Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Immunization

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they originate in the bone marrow. The humoral immune

response helps in the control and removal of pathogens such

as bacteria, viruses, fungi, and parasitesbefore they enter host

cells. The antibodies produced by the B cellsare the mediators

of this response.

The antibodies form a family of plasma proteins

referred to as immunoglobulins. They perform two major

functions. One function of an antibodyis to bind specifically

to the molecules of the foreign agent that triggered the

immune response. A second antibody function is to attract

other cells and molecules to destroy the pathogen after the

antibody molecule is bound to it.

When a foreign agent enters the body, it is engulfed by

the antigen-presenting cells, or the B cells. The B cell that has

a receptor (surface immunoglobulin) on its membrane that

corresponds to the shape of the antigenbinds to it and engulfs

it. Within the B cell, the antigen-antibody pair is partially

digested, bound to a special class of proteins called MHC-II,

and then displayed on the surface of the B cell. The helper T

cellsrecognize the pathogen bound to the MHC-II protein as

foreign and becomes activated.

These stimulated T cells then release certain chemicals

known as cytokines(or lymphokines) that act upon the primed

B cells (B cells that have already seen the antigen). The B cells

are induced to proliferate and produce several identical cells

capable of producing the same antibody. The cytokines also

signal the B cells to mature into antibody producing cells. The

activated B cells first develop into lymphoblasts and then

become plasma cells, which are essentially antibody produc-

ing factories. A subclass of B cells does not differentiate into

plasma cells. Instead, they become memory cells that are

capable of producing antibodies at a low rate. These cells

remain in the immune system for a long time, so that the body

can respond quickly if it encounters the same antigen again.

The antibody destroys the pathogen in three different

ways. In neutralization, the antibodies bind to the bacteria or

toxin and prevent it from binding and gaining entry to a host

cell. Neutralization leads to a second process called opsoniza-

tion. Once the antibody is bound to the pathogen, certain other

cells called macrophages engulf these cells and destroy them.

This process is called phagocytosis. Alternately, the

immunoglobulin IgM or IgG can bind to the surface of the

pathogen and activate a class of serum proteins called the

complement, which can cause lysis of the cells bearing that

particular antigen.

In the humoral immune response, each B cell produces

a distinct antibody molecule. There are over a million differ-

ent B lymphocytes in each individual, which are capable of

recognizing a corresponding million different antigens. Since

each antibody molecule is composed of two different proteins

(the light chain and the heavy chain), it can bind two different

antigens at the same time.

See alsoAntibody and antigen; Antibody-antigen, biochemi-

cal and molecular reactions; Antibody formation and kinetics;

Immune system; Immunity, active, passive and delayed;

Immunity, cell mediated

IImmunizationMMUNIZATION

When a foreign disease-causing agent (pathogen) enters the

body, a protective system known as the immune systemcomes

into play. This system consists of a complex network of organs

and cells that can recognize the pathogen and mount an

immune response against it.

Any substance capable of generating an immune

response is called an antigenor an immunogen. Antigens are

not the foreign bacteriaor virusesthemselves; they are sub-

stances such as toxins or enzymesthat are produced by the

microorganism. In a typical immune response, certain cells

known as the antigen-presenting cells trap the antigen and

present it to the immune cells (lymphocytes). The lympho-

cytes that have receptors specific for that antigen binds to it.

The process of binding to the antigen activates the lympho-

cytes and they secrete a variety of cytokinesthat promotes the

growth and maturation of other immune cells such as cyto-

toxic T lymphocytes. The cytokines also act on B cellsstimu-

lating them to divide and transform into antibodysecreting

cells. The foreign agent is then either killed by the cytotoxic T

cellsor neutralized by the antibodies.

The process of inducing an immune response is called

immunization. It may be either natural, i.e., acquired after

infection by a pathogen, or, the immunitymay be artificially

acquired with serum or vaccines.

In order to make vaccines for immunization, the organ-

ism, or the poisonous toxins of the microorganism that can

cause diseases, are weakened or killed. These vaccines are

injected into the body or are taken orally. The body reacts to

the presence of the vaccine(foreign agent) by making anti-

bodies. This is known as active immunity. The antibodies

accumulate and stay in the system for a very long time, some-

times for a lifetime. When antibodies from an actively immu-

nized individual are transferred to a second non-immune

subject, it is referred to as passive immunity. Active immunity

is longer lasting than passive immunity because the memory

cells remain in the body for an extended time period.

Immunizations are the most powerful and cost-effective

way to prevent infectious disease in children. Because they

have received antibodies from their mother’s blood, babies are

immune to many diseases when they are born. However, this

immunity wanes during the first year of life. Immunization

programs, therefore, are begun during the first year of life.

Each year in the United States, thousands of adults die

needlessly from vaccine-preventable diseases or their compli-

cations. Eight childhood diseases (measles, mumps, rubella,

diphtheria, tetanus, pertussis, Hemophilus influenzaetype b,

and polio) are preventable by immunization. With the excep-

tion of tetanus, all the other diseases are contagious and could

spread rapidly, resulting in epidemicsin an unvaccinated pop-

ulation. Hence, vaccinations are among the safest and most

cost-efficient public healthmeasures. Vaccinations against flu

(influenza), hepatitisA, and pneumococcal disease are also

recommended for some adolescents and adults. The vaccines

indicated for adults will vary depending on lifestyle factors,

occupation, chronic medical conditions and travel plans.

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