**3. Ocular defense mechanisms**

The surface of the eye is well protected by both mechanical and immunological defense mechanisms. To breach the defense mechanism, some form of trauma is essential. The eyelids provide mechanical protection to the surface of eyeball. The eyelashes protect against airborne particles and trauma by initiating blink reflex. The cornea is also sensitive to tactile sensation and helps in the initiation of blink reflex, which is provided by dense sensory nerve endings. The lids direct the tears, particulate debris, allergens and microbes to the lacrimal excretory system by its sweeping action over the anterior surface of the eyeball. Bell's phenomenon also provides protection to cornea as globe is turned upwards and slightly outwards during eyelid closure to avoid corneal exposure [47]. Meibomian glands secrete lipids, which provide stability to the tear film. The epithelial surface of the cornea and conjunctiva provides ana‐ tomical barrier to the pathogens. This function is further strengthened by the impermeability provided by the basement and cellular junctional complexes of the cornea. Indigenous flora of the eye also provides protection by creating a competition for colonization by the pathogens.

Immune defense mechanisms are provided by the vascular supply of the eye. Any breach in the anatomical defense system initiates the ocular inflammatory response, which helps in vasodilation and exudation of immunologically active substances and cells [1, 8, 48–52].

#### **3.1. Defenses of the tear film**

There are three layers of the tear film: oil, aqueous and mucous. Majority of the tear film is composed of aqueous layer and pH of the tear film helps in neutralization of toxic substances. Flow of tears help in mechanical flushing of the foreign particles and allergens into the lacrimal excretory system. Mucosal layer helps in entrapment of pathogens. Tear film contains various immunological active substances such as lactoferrin, lysozyme, β-lysin, ceruloplasmin, complement and immunoglobulins.

#### **3.2. Conjunctival defenses**

The conjunctival associated lymphoid tissue lies beneath the conjunctiva. It consists of both B and T lymphocytes. B and T cell precursors mature when exposed to foreign particles or allergens, then migrate to regional lymph nodes for further development, and thereafter return to the conjunctiva through blood stream to produce specific immunoglobulins and cellular defense responses.

#### **3.3. Corneal defenses**

Although the cornea is avascular, it is provided by limited defense mechanisms in the form of Langerhans cells (dendritic cells) and immunoglobulins. The surface of the cornea is covered by mucous glycoprotein, which helps in cross-linkage of the IgA and protects the anterior surface of the cornea. Immune defense mechanisms are activated whenever injury occurs, leading to recruitment of the polymorphonuclear cells, lymphocytes and fibroblasts.

#### **3.4. Cellular immune responses**

Langerhans cells are situated along the peripheral margin of the cornea and conjunctiva. These cells possess receptors, which help in phagocytosis and processing of certain antigens for presentation. Langerhans cells stimulate B and T cells to elicit a strong cellular immune response. During inflammation Langerhans cells migrate toward the cornea, causing increased release of inflammatory substances.

#### **3.5. Leukocyte defense**

Polymorphonuclear leukocytes are the hallmark of acute inflammation and are associated with oxygen-dependent pathways for the generation of free radicals that help in killing of the invading pathogens. Another immune defense mechanism operated by the production of defensins is antimicrobial proteins active against wide range of pathogens.

#### **3.6. Defensins**

tomical barrier to the pathogens. This function is further strengthened by the impermeability provided by the basement and cellular junctional complexes of the cornea. Indigenous flora of the eye also provides protection by creating a competition for colonization by the pathogens.

Immune defense mechanisms are provided by the vascular supply of the eye. Any breach in the anatomical defense system initiates the ocular inflammatory response, which helps in vasodilation and exudation of immunologically active substances and cells [1, 8, 48–52].

There are three layers of the tear film: oil, aqueous and mucous. Majority of the tear film is composed of aqueous layer and pH of the tear film helps in neutralization of toxic substances. Flow of tears help in mechanical flushing of the foreign particles and allergens into the lacrimal excretory system. Mucosal layer helps in entrapment of pathogens. Tear film contains various immunological active substances such as lactoferrin, lysozyme, β-lysin, ceruloplasmin,

The conjunctival associated lymphoid tissue lies beneath the conjunctiva. It consists of both B and T lymphocytes. B and T cell precursors mature when exposed to foreign particles or allergens, then migrate to regional lymph nodes for further development, and thereafter return to the conjunctiva through blood stream to produce specific immunoglobulins and cellular

Although the cornea is avascular, it is provided by limited defense mechanisms in the form of Langerhans cells (dendritic cells) and immunoglobulins. The surface of the cornea is covered by mucous glycoprotein, which helps in cross-linkage of the IgA and protects the anterior surface of the cornea. Immune defense mechanisms are activated whenever injury occurs,

Langerhans cells are situated along the peripheral margin of the cornea and conjunctiva. These cells possess receptors, which help in phagocytosis and processing of certain antigens for presentation. Langerhans cells stimulate B and T cells to elicit a strong cellular immune response. During inflammation Langerhans cells migrate toward the cornea, causing increased

Polymorphonuclear leukocytes are the hallmark of acute inflammation and are associated with oxygen-dependent pathways for the generation of free radicals that help in killing of the

leading to recruitment of the polymorphonuclear cells, lymphocytes and fibroblasts.

**3.1. Defenses of the tear film**

46 Advances in Common Eye Infections

complement and immunoglobulins.

**3.2. Conjunctival defenses**

defense responses.

**3.3. Corneal defenses**

**3.4. Cellular immune responses**

release of inflammatory substances.

**3.5. Leukocyte defense**

Ocular surface is constantly exposed to environment and foreign bodies, thus there are greater chances of infection. However, robust innate immune system at ocular surface protects the eye from infection. There are several peptides of defensins and cathelicidin families that are present in tear film and secreted by corneal and conjunctival cells. These are not only antimicrobial in nature but also help in the recruitment of immune cells and thus provide a link to adaptive immunity. The important defensins present in human eye are hBD-1 (human beta defensins), hBD-2, hBD-3, CAP37 (Cathelicidin-related antimicrobial peptide), LL37 (type of cathelicidin) and HNP-1, 2, 3 (human neutrophil defensins) [53].
