The Immune System

Immunology, immune system
Image by Bruno /Germany from Pixabay

Immunology is the study of the host’s defense mechanisms which we call the immune system, against a range of foreign chemicals, bacteria, viruses and other materials that might cause harm.

Immunity is the ability of the host to protect itself in particular against foreign organisms.

The immune response is the phenomenon of using the immune system to mount a defence. The immune system includes all these cells and tissues, and molecules.

A  Short History Of Immunology

Back in Ancient Greece, around 430 BC, the Peloponesian War was raging. Thucydides examines the outcomes of a great plague. he noticed that those who recovered from the disease were able to nurse other who were sick without havig the disease a second time. It was a good description of acquired immunity.

In the 1400s, the Chinese and Turks used dried crusts of smallpox as a ‘vaccine’. 

The first application of a vaccine is in 1798 when Gloucestershire born Edward Jenner uses cowpox as a vaccine to treat smallpox having noticed that milkmaids who had cowpox never caught smallpox. He tested the cowpox on an 8 year old boy by scratching him with the cowpox virus. he then exposed him to smallpox but the boy was unaffected. The term vaccine was derived from the latin word for a cow which is ‘vacca‘ (incidentally a 1st declension, feminine noun).

The Function Of The Immune System

Two immune systems operate:-

(1) The natural, non-adaptive or innate immune system relies on skin, saliva, tears, acids produced in the vagina for example and mucus. It is a common property of all living creatures. It is our first line of defense and the first line in the immune response.

(2) The adaptive or acquired immune system operates at a cellular level. It involves specialist cells, specific biomolecules such as cytokines and antibodies. It also involves specialised mucosal lymphoid tissue.

It is part of the second line of defense and works in conjunction with the innate immune response especially if this one does not immediately work.

This system is highly specific and with this system comes a ‘memory’ too. It relies on mechanisms that adapt following infection and is ‘managed’ by T- and B-lymphocytes.

The two immune systems operate to protect us against malignant microorganisms including bacteria, yeasts and fungi and viruses which are all collectively known as pathogens. It also operates against much larger organisms such as  parasites including tapeworms, helminths (flukes) and malaria parasites.

They also have another function which is to eliminate damaged or malignant cells such as cancer and tumour cells.

Antigens And Antibodies

An antigen is a substance which is introduced into the body and stimulates the production of biomolecules called antibodies. More specifically, antigens are glycoproteins which reside on the cell surface membrane of a pathogen and stimulate or elicit an immune response. 

These antibodies are highly specific to the antigen and help the immune system to eradicate the foreign body.

Complete antigens are substances that induce antibody formation by themselves and react specifically with the antibodies. There are also incomplete antigens called haptens which are substances that are unable to induce antibody formation by themselves but become ‘immunogenic’ when they are bound to proteins. These proteins are called carrier proteins and occur in tow types, simple or complex.

Our First Line Of Defense In Immunology

The first line of defense refers to the system which includes the Itegumentary System such as skin, mucous membranes and mucous itself. These all form a physical barrier which prevents access by microbes and pathogens. This is backed up by a chemical system that is the innate immune system described next.   .

Innate Immunity

Nonspecific host defenses which operate in the body do not rely on exposure to an antigen. They do not involve antibodies.

 Skin secretions tend to have an acid pH which acts as a hurdle to bacterial growth. The sebum which comes from specialist cells in the skin also produces natural antimicrobials which are toxic chemicals to bacteria. A vaginal section for example is probably the most acidic secretion known outside of the acidic pH in the stomach. .

The stomach is lined with mucosae whose secretions will kill most pathogens. The mucosa produce and then release strong acid which is hydrochloric acid (HCl) along with an acidic protease enzyme called pepsin. Pepsin is highly active low acidic pHs.

Our tears or lachrimal fluid washes the external eye surface and contains another powerful protease amongst many called lysozyme which kills bacteria.

We also produce mucous (snot) which is a sticky thick viscous fluid in our nose and other parts that literally traps microorganisms. We use cilia in nacasl cells to move these packets of fluid about which is one of the reasons we are compelled at times to clear our throat or blow our nose. 

The Second Line Of Defense In The Immune System

When pathogen enters the body, a series of secondary nonspecific defenses are put in place. The skin would normally be our first line of defense because it is an effective barrier that acts passively by preventing foreign substances entering. If there is a cut, a pathogen might enter and stimulate an inflammatory response.

In the inflammatory response, the pathogen such as bacteria stimulates an increase in blood flow to the infected area. The blood vessels expand in that area and white blood cells leak from vessels to invade the infected tissue.

The white blood cells called phagocytes engulf and destroy the bacteria. 

The infected area also becomes red and swollen, and also painful during inflammation.

The pathogen can also stimulate the immune system to release chemicals that raise body temperature. This is a fever. Increased body temperature helps slow down or even stop the reproduction of the pathogen and in turn speeds up the overall immune response.

Organs Of The Immune System

There are two distinct groups of organs involved in the immune system. They are the primary and the secondary lymphoid organs.

The primary lymphoid organs include the bone marrow, thymus which are ‘maturation’ sites.

The secondary lymphoid organs include the spleen and lymph nodes. There are also two type of lymph tissue: MALT (mucosal associated lymph tissue) and GALT (gut associated lymph tissue) 


The interferons (IFNs) are natural proteins produced by specific cells of the immune system in vertebrates. they are produced in response to a challenge by foreign substances such as parasites, viruses and tumour or cancer cells. It is said that interferons are more useful than antibodies.


Phagocytes are cells that exist as macrophages or as neutrophils. Both cells engulf and digest pathogens by fusion of the phagosome with lysosomes.


The immune system is composed of numerous different cells which include the lymphocytes. The lymphocytes are subdivided into T-lymphocytes, the B-lymphocytes or plasma cells and the natural killer lymphocytes. We also have the monocytes which are known as macrophages too.

Then there are the granulocytes which are divided into three groups – the neutrophils, eosinophils and finally the basophils.

The T and B cell lymphocytes are produced from stem cells that grow in the bone marrow. The mature in different lymphoid organs: the B cells mature in the bone marrow itself whilst the T calls mature in the thymus.

T Lymphocytes

We know of three types. One set are the helper T-lymphocytes (CD4+). These cells stimulate and activate B lymphocytes which divide and become phagocytes. These activated B lymphocytes will secrete antibodies that serve as marker proteins that enlist phagocytes to go on to kill microbes.

The second set are the cytolytic or cytotoxic T-lymphocytes (CD8+). These cells destroy abnormal cells  and infected cells containing microbes or microbial proteins using a protein called perforin.

The third set are the T memory cells which remain in the blood for many years and provide the long term protection.

B Lymphocytes

The B-lymphocytes become plasma cells. Only when they have become these plasma cells will they secrete antibodies. There are also the B memory cells which like the T memory cells remain in the blood for ever virtually and provide the long term protection.


Antibodies are also immunoglobulins which are composed of two heavy chains and two light chains. The chains are bound together using disulphide links. The protease enzyme papain is used to cleave immunoglobulins at specific sites. 

The Cell mediated Immune Response

The primary response is the production of specific clones of effector T cells and memory clones. These develop after several days following infection but does not limit the infection.

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