3. Specific Immunity: The Adaptive Line of Defense

Normal humans have an extremely specific and powerful system for resisting infectious agents called adaptive or acquired immunity, sometimes known as the third line of defense. It can be observed in the long-term protection provided by developing infections such as measles or mumps or being vaccinated for diphtheria.

The absolute need for adaptive immunity is impressively documented in children with genetic defects in this system or in AIDS patients who have lost it. Even with heroic measures to isolate the patient, combat infection, or restore lymphoid tissue, the victim can be constantly vulnerable to life-threatening infections.

Acquired adaptive immunity is the product of a dual system of specialized leukocytes – the B and T lymphocytes. During fetal development, these lymphocytes undergo a selective process that prepares them for reacting only to one specific antigen. During this time, immunocompetence, the ability of the body to react with a wide spectrum of foreign substances begins to develop. An infant is born with the theoretical potential to produce an immune response to millions of different foreign molecules or antigens. But the completion of this immunocompetence takes many years, extending into late puberty.

Antigens figure very prominently in specific immunity. They are defined as any molecules that can stimulate a response by T and B cells. They consist of protein, polysaccharide, and other compounds from cells and viruses. Environmental chemicals can also be antigens. In fact, any exposed or released substance is potentially an antigen, even those from our own cells. For reasons we discuss later, our own antigens do not usually evoke a response from our own immune systems, but they may do so in other persons.

Pathogen-associated molecular patterns (PAMPs) are molecules shared by many types of microbes that stimulate a nonspecific response. In contrast, antigens are unique molecules that stimulate specific immune responses. The two molecules do share two characteristics: (1) they are “parts” of foreign cells (microbes), and (2) they provoke a reaction by the white blood cells of the host.

Two features that most characterize adaptive immunity are specificity and memory. Unlike mechanisms such as anatomical barriers or phagocytosis, acquired immunity is selective. For example, the antibodies produced during an infection against the chickenpox virus will protect against that virus but not against the measles virus. The property of memory pertains to the rapid mobilization of lymphocytes that have been programmed to “recall” their first engagement with an invader and respond rapidly to it during subsequent exposures.

4. An Overview of Specific Immune Responses

Immune responses are highly complex and regulated, and they represent one of the most elegant and coordinated networks of cells and chemicals in the body. To present this system in an organized and clear manner, we have found it helpful to divide it into separate but related sections, each detailing some event in development or the immune response. The sections and pages of coverage are as follows:

I. Development and differentiation of the immune system

II. Lymphocytes, maturation, and the nature of antigens

III. Cooperation between lymphocytes during antigen presentation

IV. B lymphocyte responses and the production and actions of antibodies

V. T-lymphocyte responses

As we explore the information contained in these sections, we will be following a “map” of sorts. Figure 2 lays out the flow of the main stages and will serve to coordinate the presentation. It also includes figure numbers that enlarge on the topics in that section.