Cells of the Immune System

Question 1

What are hematopoietic stem cells?

What provides support for HSC renewal and differentiation?

Answer 1

progenitor cells from which all differentiated blood cell types arise during the process of hematopoiesis

HSCs are multi-potent, self-renewing sources of WBC, RBC and platelets

the bone marrow provides structural and molecular support for stem cell renewal and differentiation

Question 2

Describe HSC in the bone marrow.

How do they self-renew?/Where?

Where else are HSCs found?

Answer 2

in bone marrow HSC sit in a niche consisting of osteoblasts or sinusoidal endothelial cells

Even though stem cells have the ability to self-renew, they must be surrounded by this niche in order to do so. This is because the niches supply growth factors and other regulatory molecules that support HSC self-renewal

HSCs can also be found in the circulation, but chemical signals encourage them to “home” to the bone marrow and their niches.

Question 3

What do stromal cells do?

Answer 3

the HSCs are influenced by stromal cells that are located within the niches, and also provide factors needed for HSC maintenance.

HSCs may return to the circulation which is controlled in a circadian manner

Or, with the help of stromal cell factors, they may be pushed to differentiate
into peripheral blood progenitors which will continue down the path toward differentiation into different cell types.

there is a constant replenishing of the peripheral blood cells by the
hematopoietic stem cells, and it is critical that this balance is maintained.

Question 4

HBC may be “mobilized to differentiate into immune cells. Why?

Answer 4

to replenish peripheral WBC or RBC or platelets

during periods of stress (infection)

Question 5

What 2 main things might a HSC differentiate into?

Answer 5

A HSC will differentiate into a common myeloid or lymphoid progenitor.

From there, the cells will commit to different cell lineages and partially or fully mature in the bone marrow

Question 6

What will common myeloid progenitors differentiate into?

Answer 6

The common myeloid progenitors can differentiate into thrombocytes or red
blood cells, granulocytes such as mast cells, basophils, neutrophils, or eosinophils, or mononuclear cells that will ultimately become tissue macrophages or myeloid
dendritic cells

Question 7

What do common lymphoid progenitors differentiate into?

Answer 7

The common lymphoid progenitors can differentiate into NK cells & lymphocytes

Question 8

What determines what HSC will differentiate into?

What are some principal molecules involved in HSC differentiation for common myeloid and lymphoid progenitors? For basophils? Neutrophils? Eosinophils? Monocytes/macrophages? Dendritic cells? Tcells/Bcells?

Answer 8

The push to differentiate into different cell types comes from various cocktails of stimulating factors and cytokines.

Some of the principal molecules involved in HSC differentiation include IL‐3
& GM‐CSF for common myeloid progenitors; IL‐7 for common lymphoid progenitors

IL‐4 for basophils; G‐CSF for Neutrophils; IL‐5 for eosinophils; GM‐CSF &/or
M‐CSF for monocytes & macrophages; Flt3L for dendritic cells; IL‐2 & IL‐7 for Tcells; & many for Bcells, including IL‐3 & IL‐7.

Question 9

After differentiating from stem cells & maturing, many immune cells circulate in the
blood. Lymphocytes are responsible for setting up specific responses to pathogens, but they have to look for them.

So where do they circulate in and out of?

Describe this process.

Answer 9

So they circulate in and out of meeting places called lymphoid organs, hoping
to find their match. The most abundant lymphoid organs in the body include the
lymph nodes.

Bits of digested pathogen are brought to lymphoid organs by other immune
cells searching for their matching lymphocyte to help fight the infection

After spending time in the lymphoid organ, the lymphocytes return to the
blood circulation by traveling through the lymph system. They will then either keep
looking for their matching pathogen or go to the site of infection via the blood
stream if they met their specific pathogen in the lymphoid organ

(Most of these structures have a similar set up whereby pathogens are
displayed to the lymphocytes and a specific immune response begins.)

Question 10

There are numerous lymphoid depots where these meetings can take place. Describe.

Answer 10

1. At lymph nodes
2. In the mucosal lymphoid tissue such as those in the intestine,
3. In the spleen
4. And in tonsils and adenoids.

Question 11

Describe how lymphocytes are activated against a certain pathogen. What happens next?

Answer 11

Nonetheless, cells bringing in pathogen from the lymphatics do run into lymphocytes that had entered from the blood circulation, and that only recognize the pathogen that they are carrying. The antigen presenting cells stimulate, or “activate” the lymphocytes specifically against that pathogen. Then the activated lymphocyte leaves the lymph node in search of the area of the infection to be able to
eradicate the infection

Question 12

Describe the pathway of an activated lymphocyte. How does it return to circulation first?

If the lymphocyte had become activated, where does it go?

What happens when event is over?

Answer 12

The activated lymphocyte then returns to the circulation first via the lymph
drainage into the thoracic duct., and then dumps into the left subclavian vein

If the lymphocyte had become activated, then it will be attracted to the site of
the infection by chemical signals produced by the front line defense cells and begin
to mount a full scale, specific attack.

When the event is over, some of the lymphocytes will go on to survive as long‐lasting memory cells, that are more efficient at responding to the same pathogen in the future.

Question 13

In order for this elegant defense scheme to work, teamwork between innate and adaptive cells is paramount.

Describe innate cells - what do they do? Adaptive cells? How do they differ?

Answer 13

Innate cells are first responders keeping the infection or invasion at bay,
while the adaptive response forms. As you also saw, innate cells are responsible for presenting suspected pathogens to the adaptive cells. However, even though the innate cells become activated against pathogens, the response is not specific for an individual pathogen, there is no lasting memory involved, and the cells do not
“learn” to become more efficient against future infections with the same pathogen.

In contrast, adaptive cells are specific for only 1 pathogen and their activation greatly enhances the immune response & is responsible for tipping the scale toward eradication. The adaptive response results in memory against the specific pathogen and can confer protective immunity or more efficient responses in the future.

Question 14

Describe neutrophils. How are they identified?

How do they appear on H&E staining?

What type of cell are they?

What happens to patients who are severely neutropenic (low in neutrophils).

How long do they last?

Answer 14

Neutrophils are identified by their multi‐lobed nucleus, which also gives them their
name of Polymorphonuclear neutrophilic leukocytes, or “PMNs”. They are one of the
granulocytic myeloid cells, so they have cytoplasmic granules that are neither very basic
nor very acidic, and thus are light pink, or “neutral” on H&E staining. Neutrophils are the
most abundant white blood cells. They are the most important front‐line defense of the innate immune system. Patients who are severely neutropenic, or low in neutrophils, often succumb to otherwise non‐lethal infections without intervention.

You may think of neutrophils as the foot soldiers of the immune response. They are the most numerous and readily available front line defenders against
invasion. The usually circulate, but can move quickly from the blood stream to site
of infection where they gather in large numbers and begin to attack the pathogens.
They are short lived and usually die after 1 round of phyagocytosis. For very large
pathogens that cannot be ingested, neutrophils participate in extracellular killing.

Question 15

Describe the extracellular killing mechanisms by neutrophils.

Answer 15

We briefly discussed phagocytosis, but extracellular killing mechanisms by
neutrophils can be a powerful antimicrobial weapon as well.

They can also spill the
contents of their antimicrobial granules into the extracellular milieu, which, in addition to being antimicrobial, can also cause local tissue damage. However, this may be necessary when the pathogen in question is too large to phagocytize. They
can also release “NETs” or neutrophil extracellular traps, which are also full of their
granular enzymes & killing molecules, but also DNA elements which act to
immobilize pathogens to decrease spreading & help with phagocytosis

Question 16

What does neutrophil activation during an infection result in?

Answer 16

Neutrophil activation during an infection results clinically in…pus! The majority of pus’s content are dead neutrophils.

Question 17

Describe the macrophage. What are they filled with? What is their primary role?

Answer 17

The next myeloid cell is the macrophage, which are cells located in the tissues that
are derived from circulating monocytes. They often have an actively working nucleus
because they are transcribing many genes related to the immune response or homeostasis at any given time. These cells do not have histologically prominent granules, but they are filled with abundant lysosomes indicating their primary role in phagocytosis & intracellular killing.

Question 18

What do macrophages do in addition to being efficient phagocytes and producers of immune response molecules.

Answer 18

In addition to being efficient phagocytes & producers of immune response molecules, macrophages can present some of what they eat to T‐cells in a process
known as antigen presentation.

an antigen is any molecule that can bind to an antibody made by plasma cells. So macrophages can get the adaptive immune system involved in an infection by displaying the pathogen on its surface so that T cells can recognize it and tell the b‐cells to fire up their antibody manufacturing.

Question 19

What are tissue macrophages generated from?

When are they generated?

How do macrophages already present in tissues recognize pathogens?

Answer 19

Tissue macrophages are mainly generated from circulating monocytes, named so because they are mononuclear (as opposed to multi‐lobed) cells that are circulating in the blood.
They are continuously patrolling and produced in
increased numbers during an infection. They are chemically recruited to sites of
infection where they become tissue macrophages.

Those macrophages that are
already present in the tissues as sentinel watchdogs recognize pathogens via the
receptors on their cell surface. Early in a response, they are not numerous enough
to contain the invasion like neutrophils are, but they send out important distress
signals for immune system activation and white blood cell recruitment. Once they
call in neutrophils & adaptive cells such as T cells, they respond to signals that the T
cells send out and their anti‐microbial activity is enhanced. They also play an
important role in housekeeping, scavenging normally dead cells or debris as well as participating in the resolution of an immune response.

Question 20

What is key to macrophage ability to recognize, take up, and respond with production of inflammatory molecules?

Answer 20

Receptors on phagocytes such as macrophages are the key to their ability to recognize, take up, and respond with production of inflammatory molecules. These
receptors are not specific, but they recognize patterns that have been conserved in
pathogens & which identify them as foreign.

Question 21

Macrophages are powerful recruiters of the inflammatory response. The extent of this recruitment is exemplified when you look at the difference between
localized infection in the tissues vs infection in the blood known as bacteremia. Describe.

Answer 21

While localized infection is marked by swelling, redness, warmth, and pain at the
site of infection due to the inflammatory mediators produced by macrophages,
bacterial infection in the blood can lead to a phenomenon known as sepsis which
can rapidly lead to death. In this case, although the macrophages are doing their job in mediating inflammation, the bystander effects of their activation can be quite
detrimental to the host itself.

Question 22

Describe dendritic cells. How are they recognized?

What do they specialize in? What is their primary focus?

Answer 22

Dendritic cells are another innate cell that are recognized by their long finger‐like
processes similar to the dendrites of the nervous system from which their name is derived. Dendritic cells also specialize in phagocytosis, but instead of intracellular killing, their primary focus is to be an antigen presenting cell that stimulates the adaptive immune system to recognize a new invader. It degrades pathogens, but for the purpose of displaying them rather than destroying them.

Question 23

Describe dendritic cells - How can they take up antigens?

What is macropinocytosis?

What are they crucial for/what do they provide the crucial link between?

Answer 23

You may think of dendritic cells as the spies or the informants of the host defense system. Similar to macrophages, dendritic Cells are found in most tissues.

They are constantly sampling the environment and can take up antigen through
both phagocytosis as well as drink‐in and sample the extracellular environment via
macropinocytosis (a highly conserved endocytic process by which extracellular fluid
and its contents are internalized into cells through large, heterogeneous vesicles
known as macropinosomes).

In addition to having multiple ways to take up antigen,
they have a variety of methods by which they can process antigen. In this way, they can display antigens from a wide array of pathogens. They provide the crucial link between the innate and the adaptive immune system. Without dendritic cells, a
robust T cell response toward new microbes probably wouldn’t exist and we would
never form life long memory towards infections that we have previously encountered

dendritic cells are the masters of antigen presentation. If
there is a way to pick up and process antigen, a dendritic cell will know how

In general, not only can DCs take up antigen through phagocytosis & macropinocytosis, they can also display antigen. from their own infections as well as change the normal route of processing or share
another dendritic cells’s infection.

(there is very little that can escape recognition by the immune system due to the
incredible versatility of dendritic cells)

Question 24

What are eosinophils? How are they recognized on H & E staining?

What kind of nucleus?

What is their job?

Answer 24

Eosinophils are one of the granulocytes that are recognized by bright pink
eosinophilic staining on H&E due to their highly basic granules that bind the acid eosin stain well, and they usually have a bilobed nucleus. Eos are mainly charged with the job of neutralizing and destroying large parasitic invaders.

Question 25

What do eosinophil granules include?

What do they do?

Answer 25

Eosinophil granules include many enzymes & molecules that cause tissue destruction to pathogens & surrounding tissue, recruit further inflammation, & promote vascular permeability. For this reason, activation of eosinophils is tightly
regulated.

Activated Eosinophils release highly dangerous substances from their granules

Major Basic Protein
Toxic to parasites & to surrounding tissue
Triggers Mast Cell histamine release

Eosinophil Collagenase
Remodels connective tissue matrix

Leukotrienes

• Causes smooth muscle contraction
• Increased vascular permeability
• Increased mucus secretion

Eosinophil-Derived Neurotoxin

Question 26

Where are eos found? Describe their mode of action? Clinically what do they play an important role in?

Answer 26

Eos are scarce, but they tend to be found in the subepithelial connective tissue.

They are able to kill parasites at the site of infection with their toxic granules, and while doing this, they also synthesize chemical mediators to augment
the inflammatory response and call in help. Thus, the eosinophil can be thought of
as a lethal weapon against parasites as well as a formidable threat to healthy
surrounding tissue.

Clinically, eosinophils play an important role in allergy. They
can augment & help to sustain allergic responses causing them to be chronic. The
damage done to surrounding tissue can be significant and permanent.

Question 27

What are mast cells? What are they filled with?/ What do they release?

Answer 27

Mast cells are large, mononuclear cells that are filled with dark, basophilic granules containing mainly acidic histamine. In fact, histamine release is the specialty of masts as
many of you that may suffer from allergies might know.

Question 28

What is the main job of mast cells?

Where are they found?

What happens when they are activated? What does this lead to?

Answer 28

The main job of mast cells is to “open the vascular doors” to allow the recruited white blood cells to enter the site of infection. They are conveniently found near areas of vasculized connective tissue, and when activated, they degranulate their vasoactive substances. This leads to increased local blood flow, increased vascular permeability or leaky vessels which causes increased fluid &
antibodies to spill into the affect tissue and extravasation of needed immune cells.
The local swelling also promotes increased flow of antigen in lymph to the regional lymph nodes.

Question 29

What happens if mast cells are activated by a non-pathogenic allergen?

What happens if all the mast cells in the body become activated rapidly?

What happens in allergic urticaria?

Answer 29

Unfortunately, if mast cells are activated by a non‐pathogenic allergen, local vasodilation can lead to significant morbidity, and systemic vasodilation can lead to significant mortality!

If all the mast cells in the body become activated rapidly, it can lead to systemic vasodilation & vascular permeability with loss of blood
pressure, airway constriction & swelling of the epiglottis.

In allergic urticaria, slowly absorbed ingested allergens that reach the skin can activate mast cells and cause large, itchy, red swellings also known as hives.

Question 30

What are basophils? How are they identified?

Answer 30

can be thought of as an accomplice to mast cells and eosinophils.

They are identified by their basophilic granules similar to mast cells, but they have 2‐3 nuclear lobes that are hard to see under the basophilic stained granules. Again, they participate in anti‐parasitic & allergic responses with mast cells & eosinophils

Question 31

What are NK cells? How are they identified?

What do they attack? How do they act?

Answer 31

Natural killer cells, or “NK cells” are easily identifiable as large cells that are mononuclear and have a very distinctive granular cytoplasm compared to the previous
granulocytes that you saw earlier.

NK cells primarly attack virus infected cells or tumor
cells and release their granules that cause lysis of their target.

Question 32

Describe NK cell receptors.

Why are activated NK cells important? Describe their mode of action.

Answer 32

NK cell receptors resemble receptors found on adaptive immune cells, but
they are invariant, meaning they do not rearrange and become specific for one type
of antigen. These receptors, however, are very good at noticing when something is
fishy about the host’s cells. They can tell if a cell has been infected by a virus or if
there are obvious changes that indicate transformation of a cell into a tumorous cell.

Activated NK cells are important because they can target viral infectious quickly
while the slower adaptive immune system is gearing up. Upon release of their
cytotoxic granules, NK cell induce their target to undergo apoptosis, or organized
cell death

Question 33

Describe activated B cells. What do they make? What kind of receptors?

How do they activate?

Answer 33

Activated B‐cells make antibody. They have antigen‐specific receptors that
are also antibodies bound to their cell surface. When they encounter their particular antigen, they activate and become soluble antibody‐making machines
called plasma cells. These soluble & circulating antibodies can trap pathogen and allow the pathogen to be more readily taken up by the phagocytes. Also, after
encountering a pathogen, specific antibody will circulate for a long time and be
ready for another invasion in the future, thereby lessening the damage to the host
upon the second encounter.

Question 34

Describe the role of T cells. What happens when their receptor encounters its specific antigen?

What are the types of T cell types?

Answer 34

T-cells can be considered the commanders-in-chief of the immune system. They
have receptors that are specific for only one antigen, and when they encounter it they proliferate and differentiate into their pre-determined T cell phenotype.

Two major T cell
types include cytotoxic T cells which are similar to NK cells in that they recognize & kill
viral-infected cells. The other major T cell type are helper T cells, that, when activated
produced molecules & cytokines that direct the immune response by macrophages and Bcells
as well as augment the recruitment of immune cells to the site of infection. There
are other phenotypes of T cells in addition to the cytotoxic & helper cells