BASICS OF IMMUNITY
[COVID-19 SERIES #3]
INTRODUCTION:
Almost every living being on our planet has
some sort of ability to resist damage to itself from infections and toxins. This
protection from internal and external agents can be present as a consequence of
genetic coding in the body. Therefore, depending upon the genetic makeup it is
present in different forms. The absence of a particular genetic characteristic
makes the organism susceptible to harm. Thus, we have mice who are “genetic-knockouts”
and vulnerable to certain insults which would not affect other normal mice. There
are other types of immunity which is acquired following exposure to the deleterious
agents. Thus, we can develop resistance to certain allergic substances after a
couple of exposures. Immunity can also be passed in a passive manner. Thus,
mothers could transfer immunity to their fetus/babies or it can be injected by
plasma therapy.
The term “immune” is derived from the Latin
word “immunis” which means “free from” or “exempt”. The immune system is the
biological defense mechanism. It comprises of different cells, organs and
tissues that work together to combat infection, cellular damage and disease.
There are different methods by which
immunity can be classified. It can be regarded as innate, adaptive and passive.
It can also be classified pathologically into humoral and cellular immunity.
INNATE
IMMUNITY:
Innate immunity is the first line of
defense against non-self pathogens. It is the natural immunity with which an
individual is born. A healthy individual is able to ward off antigens due to
programmed processes present inherently in the body. It could be in the form of
antibodies, skin cells which prevent organisms from invading the body and
chemicals in the blood-stream. These are non-specific defense mechanisms that
are activated immediately or within hours following exposure to antigens.
Molecules such as lipopolysaccharides,
mannose and teichoic acid present on the surface of organisms form the “pathogen-associated
molecular patterns” (PAMPs). These molecules interact with specific receptors
known as “pattern recognition receptors” (PRRs) and activate the innate immune
system.
ADAPTIVE
IMMUNITY:
Adaptive or active immunity is acquired
over time as individuals are exposed to organisms or are immunized. This type
of immunity is only found in vertebrates. Adaptive immunity is an antigen-specific
immune response. This variety is more complex than innate immunity. Once the
body is exposed to an antigen, certain specific antibodies against the antigen
are produced. The body also develops a “memory” against the antigen making the
response more efficient. This immunity is mediated by B and T cells following
exposure to a specific antigen. Apart from memory, such a process also involves
specificity and self/non-self recognition.
The adaptive immune response is meant to
attack non-self antigens. However, in certain situations there is an error and
it starts to attack itself. Such a mechanism is seen in autoimmune diseases e.g.
systemic lupus erythematosus, rheumatoid arthritis etc.
PASSIVE
IMMUNITY:
This is “borrowed” from another source and
is effective for a short period only. Antibodies in a mother’s breast milk
provide a temporary immunity to the child against diseases to which the mother
was exposed.
HUMORAL
IMMUNITY:
Adaptive immune mechanisms function through
two mechanisms: (a) humoral immunity (b) cellular immunity. The main difference
between humoral and cellular immunity is that antigen-specific antibodies are
produced in humoral immunity, whereas antibodies are not produced in cell-mediated
immunity. Instead, T cells destroy the infected cells by inducing apoptosis.
Therefore, humoral immunity is also known
as antibody-mediated immunity. B-cells, with the assistance of helper T-cells differentiate
into plasma B-cells which produce antibodies against a specific antigen. The humoral
immune system deals with antigens from pathogens freely circulating or outside the
infected cells. Antibodies from B-cells bind to the antigens and neutralize
them or cause lysis (destruction or dissolution of cells by lysis) or
phagocytosis.
Humoral immunity involves BCR receptors and
recognizes unprocessed antigens. Once the antigen-antibody interaction occurs,
the antibodies destroy the pathogen by three mechanisms. They bind to specific
molecules on the surface of the pathogen, neutralizing the pathogen. This neutralization
prevents the pathogens from entering the cells. It also neutralizes bacterial
toxins. The pathogens which are bound to the antibodies undergo phagocytosis by
macrophages and other cells. This is called “opsonization”. Binding of
antibodies to pathogens activates the complement system. The complement system
proteins then bind to the antibody- bound pathogens and recruit phagocytic
cells.
CELLULAR
IMMUNITY:
Cellular immunity occurs within the
infected cells and is mediated by antigen-specific T-lymphocytes. The antigens
(pathogen) are expressed on the cell surface or on an antigen-presenting cell. helper
T-cells release cytokines that help activated T-cells bind to the infected
cells’ MHC-antigen (Major histocompatibility complex) complex and differentiate the T-cell into a cytotoxic
T-cell. The T-cells secrete cytokines and the infected cell then undergoes
lysis.
