle continues. Another serum protein named properdin stabilizes these protein:protein interactions during the amplification process.

Lectin pathway: This lectin pathway activates after the rec­ognition of oligosaccharide molecules on the surface of pathogen. There are five types of pattern recognition proteins (PRPs) that specifically bind with oligosaccharide moiety of pathogens, namely mannan-binding lectin (MBL), collectin-11 (CL-11), ficolin-1, ficolin-2, and ficolin-3. The first two PRPs bind with glucose, mannose, and N-acetyl-glucosamine, whereas ficolins recognize acetyl groups of bacterial membrane glycoproteins like N-acetyl-glycine, N-acetyl-cysteine, and acetyl-choline. The PRPs are complexed with MBL-associated serine proteases (MASPs), namely MASP-1, MASP-2, and MASP-3. The binding of PRP with the carbohydrate moi­ety results in the activation of MASPs. Activated MASP- 2 splits C4 into C4a and C4b. Complement factor C2 then binds with C4b to form a complex called C4b2. The bound C2 then undergoes cleavage by MASP-2 and yields C4b2b. The protease C4b2b then splits C3 into C3a and C3b.

Classicalpathway: Unlike alternative and lection pathways that involve innate immune response, the classical path­way of complement activation acts in conjunction with adaptive immune response as it is induced by antigen and antibody complexes along with other proteins like CRP, amyloid proteins, and apoptotic bodies. The major com­plement proteins of classical pathway are C1, C2, C4, C1 inhibitor (C1-Inh), and C4-binding protein (C4bp). The classical pathway activates when C1 binds with Fc portion of an Ag-Ab complex. C1 proteins have three subunits C1q, C1r, and C1s. C1q is like a strand, and two molecules each of C1r and C1s are located between the C1q strand. To become active, at least two C1q strands have to bind with antibody molecules. The interaction leads to confor­mational change in C1q, which activates C1r. C1r acts as C1s and exposes its active site to convert C1s subunits as an active enzyme. C1s then cleaves C4 into C4a and C4b. C4b acts as a receptor for C2, and C4b-bound C2 acts as a substrate for C1s and cleaves into C2a and C2b. C2a being smaller diffuses into the plasma, and larger C2b remains attached with C4b. This C4b-C2b complex cleaves C3 into C3a and C3b.

5.2.7.2.2 Amplification

The C3b thus produced by three pathways then interacts with complement factor C5 to become C3b5, which is cleaved by C3bBb into C5a and C3b5b. Factors C6 and C7 then join with C3b5b to form C5b67. Formed C5b67 then interacts with C8 to form C5b678 and further C5b6789 after combin­ing with C9. C5b6789 is the terminal complement complex (TCC) or membrane attack complex (MAC), which forms a hole in the microbial cell membrane and induces osmotic lysis of the microbes.

5.2.7.2.3 Regulation

The regulation of alternate pathway is facilitated by factor H and factor I. Factor H blocks the binding of factor B to factor C3b, and factor I inactivates C3b to iC3b. The sialic acid blocks the alternate pathway by inducing the binding of factor H with C3b, and microorganisms lacking sialic acid are killed, but the host cells that possess a sialoglycoprotein named glycophorin A are protected. The classical pathway is regulated through C1 inactivator (C1-INH), a glycoprotein that blocks C1r and C1s. CD55 or decay accelerating factor present in all blood corpuscles and endothelial cells binds with C3 and C5 convertases and induces their decay, thus protecting the normal cells from complement attack. The

Table 5.5 Functions of different complement components

C4-binding protein (C4BP) in the plasma inhibits C3 convertase (C4b2a). CD35 (CR1) expressed in RBCs, phago­cytic cells, T and B cells, kidney podocytes, and peripheral nerves helps to clear immune complexes and presents com­plement activation. There are some regulatory proteins namely protectin (CD59), clusterin, and vitronectin that inter­fere with TCC formation.

5.2.7.2.4 Functions of Complement System

Upon activation, complement system generates a wide array of products that ultimately leads to lysis of pathogens but the products of complement cascade have a wide range of functions detailed in Table 5.5.

5.2.7 Cytokines

Cytokines are the low-molecular-weight (smaller than 30 kDa) proteins or glycoproteins synthesized from leukocytes or other cells of the body and act as soluble mediators to regulate immunity. The cytokines exert its action after binding with its receptors in the target cells and subsequently activate the intracellular signaling cascade that ultimately alters the gene expression of target cells and causes differentiation, proliferation, and activation of the target cells. The cytokines act in autocrine, paracrine, and endocrine fashion.

5.2.8.1 Properties of Cytokines

The cytokines have properties like the following:

Pleiotropy: When a cytokine has different effects on different types of target cells, the cytokine is said to be pleiotropic. IL-4 produced from activated T_H cells causes proliferation, differentiation, and activation of B cells but only proliferates thymocytes and macrophages.

Redundancy: When two or more cytokines exert similar effect on single target cells, the effect is said to be redun­dant. IL-2, IL-4, and IL-5 produced from T_H cells cause proliferation of B cells.

Synergy: It is the cooperative effect of cytokines. Here, the cytokines in combinations have more pronounced effect compared to their individual effect. IL-4 and IL-5 pro­duced from T_H cells induce B cells to produce IgE, but neither IL-4 nor IL-5 has individual effect to induce B cells for IgE production.

Antagonism: When the effect of a cytokine is inhibited by another cytokine, then the effect is called antagonism. The effect of IL-4 on B cells is inhibited by IFN-γ.

Cascade reaction: When one cytokine induces a target cell to produce one or more cytokines that in turn stimulates another target cell to produce other cytokines, it is called cascade reaction. IFN-γ produced from activated T_h cells stimulates macrophages to secrete IL-12 that in turn stimulates activated TH cells for IFN-γ, TNF, and IL-2 secretion.

5.2.8.2 Classification of Cytokines

There are six different cytokine families, namely interleukins, chemokines, interferons, tumor necrosis factors (TNF), colony-stimulating factors (CSF), and transforming growth factor-β. There are several subgroups under different families.

Interleukins: They are so named with a thought that it was synthesized by leukocytes but later, it was found that interleukins can be produced from a variety of cells. They play a pivotal role in hematopoiesis, activation, and differentiation of immune cells. They also have pro-inflammatory properties and help in leukocyte migra­tion and adhesions. Till date, 40 interleukins have been identified and named alphabetically from IL-1 to IL-40.

Chemokines: These are chemotactic cytokines and attract the leukocytes to the site of infection. Structurally, chemokines are subdivided into four families based on the N-terminal cysteine residue.

CC chemokines: They have two adjacent cysteine residues at N-terminal region. Twenty-eight CC chemokines have been identified (named from CCL-1 to CCL-28) so far, and majority of them are chemotactic for monocytes.

CXC chemokines: They are characterized by two cysteine residues separated by an amino acid at the N-terminus. There are 17 CXC cytokines, namely CXCL-1 to CXC-17. They attract neutrophils at the site of infection.

C chemokines: They have two cysteine residues, one at N-terminal region and another at downstream. There are two C chemokines (XCL-1 and XCL-2).

CX3C chemokines: CX3C chemokines are having two cysteine residues at N-terminus separated by three amino acids. Besides their role in chemotaxis, they are also involved in cell adhesion. Till date, a single CX3C chemokine has been identified (CX3CL1).

Interferons (IFN): They emerged as antiviral proteins, but later, their roles in immunomodulation and cancer immu­nology have been identified. IFNs are classified into three types, type I (IFN-α and IFN-β), type II (IFN-γ), and type III (IFN-λ1, 2, and 3).

Tumor necrosis factor (TNF): TNF is produced from activated natural killer (NK) cells, macrophages, and T lymphocytes with diverse physiological functions in cell proliferation, differentiation, and carcinogenesis. It is also a pro-inflammatory cytokine. The TNF is classified into TNF-α and TNF-β. TNF-α is produced from monocytes, macrophages, and T cells and has functions as inflamma­tory mediators and cell adhesions. TNF-β is produced mainly from activated lymphocytes and has functions similar to TNF-α.

Colony-stimulating factor (CSF): These are responsible for differentiation of leukocytes in the bone marrow. There are four families of CSF, namely granulocyte-colony­stimulating factor (G-CSF), macrophage-colony­

stimulating factor (M-CSF), granulocyte-macrophage-col­ony-stimulating factor (GM-CSF), and multiple-colony­stimulating factor (also called IL 3). M-CSF is responsible for differentiation of macrophage precursors. G-CSF is responsible for differentiation of granulocyte precursors. GM-CSF is produced from lymphoid and nonlymphoid cells and helps in maturation and differentiation of both granulocytes and monocytes. The role of multiple-colony­stimulating factors is the differentiation of hematopoietic stem cells into myeloid progenitor cells. There are two other CSF like erythropoietin and thrombopoietin.

Transforming growth factor-β (TGF-β): It can be produced from a variety of cells including T cells and monocytes. The main function of TGF-β is the inhibition of cellular growth and production of extracellular matrix. TGF-β also acts as a negative regulator of T-cell and macrophage activation.

5.2.8.3 Pro- and Anti-inflammatory Cytokines

The cytokines secreted in response to an infection mainly by the macrophages and that upregulate the inflammatory response are called pro-inflammatory cytokines. They are pyrogenic in nature and stimulate acute-phase reactions. IL-1 and TNF-α are predominant pro-inflammatory cytokines. The anti-inflammatory cytokines are responsible for the downregulation of inflammatory process. IL-6 is a potent anti-inflammatory cytokine that inhibits the effects of IL-1 and TNF-α. IL-4 and IL-10 are also anti-inflammatory cytokines. The overproduction of pro-inflammatory cytokines compared to anti-inflammatory cytokines leads to autoimmune diseases.

5.2.8.4 Mechanism of Action of Cytokines

The cytokines exert its effects after binding with target cell receptors. There are six classes of cytokine receptors, namely type I cytokine receptors, type II cytokine receptors, chemo- kine receptors, tumor necrosis factor receptor (TNFR) super­family, TGF-beta receptors, and immunoglobulin (Ig) superfamily. The receptors are associated with tyrosine kinases called Janus kinases (JAKs) and transcription factors called signal transducer and activator of transcription (STAT). The binding of cytokine with its receptor leads to the activation of JAK by phosphorylation. Phosphorylated JAK combines with STAT, and the complex then moves to nucleus, binds with DNA regulatory site, and activates tran­scription. The transcription of DNA causes protein synthesis, and the response of cytokine is generated.

5.2.8.5 Functions of Cytokines

The functions of different cytokines, their sources, and target cells have been detailed in Table 5.6.

5.2.8.6 Pathogen Recognition and Inflammatory Signaling in Innate Immune System

Innate immune system, being the first line of defense, recognizes the pathogen during the initial stage of infection and subsequently eliminates the pathogens from the host body. To achieve this, the innate immune system utilizes pattern recognition receptors (PRRs) that recognize patho­gen-associated molecular patterns (PAMPs), distinct struc­ture present on the pathogen (Table 5.7).

Table 5.6 Cytokines, their sources, target cells, and functions

(continued)

Table 5.6 (continued)

Table 5.7 Pathogen-associated molecular pattern and pathogen recog­nition receptor (PRR)

Besides PAMPs, the PRRs of innate immune system also recognize damage-associated molecular patterns (DAMPs). DAMPs are endogenous molecules released in response to stress or tissue injury and are potent stimulators for noninfec- tious inflammation. Different DAMPs and their receptors are presented in Table 5.8.

Through the binding of ligands (PAMPs or DAMPs), PRRs are stimulated, facilitate downstream signal transduc­tion, and lead to transcriptional activation of genes for pro-inflammatory cytokines, chemokines, cell adhesion mol­ecule, and IFNs. This pro-inflammatory signaling pathway also activates the adaptive immune response.

5.2.8 Inflammation

It is a complex tissue reaction against tissue damage or pathogenic microorganisms that results in clearance of the invading pathogens through the activation of the components of the innate immune system. Rubor (redness), calor (heat), dolor (pain), and tumor (swelling) are four cardinal signs of inflammation stated by the Roman physician Celsus. These signs can be well explained by the major events of inflamma­tion such as the following:

Vasodilation: It is mediated by nitric oxide (NO) and vasodilatory prostaglandins. Pro-inflammatory cytokines such as IL-1 and TNF-α produced from activated leukocytes stimulate inducible nitric oxide synthase (iNOS) and cyclo-oxygenase (COX-1 and -2). iNOS produces NO from L-arginine. NO in turn causes smooth muscle relaxation. Prostaglandins (PGI2, PGD2, PGE2, and PGF2α) and prostacyclins are the vasodilatory prostaglandins synthesized from arachidonic acid by the action of COX-1 and -2. Both these NO and prostaglandins cause vasodilation, and engorgement of the capillary network leads to redness (rubor) and increased tissue temperature (calor).

Increased capillary permeability: The alteration in the capil­lary permeability is mediated by the release of certain inflammatory mediators such as histamine, bradykinin, leukotrienes, and platelet-activating factor (PAF). Together, the increased vascular permeability and capil­lary hydrostatic pressure lead to leakage of protein-rich fluid (exudate) in the interstitium of the inflammatory site. Accumulation of exudates causes edema or swelling that allows the delivery of antibodies and other acute-phase proteins in inflamed site.

Leukocyte migration: See functions of neutrophils.

5.2.9 Phagocytosis

Phagocytosis is the ability to ingest or engulf other cells and particles by some specialized cells called phagocytes. The process was discovered by Elie Metchnikoff (1845-1916) during his studies on some marine organisms. He was the pioneer to develop the concept cellular immunity and was awarded the Nobel Prize in 1908 together with Paul Ehrlich for notable contribution in the field of immunology. In uni­cellular organisms, phagocytosis is a process of cell nutrition, but for multicellular organisms, it is a means to kill the

Table 5.8 Damage-associated molecular pattern (DAMP) and pathogen recognition receptor (PRR)

pathogen and cellular debris and thus plays an important role in innate immunity as well as tissue homeostasis. Based on these two functions, phagocytes can be classified as preferen­tial phagocytes (neutrophils, macrophages, monocytes, den­dritic cells, and osteoclasts) that act to eliminate pathogens. The nonprofessional phagocytes (epithelial cells, fibroblasts, and endothelial cells) are mainly involved in the elimination of apoptotic bodies.

A phagocyte can recognize a pathogen either directly by PRRs present on them or indirectly through some molecules that form a bridge between the phagocyte and the particle to be ingested. These are called opsonins such as antibodies (IgG) and complement components, and the process is called opsonization. The direct PRRs of phagocytes are Dectin-1, mannose receptors, CD14, and scavenger receptor A (SR-A) that recognize polysaccharides, mannans, lipopolysaccharide, and lipoteichoic acid, respectively. Fcγ receptors (FcγR) are the opsonic receptors that bind with the Fc portion of IgG molecules.

After the interaction between phagocytes and ingesting particles, a series of signaling events initiate that leads to the remodeling of membrane and cytoskeleton of the phagocytes to form pseudopods that cover the particle and a depression called phagocytic cup is formed at their point of contact. The target particle is then surrounded by the mem­brane, and it closes at the distal end to form phagosome. The phagosome thus formed interacts with endosomes and lysosomes and finally fuses to form phagolysosome.

The cytotoxic effects of phagocytes are achieved through oxygen-dependent and oxygen-independent mechanisms. The phagocytes produce a number of reactive oxygen and nitrogen intermediates with potent microbial activity. The production of reactive oxygen and nitrogen intermediates occurs through a metabolic process called respiratory burst, which activates peroxidase enzymes. The reactive oxygen intermediates are superoxide anion (O₂'^-), hydroxyl radicals (OH'), singlet oxygen (¹O₂), hydrogen peroxide (H₂O₂), hypochlorous acid (HOCl), and monochloramine (NH₂Cl). The reactive nitrogen intermediates are nitric oxide (NO), nitrogen dioxide (NO2), and nitrous acid (HNO2). In oxygen-independent mechanism, the killing of pathogens is achieved through defensins, tumor necrosis factor, and hydrolytic enzymes.

5.3