Ritu's Biology Hub

Continued from Part 3

Innate Barriers

1. Eye

The conjunctiva is a specialized, mucus secreting membrane lining the interior of eyelids and the exposed surface of eyeballs. The continuous flushing action of tears from lacrymal glands keeps away microorganisms from the eye. Tears contain lysozyme, lactoferrin and sIgA and thus provides a chemical and physical protection.

2. Skin

The intact skin is an effective physical barrier. The skin contains tightly packed group of tiny cells called keratinocytes. Keratinocytes contain a special protein called keratin which are scleroproteins that form a major component of hair, nails and the human skin. These parts of the human body continually shed these cells removing any grime or microbes sticking to them. The pH of the skin is also very acidic due to secretion of oil glands, sweat glands and OA (organic acids) production by Staphylococcus. The skin also contains concentrated amounts of NaCl protecting the body from a wide number of microbes. However, sometimes microbes are successful in invading the dermis tissue underlying the epidermis. Here, they encounter a special group of cells called as SALT (Skin Associated Lymphoid Tissue). Their main function is to localise the microbes to the area immediately underlying the epidermis and preventing them from entering the bloodstream. One type of SALT cells is the Langerhan cells that phagocytose the pathogens. Once they have internalized the pathogen they translocate to nearby lymph nodes and mature into dendritic cells. These then present the Ags on their surfaces that are recognized by lymphocytes which in turn trigger the specific immune response. Another kind of cells in the SALT system is the intraepidermal lymphocytes. They are present on the skin to intercept the microbes that breach the 1^st line of defense. These resemble T cells but unlike T cells they do not have diversity in receptors and are specialized to recognize a common pathogen pattern. There are tissue macrophages in the dermis that can phagocytose most microorganisms.

3. Mucus membrane

The mucus membrane has intact stratified epithelial cells and mucus secretions that form a protective covering and resist penetration while entrapping microorganisms. Furthermore, some mucosal surfaces are bathed in antimicrobial secretions e.g. cervical mucus, prostatic fluid and tears which contain an antimicrobial substances – lysozyme (muramidase) which hydrolyses the β (1 -> 4) linkage between NAM and NAG in bacterial cell wall peptidoglycan especially in Gram postives. They also contain some special proteins which help prevent establishment of microbes. These mucus secretions contain iron-binding proteins called as lactoferrin. Lactoferrin sequesters iron from the plasma, reducing the amount of iron available to the invading pathogens thereby limiting their ability to multiply. Moreover, the mucus membrane also produces Lactoperoxiadase an enzyme that catalyses the production of super oxide radicals, a reactive O₂ intermediate toxic to many microbes.

The mucosal surface has a specialized set of cells – MALT (Mucus Associated Lymphoid Tissue). The commonly studied MALT is GALT which includes tonsils, adenoids, diffused lymphoid areas along the gut and special regions in the intestine – Peyer’s patch less well organized MALT – BALT. The GALT can operate by 2 mechanisms – First when the Ag arrives at the mucus membrane, the epithelial cells contacts with M cells. The M cells contain special pockets comprising of B cells, T cells and macrophages. The M cells phagocytoze pathogens and transport it to the pockets where macrophages engulf them and destroy. The other way could be that after phagocytosing, M cells transport the vesicles to lymphoid follicles. Here the B cells recognize the specific Ag and mature into Ab producing plasma cells. They then leave the follicle, come into the gut and interact and secrete mucus associated antibodies also called as sIgA. These Abs then enter the lumen of the gut and interact with the microorganisms. Another mechanism involves the presence of inter and intra epithelial lymphocytes scattered throughout the mucosal layer of the gut which are mainly T-cells that intercept any kind of pathogen should the intestinal lining be breached.