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Innate immunity is conferred by all those elements with which an individual is born and that are always present and available at very short notice to protect the individual from challenges by foreign invaders. The major properties of the innate immune system are discussed in Chapter 3. Table 1.1 summarizes and compares some of the features of the innate and adaptive immune systems. Elements of the innate system include body surfaces and internal components, such as the skin, the mucous membranes, and the cough reflex, which present effective barriers to environmental agents. Chemical influences, such as pH and secreted fatty acids, constitute effective barriers against invasion by many microorganisms. Another noncellular element of the innate immune system is the complement system. As in the previous editions of this book, we cover the subject of complement in Chapter 4.

Numerous other components are also features of innate immunity: fever, interferons, other substances released by leukocytes, and pattern recognition molecules (innate receptors), which can bind to various microorganisms (e.g., Toll‐like receptors or TLRs; see Chapter 3), as well as serum proteins such as β‐lysin, the enzyme lysozyme, polyamines, and the kinins, among others. All of these elements either affect pathogenic invaders directly or enhance the effectiveness of host reactions to them. Other internal elements of innate immunity include phagocytic cells such as granulocytes, macrophages, and microglial cells of the central nervous system, which participate in the destruction and elimination of foreign material that has penetrated the physical and chemical barriers.

TABLE 1.1. Major Properties of the Innate and Adaptive Immune Systems

Property	Innate	Adaptive
Characteristics	Antigen nonspecific	Antigen specific
	Rapid response (minutes to hours)	Slow response (days)
	No memory	Memory
Immune components	Natural barriers (e.g., skin, mucous membranes)	Lymphocytes
	Phagocytes and natural killer cells	Antigen recognition molecules (B and T cell receptors)
	Soluble mediators (e.g., interferons, complement)	Secreted molecules (e.g., antibody)
	Pattern recognition molecules
Major protective mechanisms	Inflammation and antiviral defenses	Antigen specificity and lifelong memory responses