Chapter 20 – The Lymphoid System and Immunity

Chapter 20 – The Lymphoid System and Immunity

Functions of lymphoid system

• Production (along with red bone marrow) and distribution of lymphocytes

• Fluid balance – Cardiovascular system delivers more fluid to

peripheral tissues (via capillaries) than it drains away • Excess fluid enters lymphatic system, and is then

returned to cardiovascular system

Components of lymphoid system • Lymph – Has lower

concentration of proteins than plasma

• Lymphatic vessels – Collect fluid from

tissues and return to venous system

• Lymphoid tissues and organs

Lymphatic capillaries • Smallest vessel type • Originate as pockets (not

continuous as in cardiovascular system)

• Endothelial cells overlap and form valves – Allows entry into lymphatic

system, but prevents re-entry into intercellular spaces

• Lacteals are large capillaries in small intestine – Involved with lipid transport

Small lymphatic vessels • Lead toward body’s

trunk • Similar in structure

to veins – Have valves, which

form bulges in the vessel

– Valves are closer together than in veins

Major vessels

• Superficial lymphatics– Dermal layer of skin; areolar layer of GI tract;

respiratory, urinary, and reproductive systems • Deep lymphatics – Accompany arteries and veins of skeletal muscles,

deeper organs• Superficial and deep converge to form

lymphatic trunks

Major vessels cont • Trunks converge to form:

– Thoracic duct • Collects lymph from body inferior to diaphragm and left side of body superior to

diaphragm – Cisterna chyli – sac-like area at base of thoracic duct

» Receives lymph from lower regions

– Right lymphatic duct • Collects lymph from right side of body superior to diaphragm

Lymphoid organs and tissue

• Organs– Lymph nodes – Thymus – Spleen

• Tissues – MALT (mucosa-associated lymphoid tissue)– Tonsils

Lymph nodes • Kidney bean shape • Afferent lymphatic carries

lymph into nodes; efferent lymphatic carries out

• Purifies lymph before returning to venous circulation– 99% of foreign antigens are

removed – Macrophages in walls

remove debris • Serve as antigen-presenting

cells

• Largest nodes are cervical, axillary, and inguinal

Thymus • Largest (relative to body

size) during young childhood (1 – 2 yrs)

• After puberty, starts to become fibrous– Involution

• Two lobes – Septa divide each into

smaller, multiple lobules – Outer region = cortex;

inner region = medulla• Site of T cell maturation

Spleen • Removes abnormal blood

cells from phagocytosis • Stores iron (recycled from

blood cells)• Initiation of immune

responses by T and B cells to antigens in circulating blood

• Reticular connective tissue– Red pulp – contains RBCs

• All circulating components of blood, and macrophages

– White pulp – contains lymphocytes (stains purple)

Lymphoid tissues • MALT (mucosa-

associated lymphoid tissue)– Peyers patches in

intestinal walls; walls in vermiform appendix

• Tonsils – Pharyngeal – singular

(“adenoid”)– Palatine – paired – Lingual – paired

Innate defenses

• First line of defense against infection – Bacteria, viruses, other pathogens

• Present before an exposure occurs

• Non-specific

Innate defenses cont

• Skin – Lysozyme – digests bacterial cell walls

• Mucous membranes – Traps microbes; stomach acids kill most bacteria

• Neutrophils/macrophages – Engulf microbes by phagocytosis

• Natural killer cells – Attack cancer cell and viral-infected cells by releasing

chemicals to promote apoptosis

Innate defenses cont

• Interferons – Produced by viral-infects cells that help other cells

resist infection • Complement – ~30 proteins in plasma – Circulate in an inactive form – activated by

exposure to microbes – Leads to lysis of invader and an inflammatory

response

Innate defenses cont • Inflammatory response – Histamine released by damages tissue causes nearby blood

vessels to dilate and become ‘leaky’– Complement attracts phagocytes to area – Clotting proteins and platelets form local clot to seal off

infected area

Acquired immunity • Fully develops AFTER exposure to a pathogen – Can amplify non-specific responses (inflammation,

complement)• Responds to the presence of foreign antigens– Increase the number of cells that fight invader, and

produce defense proteins called antibodies • Has memory – A second exposure to a previously encountered antigen

is faster and stronger • Acquired by:– Natural exposure – Vaccination/immunization

Types of acquired immunity • Active – Person’s own immune system actively produces

antibodies – Either due to natural exposure or artificial (vaccine)

• Passive – Receive pre-made antibodies • Maternal antibodies to fetus through placenta• Breast milk to babies • Shot of antibodies after high-risk exposure

– Lasts only as long as antibody “lifespan”• Weeks to months

Lymphocytes – B cells • B cells differentiate in

bone marrow • Humoral immunity – Secrete antibodies which

circulate in blood and lymph

– Defend against bacteria and viruses in body fluids

– Antibodies join with antigen • Complex recognized by

macrophages

Antigens • Have specific regions

where antibodies bind to them

• Antigenic determinant or epitope – Small region on antigen’s

surface– Recognized by antibody – Single antigen may be

recognized by different antibodies at different sites

Antibody structure

• 4 polypeptide chains • Each arm of Y has 2

variable regions that form a specific antigen recognition site

• C region sequences determine class of antibody

Classes of antibodies • IgG– Most abundant– Only one that can cross the placenta – Fights bacteria, viruses, toxins; activates complement

• IgM– First type produced by newborn – Antibodies for ABO antigens; activates complement

• IgA– Found in saliva, tears, colostrum, mucous membranes – Primary defense against local infections of mucous membranes

• Prevents attachment of viruses and bacteria to epithelial cells

Classes of antibodies cont

• IgD– Cell membranes of B lymphocytes – Antigen receptor for initiation of B cell differentiation

• IgE– Involved in inflammation, allergic responses,

parasitic infections – Binds to mast cells and basophils

• Triggers release of histamine and other inflammation & allergy mediators

Antibodies mark antigens for elimination

Clonal selection

Primary and secondary immune response

Lymphocytes – T cells

• Differentiate in thymus • Cell-mediated

immunity – Circulate in blood and

attack cells that are infected

– Also protects against fungal and protist invaders

– Aid in elimination of cancerous cells

T cells

• Helper T cells (CD4)– Interact with other WBCs that function as antigen-

presenting cells • Activation of helper T stimulates other cells of

immunity

• Cytotoxic T cells (CD8)– Attack body cells that are infected by pathogens

Hypersensitivity

• Excessive or inappropriate activation of the immune response

• The body is damaged by the immune response, rather than by the antigen

• Type I hypersensitivity – Commonly called ‘allergic reactions’– Mediated by IgE

Anaphylaxis

• Systemic response to the inflammatory mediators released in type I hypersensitivity – Histamine, acetylcholine, kinins, leukotrienes, and

prostaglandins all cause vasodilation – Acteylcholine, kinins, leukotrienes, and

prostaglandins all can cause bronchoconstriction

Autoimmune diseases

• Immune system attacks self-antigens • Normally, self-reactive immune cells are killed in

the lymphoid organs or suppressed by regulatory T cells

• In autoimmunity, this self-tolerance breaks down • Immune system destroys body tissues • Anti-tissue antibodies appear in blood (ex: anti-

thyroid antibodies)

HIV – human immunodeficiency virus

• HIV is a retrovirus – Has RNA instead of DNA

as its genetic material – Uses the enzyme reverse

transcriptase to make DNA from RNA

• Binds to CD4 receptors

HIV cont • New viral particles can be

shipped from cell via exocytosis

• Mutation then causes HIV to destroy helper T cells (cells lyse)

• AIDS – helper T cell population is dangerously low– Affects humoral and cell

mediated immunity