Here I summarize some important properties of the innate immune system, compared with
those of the adaptive immune system.
First, innate immunity receptors recognize PAMPs and DAMPs, and thus their specificity is
broad.
This is also underlined by the fact that the innate immunity receptors are encoded in the
germline and do not mutate as frequently as B-cell receptors and T-cell receptors, both of which
are encoded by genes that undergo gene rearrangements and somatic mutations.
So, basically, all cells have more or less the same innate immunity receptors, whereas, as I
will explain to you later, each lymphocyte expresses a unique receptor, because lymphocytes
respond to antigens on the principle of clonal selection.
Therefore, innate immunity receptors have limited diversity compared with antigen-specific
receptors of the adaptive immune system, while both types of receptors can discriminate self
from non-self molecules.
OK, so much for the innate immune system.
I will now focus on the adaptive immune system that is more specific, more adaptable, and
hence, more potent than the innate immune system.
As you see here, the major cellular components of this system are T and B lymphocytes.
T cells express specific antigen receptors called T-cell receptor, and upon activation, they can
differentiate into several subsets, such as Th1, Th2, Th17, Treg and Tc.
These subsets are diverse in terms of how they are activated and what kind of cytokines they
produce.
On the other hand, B cells express antigen specific receptors called B cell receptors or BCRs
and consist of two major subsets, B1 and B2.
Both subsets have the potential to produce antibodies but of different properties.
Here I show you antigen receptors of T cells and B cells.
They are similar in a sense that they are polypeptides recognizing a specific antigen.
However, they are very different in structure and different in the nature of the antigen they
recognize.
For instance, a T-cell receptor is alpha-beta heterodimer and recognizes only peptides with the
aid of dendritic cells.
As I will explain to you later, a T-cell receptor recognizes so-called peptide-MHC complexes
presented by dendritic cells.
In contrast, a B-cell receptor is tetramer consisting of four polypeptides, namely two heavy
chains and two light chains.
A B-cell receptor can recognize such antigens as proteins and sugars in the absence of dendritic
cells.
This is an important point.
In the adaptive immune system, T cells and B cells have their individual function.
B cells mediate so-called humoral immunity where B cells respond to extracellular microbes by
producing antibodies that can neutralize and eliminate these molecules such as microbes and
microbial toxins.
In contrast, T cells mediate cell-mediated immunity in response to microbes or parasites that are
internalized into or synthesized in cells.
Helper T cells eliminate phagocytosed microbes by secreting cytokines and activating
macrophages, whereas cytotoxic T cells kill infected cells and eliminate reservoirs of infection.
Here I summarize important properties of the adaptive immune responses.
The responses are highly specific, because the responses are mediated by lymphocyte clones
expressing specific antigen receptors.
I will tell you more about clones a little later.
The adaptive immune responses are also highly diverse, which makes the adaptive immune
system to respond to a large variety of antigens successfully.
The adaptive immune responses can also generate memory.
When the adaptive immune system is exposed to the same antigen it has encountered before, it
remembers this and shows a more rapid and more effective response, and this is why vaccines
can confer long-lasting protection against infections.
The adaptive immune responses are self-limited and decline as the infection is eliminated.
The last not least important property of the adaptive immune system is that, although it can
respond to a whole variety of microbes and other foreign antigens, it normally does not react
with self antigens.
As you will hear later in this lecture series, self-reactivity is normally down-regulated so that no
concomitant tissue injury occurs when this system responds to foreign antigens.
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