The immune system is responsible
for fighting infection and disease.
It is comprised of many specialized cell types,
all which work together to keep your body healthy.
In this short video,
you will be introduced to the major cellular players of the immune system.
Let’s start by introducing the two major arms of the immune system:
innate and adaptive immunity.
Innate immunity serves as the first line of defense
and is a more general immune response.
You might recognize some of the classic symptoms of innate immunity,
such as fever and inflammation.
Adaptive immunity allows for very specific detection
and elimination of pathogens.
However, it takes a longer time to ramp up than innate immunity.
Importantly, the adaptive immune system provides immunological memory --
the ability of our immune cells to remember previous infections and clear them more quickly in the future.
All immune cells develop from a single pluripotent cell
in the bone marrow,
the hematopoietic stem cell.
Hematopoietic stem cells give rise to
lymphoid and myeloid progenitors,
each of which differentiate into a range of immune cell types.
We refer to the cells on the top as the lymphoid lineage,
whereas the cells on the bottom are classified as the myeloid lineage.
The myeloid progenitors also give rise to red blood cells and platelets,
although we won’t focus on those here.
Let’s now look closely at the lymphoid lineage.
The lymphoid progenitor differentiates into three cell classes.
We’ll start by introducing B cells.
Upon activation, mature B cells differentiate into memory cells or plasma cells.
Plasma cells are the immune cells that are responsible for secreting antibodies,
an important component of adaptive immunity.
Natural killer cells are cytotoxic cells of the innate immune system.
They detect virus-infected cells and kill them.
T cells arise from a common progenitor.
There are many types of mature T cells,
but the best known ones -- those which we’ll cover here -- include:
memory T cells, cytotoxic T cells,
and helper T cells.
Like memory B cells,
a fraction of mature T cells remain in the body as memory T cells
to help the body mount a faster immune response in future infections.
Cytotoxic T cells recognize antigen,
or small pathogen-derived particles,
on infected cells and kill them in a pathogen-specific manner.
Lastly, upon activation by specialized cells in the body,
helper T cells secrete cytokines that boost the adaptive immune response.
For example, helper T cells play an important role in B cell activation.
Now let’s take a look at the myeloid lineage.
The myeloid lineage produces most cells of the innate immune system
as well as important antigen-presenting cells that prime the adaptive immune response.
The myeloid progenitor gives rise to neutrophils,
which are innate immune cells that specialize in the capture and killing of microorganisms
throughout the body;
eosinophils, which are a type of granulocyte
that releases cytokines to defend against parasites;
and monocytes, which further differentiate into dendritic cells and macrophages.
Dendritic cells are a specialized type of phagocytic cell
that bridges innate and adaptive immunity.
Macrophages are tissue resident phagocytic cells.
They patrol the body and assist in cleaning up infection
and activating other immune cells.
Other cells that arise from the myeloid progenitor
include mast cells, another type of granulocyte that are implicated in allergy;
as well as basophils,
which are a less well-understood cell type that is involved in the immune response to parasites.
So how are immune cells activated?
Here, we’ll briefly cover the molecular paradigms
for innate and adaptive immune activation.
Cells of the innate immune system
express molecules known as pattern recognition receptors at their surface.
These receptors bind pathogens or parts of pathogens,
which induce intracellular signals
that activate an innate immune response.
The particles recognized by these receptors
are common amongst pathogens.
To activate adaptive immunity,
cells present antigen
-- small peptide fragments of pathogens --
to T cells to inform a specific immune response.
Antigen is presented by two types of surface molecules:
MHC class I and MHC class II.
MHC class I molecules are expressed by all cells in the body
and are used in defense against intracellular pathogens such as viruses.
They do this by presenting endogenous, or intracellular, antigens
to cytotoxic T cells.
MHC class II molecules present exogenous antigen,
which is antigen found on pathogens outside of cells,
and activate helper T cells.
MHC class II molecules are expressed
by what are known as professional antigen-presenting cells,
which include dendritic cells, macrophages, and B cells.
There are safeguards in place to ensure that the immune system
isn’t activated against self antigens.
The mammalian immune system
is comprised of a complex set of cells
that contribute to innate and adaptive immunity.
The foundational understanding you’ve gained from this video
serves as a starting point for a deeper look at this topic.