Immune System and the Body’s Defenses

Introduction

The immune system is a complex network of cells, secreted molecules, and plasma proteins that work together to protect the body from infectious agents and other potentially harmful substances. Its primary function is to provide us with immunity.

• Immunity: The ability of the body to resist infection and disease.

• Infectious agents: Organisms or substances that can cause disease.

Categories of Infectious Agents

There are five major categories of infectious agents:

1. Bacteria: Single-celled prokaryotes. Most are harmless, but some are virulent.

2. Viruses: Pieces of DNA or RNA in a protein shell. Viruses are not alive and require a host to reproduce.

3. Fungi: Eukaryotic cells that produce spores, including molds and yeasts.

4. Protozoans: Eukaryotic cells that are intracellular or extracellular parasites.

5. Multicellular parasites: Nonmicroscopic organisms that reside within a host.

Types of White Blood Cells (Leukocytes)

Immune cells are a special class of white blood cells. Major types include:

1. Neutrophils

2. Eosinophils

3. Basophils

4. Monocytes

5. Lymphocytes: Includes T-lymphocytes, B-lymphocytes, and Natural Killer (NK) cells.

Immunity

Types of Immunity

The immune system’s physiologic processes are organized into two categories:

1. Innate immunity: Non-specific immunity present from birth that protects against a variety of different substances.

2. Adaptive immunity: Specific immunity that develops in response to exposure to particular antigens.

Innate Immunity

Characteristics

• Prevents entry of potentially harmful substances.

• Responds non-specifically to a range of harmful substances.

Innate immunity can be further divided based on mechanism of action:

1. Preventing entry (first line of defense)

2. Internal defenses (second line of defense)

First Line of Defense: Preventing Entry

Physical and chemical barriers prevent pathogens from entering the body. These include:

• Skin: Composed of the dermis and epidermis, forms a physical, chemical, and biological barrier. Plays a significant role in preventing entry as long as it is intact.

• Mucous membranes: Line the body openings and produce mucus and release antimicrobial substances.

Second Line of Defense: Internal Nonspecific Defenses

If pathogens bypass the first line, internal defenses are activated. These include:

• Immune cells: Phagocytes and other cells that attack invaders.

• Chemicals: Antimicrobial proteins and enzymes.

• Physiological responses: Inflammation and fever.

Phagocytic Cells

Phagocytes engulf and remove unwanted substances through phagocytosis.

• Phagocytosis: "Cellular eating," a form of endocytosis by which cells ingest and digest solid substances.

• Phagocytic cells include:

  1. Neutrophils

  2. Macrophages

  3. Dendritic cells

Apoptosis-Initiating Cells

Apoptosis is programmed cell death. The only type of apoptosis-initiating cell in the body is:

• Natural Killer (NK) cells

Chemical Defenses

Chemical, nonspecific line of defense includes antimicrobial proteins:

• Interferons: Proteins released by virus-infected cells that interfere with viral replication and help stop the spread of viruses.

• Complement system: Proteins made in the liver that enhance the effectiveness of both innate and adaptive immune systems.

Physiologic Responses

Physiologic responses in the innate immune system include:

• Inflammation: An immediate local, nonspecific event that occurs in vascularized tissue against a variety of injury-causing stimuli.

• Fever: An abnormal elevation of core body temperature of at least 1 degree C (1.8 degrees F).

Benefits of Inflammation

1. Disposes of cellular debris and pathogens

2. Alerts the adaptive immune system

3. Sets the stage for repair

There are 5 cardinal signs of acute inflammation:

1. Redness

2. Swelling

3. Pain

4. Heat

5. Loss of function

Other events of inflammation include:

• Release of chemicals

• Vascular changes

• Recruitment of leukocytes

• Delivery of plasma proteins

Benefits and Risks of Fever

• Inhibits reproduction of bacteria and viruses

• Accelerates tissue repair

• Increases activity of adaptive immunity

• Risks: Changes in metabolic pathways, possible seizures, irreversible brain damage

Adaptive Immunity

Overview

Adaptive immunity is an acquired and specific immune response. The primary cell involved is the lymphocyte.

Types of Adaptive Immunity

• Antibody-mediated (humoral) immunity

• Cell-mediated immunity

Both adaptive immune responses require recognition of an antigen.

• Antigen: Large, complex molecules that react with antibodies or lymphocytes to cause an immune response.

Lymphocyte Life Cycle

Both types of lymphocytes (T-cells and B-cells) have common steps in their life cycle:

1. Origin

2. Maturation

3. Circulation

4. Antigen encounter and activation

5. Proliferation

Antibody-Mediated (B-cell) Immunity

Antibody-mediated immunity involves B-lymphocytes. B-lymphocytes can recognize one kind of foreign antigen and produce antibodies against it.

Antibody-mediated immunity occurs in 4 steps:

1. B-lymphocyte recognizes foreign antigen, which is confirmed by the helper T-lymphocyte.

2. B-lymphocyte undergoes mitosis, differentiating into plasma cells and memory B-lymphocytes.

3. Plasma cells produce and secrete large amounts of antibodies.

4. Memory B-lymphocytes: Confer immunity to certain antigens for many years or a lifetime.

Antibody Functions

Cell-Mediated Immunity

Cell-mediated immunity involves T-lymphocytes. T-lymphocytes make up about 70-85% of body lymphocytes and there are two different types:

• Helper T-lymphocytes: Oversee immune response by presenting antigens to other lymphatic cells and secreting cytokines.

• Cytotoxic T-lymphocytes: Must be activated by a helper T-cell and respond to one type of antigen only. They release cytotoxic chemicals to destroy unhealthy cells.

Immunologic Memory

After an immune response, interactions between a lymphocyte and an antigen lead to the formation of memory cells. Memory cells are lymphocytes that produce a rapid and strong immune response following a secondary antigen exposure. This process is the basis for vaccination.

Example: Vaccines introduce antigens to the body, stimulating the formation of memory cells without causing disease, thus providing immunity against future exposures.