Overview of the Immune System

What the USMLE Requires You To Know

•Components ofthe innate and adaptive immune responses

•Attributes of innate and adaptive immune responses

•Interactions between innate and adaptive immune responses

The immune system is designed to produce a coordinated response to the introduc­ tion of foreign substances or antigens into the body. It is organizationally divided into two complementary arms: the innate (or native or natural) immune system and the adaptive (or acquired or specific) immune system.

Innate immunity provides the body's early line of defense against microbial invaders. It comprises 4 types of defensive barriers:

•Anatomic or physical (skin, mucous membranes)

•Physiologic (temperature, pH, and chemicals such as lysozyme, comple­ ment, and some interferons)

•Phagocytic (monocytes, neutrophils, macrophages)

•Inflammatory events

Innate immune defenses have in common that they:

•Are present intrinsically with or without previous stimulation

•Have limited specificity for shared structures of microbes

•Are not enhanced in activity by repeated exposure

•Have limited diversity of expression

Once the barriers of the innate immune response have been breached, the adaptive immune response is activated in an antigen-specific fashion to provide for the elimi­ nation of antigen and lasting protection from future challenge. The components of the adaptive immune system are:

•Lymphocytes (T cells and B cells) and plasma cells (end cells of B-lymphocyte differentiation)

•Antigen-presenting cells (macrophages, B cells, and dendritic cells)

Adaptive immune defenses have in common that they are:

•Specific for particular antigens and are specialized to provide the best pro- tection

•Diverse in their specificity

•Enhanced with each repeated exposure (express immunologic memory)

•Capable of self/non-self recognition

•Self-limiting

In a Nutshell

The immune system has two arms:

•Innate

•Adaptive

In a Nutshell

The Innate Arm (Anatomic, Physiologic,

Phagocytic, Inflammatory)

•Present intrinsically

•Nonspecific

•No memory

•Limited diversity

In a Nutshell

The Adaptive Arm (Lymphocytes and Their

Products)

•Inducible

•Specific

•Memory

•Extensive diversity

•Selfversus non-self distinction

•Self-limiting

Section I • Immunology

In a Nutshell

•Antibodies and complement enhance phagocytosis.

•Antibodies activate complement.

•Cytokines stimulate adaptive and in­ nate responses.

These features of adaptive immunity are designed to give the individual the best pos­ sible defense against disease. Specificity is required, along with memory, to protect against persistent or recurrent challenge. Diversity is required to protect against the maximum number of potential pathogens. Specialization of function is necessary so that the most effective defense can be mounted against diverse challenges. The ability to distinguish between invaders and one's own cells and tissues (self versus non-self) is vital in inhibiting a response to one's own cells (autoimmunity). Self-limitation allows the system to return to a basal resting state after a challenge to conserve energy and prepare for the challenge by new microbes.

Table l-1-1. Comparison of Innate and Adaptive Immunity

The innate and adaptive arms of the immune response do not operate independently of one another.

•Phagocytic cells process and display antigen to facilitate stimulation of spe­ cific T lymphocytes.

•Macrophages secrete immunoregulatory molecules (cytokines), which help trigger the initiation of specific immune responses.

•T lymphocytes produce cytokines, which enhance the microbicidal activities of phagocytes.

•Antibodies produced by plasma cells bind to pathogens and activate the complement system to result in the destruction of the invaders.

•Antibodies produced by B lymphocytes bind to pathogens and assist with phagocytosis (opsonization).