Introduction

The capacity of animals to run from or disable poten­tial predators is the essence of survival. Perhaps less apparent is the need to protect cells and tissues from attack by harmful agents—parasites, bacterial patho­gens, toxins, or cells that have become cancerous.

Some everyday examples help us understand phys­iological processes involved in protection of animals from these various agents. Let's suppose that your puppy is playing in the yard, and he is splashed by your little brother as your brother rides his bicycle through a puddle of debris from the gutter. Secretions from the puppy's eyes and mucus membranes of his nostrils act to flush material away to prevent the chance of infection. Similarly, his hair covering, eye­lashes, and thick skin prevent bacteria and debris from the dirty water from gaining entrance. Even sebaceous secretions of his skin or ear canals can produce condi­tions that limit the potential growth of harmful bacte­ria. These are all examples of nonspecific defenses.

Let's suppose that your puppy simultaneously steps on a broken bottle as he runs through the puddle chasing your brother. He manages to soak the cut with dirty bacterial-filled water. However, fortunately for him, he has received a good supply of antibodies from his mother's colostrum. This means that he likely has specific antibodies in his bloodstream from his mother. This allows his fledgling immune system to recognize and ultimately kill bacteria present in the dirty water. This would be an example of specific but passive immu­nity. As the puppy grows and is exposed to various materials in his environment he will develop his own complement of specific antibodies and the cells neces­sary to regenerate additional supplies of specific anti­bodies. Thus, defense mechanisms are divided into nonspecific and specific divisions. The immune system primarily concerns specific protection.

The basis for specific immunity arose from observa­tions in the late 1800s, when scientists discovered that animals that had survived a bacterial infection had protective agents in their blood (now known to be immunoglobulins or antibodies) that defended the animal. It was also shown that if antibody-containing serum from the surviving animals was given to animals that had not been exposed to the pathogen, these animals were also protected against attack. It was initially believed that production of antibodies was the only critical requirement for protection. How­ever, in cases where transfer of antibody-containing serum failed to provide protection, transfer of the donor's white blood cells often did provide protec­tion. As experiments became more elaborate, it became clear that immunity involved both circulating agents as well as populations of white blood cells or leuko­cytes. Thus, the two overlapping arms of specific defense are humoral immunity (meaning blood derived) and cell-mediated immunity (Rodriguez et al., 2012).

Anatomy and Physiology of Domestic Animals, Second Edition. R. Michael Akers and D. Michael Denbow. © 2013 John Wiley & Sons, Inc. Published 2013 by John Wiley & Sons, Inc.

One of the major types of white blood cells includes the lymphocytes. Furthermore, there are two broad classes of lymphocytes. When they are activated, B lymphocytes, usually simply called B cells, are induced to divide to generate clones of cells. You may have read or heard of polyclonal or monoclonal antibodies being used in scientific experiments or more recently in some cancer treatments (Herceptin). Regardless, some of these clonal cells are retained in the body as memory B cells. Other B cells differentiate into plasma cells that synthesize and secrete large quantities of anti­bodies. The presence of the memory B cells explains the marked increase in antibody concentration (titer) in the blood when animals are exposed to the same pathogen or antigen a second time. Essentially, the pieces required for more antibody production are already in place.

Fig. 15.1. Overview of defense mechanisms. Nonspecific or innate protection provides a first line of defense. The barrier derived from the epidermis of the skin or the stratified squamous epithelium of body openings and their secretions are a first line of protection. General effects of phagocytes, natural killer T cells, and antimicrobial proteins are also important. Humoral immunity involves activation of clones of B lymphocytes to produce memory B cells and daughter B cells that differentiate into plasma cells that synthesize and secrete large amounts of antibodies.

Practically speaking, the actions of the two divi­sions of the immune system are closely interwoven. But generally, humoral immunity is most effective against bacteria and their toxins or free viruses because antibody binding can directly inactivate these attack­ers for example, causing precipitation or agglutina­tion, or masking surfaces, to make them susceptible to destruction by phagocytic cells or complement acti­vation. Cell-mediated immunity is a better weapon against cellular targets; examples include cells that have been infected by viruses or parasites, or perhaps cancer cells.

In subsequent sections we will consider some of these elements of the immune system in more detail. But let's begin by considering defense attributes of the immune system and physiological systems generally. Defense of the internal environment centers on three main functions:

• destruction or neutralization of bacterial, viral, or parasitic pathogens

• destruction of aged or damaged ; that is, consider the finite life of red blood cells

• surveillance to detect and eliminate abnormal, cancerous, or virally infected cells (Fig. 15.1).