15-9 Animal defenses can be classified into innate and adaptive immune systems

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• The innate immune system is always present and cannot be induced by exposure to pathogens.
• Innate immunity consists of anatomical structures, antimicrobial secretions, the normal flora, complement, the inflammation response and phagocytes.

Host defenses can be divided into two categories, innate and adaptive immunity. Innate immunity involves general mechanisms in a healthy animal that prevent colonization by microorganisms and antagonize or kill those that do enter the host. They are always present and the strength of their response does not increase with repeated exposure to the inducing microbe. Adaptive immunity develops through the mechanisms that are turned on in response to a pathogen. This involves activation of the immune system, where the initial response to a pathogen is weak but becomes quite vigorous over a period of a few days. Adaptive defenses also have a memory of encountered pathogens such that a second infection by a pathogen is met with a more rapid and vigorous immune response.

Innate immune system

Innate immunity is the first line of defense against a pathogen. The system must be somehow circumvented by the pathogen before it can enter into the host. In most cases, the adaptive immune system is only activated after the innate immune system has been breached. Here we examine various structures, products and cells of the innate response as shown in Figure 15-6.

Microbial resistance in anatomical structures

Figure 15-7 summarizes the various anatomical defenses of the body. The skin is an extremely effective barrier to microorganisms. Besides preventing the cells of our body from escaping, it also prevents microorganisms from getting in. Skin contains several layers of tightly packed, heavily keratinized cells (keratin is a fibrous protein that gives skin, hair and nails its toughness). It is very difficult for most organisms to squeeze in between skin cells. Skin cells are also continually being shed and replaced by new ones, thus removing any microorganisms attached to them. The skin surface is also very hydrophobic and dry, which prevents the growth of many microorganisms. Sebaceous glands are present throughout the skin and they secrete hydrophobic oils that further repel water and microorganisms. The oil also helps keep the skin supple and flexible, preventing cracking that might allow microbial access to internal layers. Finally, melanin in the skin also helps to reduce the harmful impact of UV light by absorbing it. UV light can be damaging to all cells, including cells of the immune system.

Hair helps to restrict access of airborne pathogens to the skin. It protects the most sensitive or exposed body orifices, including the nasal cavity, the eyes and ears. Hair also serves as a cushion lessening the severity of cuts and grazes, which decreases the depth and number of breaks in the skin.

The movement of various body parts can help to rid the body of microorganisms trying to colonize. Blinking the eyelids constantly sweeps microorganisms out of the eye. The entire respiratory tract, except bronchioles and alveoli, is coated with cilia that beat upward and push microorganisms out. The peristaltic action of the gut not only moves food along our digestive tract, but also flushes microbes out of our system. If microorganisms do not have a method of attachment, they rapidly leave the gastrointestinal tract. The constant flushing of the urethra by urine helps to keep the bladder free of bacteria. All of the above movements continually remove microbes from our bodies.

Antimicrobial secretions

The body produces a number of antimicrobial substances that inhibit or kill microorganisms. Collectively these substances are known as tissue bactericides. Tear ducts, sebaceous glands, ears, nose, lungs, mouth and digestive tract all secrete antimicrobial substances. Figure 15-8 lists some of the common tissue bactericides, their mode of action and location.

One important group of broad-spectrum antimicrobial peptides is the defensins, which participate in the host defense of mammals, birds, plants and insects. Their presence in a wide variety of species probably indicates that they are an ancient form of antimicrobial antagonism. Defensins form a family of cysteine-rich, cationic and structured polypeptides of 29-42 amino acids that contain three or four disulfide bridges. They work by disrupting the membrane of a wide variety of pathogens, including gram-negative and gram-positive bacteria, fungi and some enveloped viruses. They also serve as chemokines, which attract elements of the adaptive immune system. Defensins are expressed by many different tissues and it is now clear that they are a vital part of innate immunity.

Microbial antagonism

As mentioned in Chapter 14, the normal non-pathogenic microorganisms that live inside and on our bodies as part of normal flora have an important role in defending us against pathogens. This is accomplished in three ways.

1. The normal flora are well adapted to the tissues they live on and they out-compete pathogenic microbes for these sites of colonization, thus preventing harmful bacteria from gaining an initial foothold on our bodies.
2. The normal flora produce specific antimicrobial compounds (called bacteriocins) that kill or inhibit closely related species. The normal flora also produce non-specific compounds such as the end products of their metabolism that may inhibit pathogenic bacteria.
3. They utilize nutrients, limiting their availability to pathogens.

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