The human immune system has evolved to protect the body from attacks of invaders like disease-causing bacteria, fungi, viruses and parasites. It also protects the body from mutinies like cancers. The system provides immunity (from the Latin, immunis) which means a “state of resistance from infectious diseases.” The immune system has two arms, the non-specific (or innate) arm and the specific (or adaptive) arm.
Innate Immunity
In contrast to adaptive immunity, which recognizes the enemy and gives a response accordingly, innate immunity protects the body non-specifically. That is, it is not capable of the elaborate mechanism (of the adaptive immunity) of recognizing the invader but will give the same type of response to any pathogen. The innate immunity can be categorized into four major barriers.
Anatomic Barrier
This barrier consists of the skin and the mucous membrane. It prevents the entry of pathogens and is the body's first line of defense. The skin consists of two distinct layers; a thinner outer layer called the epidermis (the outer layer of which consists of dead cells and is filled with a protein called keratin, which makes the skin waterproof) and a thicker layer called the dermis.
Special glands called sebaceous glands, associated with hair follicles secrete an oily substance called sebum. Sebum spreads throughout the skin and keeps it moisturized. It is acidic and hence prevents growth of microorganisms.
The conjunctivae and the alimentary, urogenital and respiratory tracts are lined by mucous membranes. Saliva, tears and mucous secretions tend to wash away pathogens and they contain antibacterial and antiviral substances. Mucus entraps pathogens and the ciliary escalator throws any trapped particles upwards.
Physiologic Barrier
This barrier includes pH (gastric acidity), temperature, and a variety of chemicals. Body temperatures of many animals are not suitable for the growth of many pathogens and hence these animals are naturally resistant to these organisms. For example, chicken are resistant to anthrax because their high body temperature inhibits the growth of anthrax bacilli. Gastric acidity is also an important innate defense mechanism since very few organisms survive the low pH environment of the stomach. Many soluble factors like lysozyme, interferons and complement contribute to innate immunity.
Phagocytic Barrier
A few types of cells (phagocytes) in the body have the ability to move towards and capture foreign microorganisms by a process called phagocytosis. In this process, the phagocyte moves towards the foreign microorganism, adheres to its surface and ingests it. Once inside the phagocyte, the microorganism is digested by the action of many enzymes and finally the digested material is egested. White blood cells like neutrophils, eosinophils and macrophages are the phagocytes of the body.
Inflammatory Barrier
Tissue damage caused either by a wound or by an invading pathogen elicits a complex series of events collectively termed as inflammatory response. In this process, cells of the immune system are assembled at the site of tissue damage. They cordon off the region to prevent the escape of the organisms to other parts of the body and surround the pathogens and kill them. This process leads to some tissue injury as well and is characterized by signs like redness, swelling, heat and pain.
Innate immunity provides the first line of defense against infection. Many components of innate immunity are already present before the commencement of infection and they comprise a set of disease-resistance mechanisms that are not specific for any particular pathogen but provides non-specific protection to our body.
Reference : Immunology by Goldsby, Kindt, Osborne and Kuby (5th Ed.) W. H. Freeman, NY.
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