Cells and Organs of the Adaptive Immune System

Characteristics of adaptive immune responses

• specific - tailored response with increased effectiveness

• memory - increased speed of response and a more robust response after the first offense

What are the two main lymphocytes of the adaptive immune system?

B cells and T cells

What are the main antigen presenting cells of the adaptive immune system?

Macrophages and dendritic cells

Where to B cells mature?

Bone marrow in mammals, or the bursa of fabricius in birds

Where do T cells mature?

Thymus 

What are primary lymphoid organs?

• organs where lymphocytes are formed and mature

• where B and T cells acquire their specific antigen receptor

• includes the bone marrow, bursa of fabricius, and the thymus 

What are secondary lymphoid organs?

• organs where mature lymphocytes are maintained

• where adaptive immune responses are initiated

• includes lymphatics and lymph nodes, spleen, and MALT

In which part of the bone marrow are lymphocytes formed?

Hematopoiesis occurs in the red marrow

Describe the B cell life cycle

• generated in bone marrow

• exit the bone marrow as naive, mature B cells

• travel to secondary lymphoid organs to participate in immune responses

• become antibody-producing plasma cells 

What does “mature” mean in terms of lymphocytes?

an immune cell that is fully developed

What does “naive” mean in terms of lymphocytes?

an adaptive immune cell that has not yet encountered the specific antigen it will bind 

Which lymphocytes require antigen processing?

T cells

What happens to B cells as they mature?

They develop antigen receptors (BCRs) that confer specificity and help them recognize antigens

How are B cells activated?

• once they bind antigen, B cells multiply in a process called clonal expansion

  • daughter cells have the same BCR as the progenitor 

• after clonal expansion, some mature B cells become plasma cells that secrete abundant antibodies 

Describe the T cell life cycle

• T cells exit the bone marrow after formation and travel to the thymus, where they differentiate into immature T cells called thymocytes

• thymocytes exit the thymus as naive, mature T cells

• in secondary lymphoid organs, T cells participate in immune responses and become effector T cells

Describe the thymus and its role in T cell maturation

• immature thymocytes interact with epithelial cells in the thymus cortex

• these cells mature as they move toward the medulla and interact with medullary epithelial cells, macrophages, and dendritic cells 

What type of peptides to T cells recognize?

linear peptides that have been processed intracellularly and presented by antigen presenting cells (APCs)

Describe T cell activation

• T cells bind antigen and multiply through clonal expansion

  • daughter cells have the same TCR

• APC presents antigen on MHC

Cell-mediated immunity refers to ______ while humoral immunity refers to _______.

1. T cells

2. B cells 

What types of pathogens do T cells generally target?

intracellular pathogens

CD8+ T cells

cytotoxic T cells that directly kill infected cells

CD4+ T cells

helper T cells that produce cytokines, activate humoral immunity and innate immune cells 

Generally, what type of pathogens do B cells target?

They target extracellular bacteria, toxins, and viruses via production of antibodies

Why are lymph nodes important in the adaptive immune response?

They help expose the lymphocytes to antigen

Describe the flow of lymph and what it brings to the lymph nodes

• lymphatic fluid carries in antigen, pathogens, debris, toxins, etc. 

• dendritic cells bring antigen to the lymph nodes through lymphatics or blood vessels

• blood delivers B cells and T cells 

Steps of lymph travel

1. afferent lymphatic vessels

2. marginal/medullary sinuses

3. paracortical region

4. primary follicles

5. secondary follicles (germinal centers)

6. medullary sinus 

7. efferent lymphatic vessels 

What is the spleen’s function and structure?

• the spleen is a secondary lymphoid organ that mounts immune responses to blood borne pathogens

• resident macrophages help recycle senescent RBCs

• red pulp: contains RBCs and macrophages; where RBC recycling occurs

• white pulp: lymphoid tissue that surrounds arterioles, structures similar to those found in lymph nodes

White pulp organization

• periarterial lymphoid sheath (PALs) consist mostly of T cells and surround arterioles

• primary and secondary follicles (B cells) are near the PALs

• the marginal zone contains B cells and is an area where particular antigens get trapped at the interface between the red and white pulp 

MALT

• mucosal-associated lymphoid tissue

• nasopharyngeal, bronchus, and gut

• all mucosa contain specialized lymphoid aggregates that monitor and respond to potential pathogens 

Where do adaptive immune responses begin?

in the secondary lymphoid organs