Chapter 22: The Lymphatic System and Immunity

Define immunity

Immunity (or resistance) is the body's ability to ward off damage or disease through its various defenses

• Vulnerability or a lack of this resistance is known as susceptibility

Compare the two basic types of immunity

The two types are innate (nonspecific) immunity and adaptive (specific) immunity

(1) Innate immunity is present at birth, lacks specific recognition of microbes, and acts against all invaders in the same way

(2) Adaptive immunity involves specific recognition of a microbe once it has breached innate defenses and adapts to handle a particular pathogen using T and B lymphocytes

List the components of the lymphatic system

The lymphatic system consists of a fluid called lymph, lymphatic vessels that transport it, structures and organs containing lymphatic tissue (lymphocytes within a filtering tissue), and red bone marrow

Describe the functions of the lymphatic system

The system has three primary functions:

1. Draining excess interstitial fluid from tissue spaces and returning it to the blood

2. Transporting dietary lipids and lipid-soluble vitamins (A, D, E, and K) absorbed by the GI tract

3. Carrying out immune responses directed against particular microbes or abnormal cells

Describe the organization of lymphatic vessels

• Vessels begin as lymphatic capillaries (closed at one end) located between cells.

• These unite to form larger lymphatic vessels, which pass through lymph nodes

• Vessels exiting nodes unite into lymph trunks (e.g., lumbar, intestinal), which finally drain into the thoracic duct or right lymphatic duct before entering the venous system

Explain the formation and flow of lymph

More fluid filters out of blood capillaries than is reabsorbed

• This excess (~3 liters/day) enters lymphatic capillaries to become lymph

Flow is maintained by the same "pumps" as venous blood:

• The skeletal muscle pump (contractions compress vessels) and the respiratory pump (pressure changes during inhalation)

• Valves ensure one-way flow

Distinguish between primary and secondary lymphatic organs

• Primary lymphatic organs (red bone marrow and the thymus) are sites where stem cells divide and become immunocompetent, meaning they are capable of mounting an immune response

• Secondary lymphatic organs and tissues (lymph nodes, spleen, and lymphatic nodules) are the sites where most actual immune responses occur

Describe the components of innate immunity.

It includes a first line of defense (physical and chemical barriers of the skin and mucous membranes) and a second line of defense (antimicrobial substances like interferons and complement, natural killer cells, phagocytes, inflammation, and fever).

Describe how T cells and B cells arise and function in adaptive immunity.

Both arise from pluripotent stem cells in red bone marrow

• B cells complete their development in red bone marrow, while pre-T cells migrate to and mature in the thymus

• B cells function in antibody-mediated immunity (secreting antibodies), while cytotoxic T cells function in cell-mediated immunity (directly attacking invaders)

Explain the relationship between an antigen and an antibody

• An antigen is a substance (usually protein) recognized as foreign that provokes an immune response.

• An antibody is a protein produced by plasma cells (derived from B cells) that can specifically bind to and inactivate that specific antigen

Compare cell-mediated and antibody-mediated immunity

• In cell-mediated immunity, cytotoxic T cells directly attack intracellular pathogens (inside cells), some cancer cells, and foreign tissue transplants

• Antibody-mediated (humoral) immunity involves B cells transforming into plasma cells that secrete antibodies to attack extracellular pathogens found in body fluids

Outline the steps in a cell-mediated immune response

The response involves:

1) Antigen recognition by T-cell receptors (TCRs) binding to antigen-MHC complexes

2) Co-stimulation (a second signal like IL-2)

3) Clonal selection, where the T cell proliferates and differentiates into active effector cells and memory cells

4) Elimination of invaders by active cytotoxic T cells.

Distinguish between the action of natural killer cells and cytotoxic T cells

Natural killer (NK) cells are innate

• They lack specific receptors and kill a wide variety of infected body cells and tumor cells that display abnormal membrane proteins

Cytotoxic T cells are adaptive

• They have specific receptors for one particular microbe and only kill target cells infected with that specific pathogen

Define immunological surveillance.

It is a process carried out by cytotoxic T cells, macrophages, and NK cells that involves recognizing and destroying cancerous cells that display novel cell surface components called tumor antigens.

Describe the steps in an antibody-mediated immune response

1) A B cell recognizes an antigen through its receptors

2) The B cell processes the antigen and presents it to a helper T cell

3) Helper T cells provide co-stimulation (cytokines)

4) The B cell undergoes clonal selection, forming a clone of plasma cells (which secrete antibodies) and memory B cells

List the chemical characteristics and actions of antibodies

Antibodies (immunoglobulins) are "Y" or "T" shaped glycoproteins with heavy and light chains, having variable regions (antigen-binding sites) and constant regions

• Their actions include neutralizing antigens, immobilizing bacteria, agglutinating/precipitating antigens, activating complement, and enhancing phagocytosis

Explain how the complement system operates

This system consists of over 30 proteins that act in a cascade

• When activated (via classical, alternative, or lectin pathways), they enhance immune reactions by causing cytolysis (bursting) of microbes, promoting phagocytosis through opsonization, and contributing to inflammation

Distinguish between a primary and a secondary response to infection

• A primary response occurs after the first exposure; it is slow (days to build) and produces a low antibody titer

• A secondary response occurs after subsequent exposures; it is much faster and more intense because thousands of memory cells quickly proliferate and differentiate, providing a much higher antibody titer (mainly IgG)

Describe how self-recognition and self-tolerance develop

Development occurs in the thymus (for T cells) via two processes:

1) Positive selection allows T cells to survive only if they can recognize self-MHC proteins (self-recognition)

2) Negative selection eliminates T cells that react to self-peptide fragments, ensuring the immune system does not attack the body's own tissues (self-tolerance)

• B cells also undergo similar selection in bone marrow