Microbiology and Immunology

Immunity, cell mediated WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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horse in response to the inactivated tetanus toxin. This proce-

dure is typically done if someone has been exposed to a situa-

tion where the possibility of contracting tetanus exists. Rather

than rely on the individual’s immune system to respond to the

presence of the toxin, neutralizing antibodies are administered

right away.

Active and passive immunity are versions of what is

known as antibody-mediated immunity. That is, antibodies

bind to the antigen and this binding further stimulates the

immune system to respond to the antigen threat. Antibody-

mediated immunity is also called humoral immunity.

A third type of immunity, which is known as delayed

immunity or delayed-type hypersensitivity, is represents a dif-

ferent sort of immunity. Delayed immunity is a so-called cell-

mediated immunity. Here, immune components called T-cells

bind to the surface of other cells that contain the antigen on

their surface. This binding triggers a further response by the

immune system to the foreign antigen. The response can

involve components such as white blood cells.

An example of delayed immunity is the tuberculin test

(or the Mantoux test), which tests for the presence of

Mycobacterium tuberculosis, the bacterium that causes tuber-

culosis. A small amount of bacterial protein is injected into the

skin. If the individual is infected with the bacteria, or has ever

been infected, the injection site becomes inflamed within 24

hours. The response is delayed in time, relative to the imme-

diate response of antibody-based immunity. Hence, the name

of the immunity.

See alsoAntibody formation and kinetics; Immunization

IImmunity, cell mediatedMMUNITY, CELL MEDIATED

The immune systemis a network of cells and organs that work

together to protect the body from infectious organisms. Many

different types of organisms such as bacteria, viruses, fungi,

and parasitesare capable of entering the human body and

causing disease. It is the immune system’s job to recognize

these agents as foreign and destroy them.

The immune system can respond to the presence of a

foreign agent in one of two ways. It can either produce solu-

ble proteins called antibodies, which can bind to the foreign

agent and mark them for destruction by other cells. This type

of response is called a humoral response or an antibody

response. Alternately, the immune system can mount a cell-

mediated immune response. This involves the production of

special cells that can react with the foreign agent. The reacting

cell can either destroy the foreign agents, or it can secrete

chemical signals that will activate other cells to destroy the

foreign agent.

During the 1960s, it was discovered that different types

of cells mediate the two major classes of immune responses.

The T lymphocytes, which are the main effectors of the cell-

mediated response, mature in the thymus, thus the name T cell.

The B cells, which develop in the adult bone marrow, are

responsible for producing antibodies. There are several differ-

ent types of T cellsperforming different functions. These

diverse responses of the different T cells are collectively

called the “cell-mediated immune responses.”

There are several steps involved in the cell-mediated

response. The pathogen (bacteria, virus, fungi, or a parasite),

or foreign agent, enters the body through the blood stream, dif-

ferent tissues, or the respiratory tract. Once inside the body,

the foreign agents are carried to the spleen, lymph nodes, or

the mucus-associated lymphoid tissue (MALT) where they

will come in contact with specialized cells known as antigen-

presenting cells (APC). When the foreign agent encounters the

antigen-presenting cells, an immune response is triggered.

These antigenpresenting cells digest the engulfed material,

and display it on their surface complexed with certain other

proteins known as the Major HistocompatibilityClass (MHC)

of proteins.

Next, the T cells must recognize the antigen.

Specialized receptors found on some T cells are capable of

recognizing the MHC-antigen complexes as foreign and bind-

ing to them. Each T cell has a different receptor in the cell

membrane that is capable of binding a specific antigen. Once

the T cell receptor binds to the antigen, it is stimulated to

divide and produce large amounts of identical cells that are

specific for that particular foreign antigen. The T lymphocytes

also secrete various chemicals (cytokines) that can stimulate

this proliferation. The cytokines are also capable of amplify-

ing the immune defense functions that can eventually destroy

and remove the antigen.

In cell-mediated immunity, a subclass of the T cells

mature into cytotoxic T cells that can kill cells having the for-

eign antigen on their surface, such as virus-infected cells, bac-

terial-infected cells, and tumor cells. Another subclass of T cells

called helper T cells activates the B cells to produce antibodies

that can react with the original antigen. A third group of T cells

called the suppressor T cells is responsible for regulating the

immune response by turning it on only in response to an antigen

and turning it off once the antigen has been removed.

Some of the B and T lymphocytes become “memory

cells,” that are capable of remembering the original antigen. If

that same antigen enters the body again while the memory

cells are present, the response against it will be rapid and

heightened. This is the reason the body develops permanent

immunity to an infectious disease after being exposed to it.

This is also the principle behind immunization.

See alsoAntibody and antigen; Antibody-antigen, biochemi-

cal and molecular reactions; Antibody formation and kinetics;

Antibody, monoclonal; Antigenic mimicry; Immune stimula-

tion, as a vaccine; Immune synapse; Immune system;

Immunity, active, passive and delayed; Immunity, humoral

regulation; Immunization; Immunochemistry

IImmunity, humoral regulationMMUNITY, HUMORAL REGULATION

One way in which the immune systemresponds to pathogens

is by producing soluble proteins called antibodies. This is

known as the humoral response and involves the activation of

a special set of cells known as the B lymphocytes, because

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