For the last two years (COVID-19 time) we all have been talking about immunity. The word ‘immunity’ has become very familiar to us. Immunity is the capability of the organisms to resist harmful microorganisms. Immunology is a branch of biological sciences which deals with the immunity and the immune system of the body. A basic understanding of the subject is very essential for every student of biology. Though a large number of books are available on the subject but there is always a scope for a book which may not only explain the concepts of immunology in a simple manner but could also create an interest and zeal among the students for the subject. ILLUSTRATED IMMUNOLOGY is a unique approach to understand the fundamentals and emerging trends of immunology in a very simple and self-explanatory form. The book has been designed keeping students’ interest and needs in mind. The book covers all essential aspects of immunology and aims to provide the latest knowledge to the readers in an easy and lucid manner. The book includes a large number of simple, hand-drawn and self explanatory illustrations. This would help the students to grasp the concepts firmly, deeply and in an easy manner. A comprehensive Glossary of important terms has also been included in the book for quick reference.

The study of the immune system, in its various aspects, is called immunology. Immunology is the branch of bioscience which deals with the defense mechanisms including all physical, chemical and biological properties of the organism that help it to combat its susceptibility to foreign agents and microorganisms. Immunology includes the development and functions of various components of immune system by which the body responds, destroys or neutralizes foreign agents or pathogens. It includes the study at cellular as well as molecular level. Immunology is an interdisciplinary subject which incorporates and requires the knowledge of many branches of biology like microbiology, biochemistry, molecular biology and clinical medicine.

The major components of the immune system are the lymphocytes which constitute lymphoid system (Figure 2.1).The lymphoid organs are classified into two categories: primary and secondary lymphoid organs.

Innate immunity or innate immune response is a set of non-specific or less specific mechanisms which provides the first and rapid line of defense. All such mechanisms exist prior to the exposure to a pathogen i.e. before the onset of the infection. It is also called inborn or innate immunity. The different components of the innate immunity like physical barriers, chemical barriers, phagocytosis, inflammation etc. act to prevent the entry or elimination of the microorganisms in a non- specific manner.

Innate immunity provides the first line of defense against invading pathogens. But if the pathogens are able to escape from the first line of defense mechanisms, the body uses, another set of immune mechanisms which provide defense in a more specialized manner. These are called second line of defense mechanisms. The second line of defense mechanisms are highly specific in nature. These are called adaptive immune response and provide adaptive immunity. It is also called acquired immunity since these immune responses are not present prior to the exposure of the microbes. When the body is exposed to foreign substances or microorganisms, body develops adaptive immune responses. Adaptive immunity is a set of specialized cells and processes that function to eliminate pathogens or prevent their growth.

When the body is exposed to a microbe or a foreign substance, the body makes some response which is called immune response. Immune responses are initiated by the interaction between a ligand, present on the surface of the microbe or secreted by the microbe or a foreign substance, and a receptor protein on the cell’s surface or a soluble receptor. There are several factors such as shape, charge and presence of other receptors which influence this interaction between the ligand and receptor. This ligand which is recognized by the immune system is called antigen. An antigen can be defined as an organism, a molecule, or part of a molecule which is recognized by the immune system. Often, the term immunogen is also used for antigen. Though, antigen and immunogen are used as synonyms but specifically, they are different from each other.

Antibodies, also called immunoglobulins, are the proteins produced by the immune system against antigens. These are the protective proteins which interact with antigens and neutralize their effects. Antibodies are produced by B lymphocytes. An immunoglobulin has a Y-shaped structure consisting of four polypeptide chains.

Humoral immune response is the immune response which is mediated by antibodies.. It is called humoral immune response because antibodies are transferred with the ‘humour’ that is fluid or blood. Antibodies are the proteins produced by B lymphocytes.

Cell mediated immune response (CMI) is the major defense mechanism against intracellular microbes and tumor cells. It is mediated mainly by T lymphocytes. T cells originate in bone marrow but the maturation of T cells occurs in thymus.

The interactions between antigens and antibodies are specific in nature in which an antibody recognizes an antigen through complementarities of shapes on paratope and epitope. An antigen reacts only with antibodies produced by itself or with closely related antigens. The reaction is similar to a lock and key. As a standard lock can be opened by its own key only similarly, an antibody can react only with its own antigen. Antibodies recognize molecular shapes (epitopes) on antigens.

The complement system is a group of about 30 heat labile plasma proteins which constitute nearly 15% of total serum proteins. It is a system of plasma and membrane-associated serum proteins which can elicit a highly efficient and regulated inflammatory and cytolytic immune responses to different types of infectious organisms.

Major Histocompetibility Complex (MHC) is a closely linked cluster of genes which have codes for cell surface proteins known as MHC molecules. MHC molecules were previously called as HLA molecules. Since these molecules were first discovered on the leucocytes, these were called human leukocyte antigen or HLA molecules.

Cytokines are the short lived chemical messenger molecules, released by a group of cells which can modulate the functions of the same cell(by which they are produced) or other cells.

The basic purpose of the immune system is to provide defense to the body from various types of infections. But, sometimes, the system which is aimed to provide protection become damaging or even life-threatening. There are situations when immune response against harmless and inert antigens is exaggerated and cause diseases.

Infectious diseases are the result of a complex interaction between the host and the microbes. Evasions of protective immunity by the parasites critically determine the survival and pathogenicity of the microbes. The invasion of the body by harmful microorganisms is called infection.

One of the greatest achievements in science is the development of vaccines. Vaccines have been revolutionary for the prevention of infectious diseases. They have prevented a large number of deaths caused, every year, by dreadful diseases like smallpox, polio, hepatitis etc. Diseases like smallpox and polio have been successfully eradicated by vaccination.

Transplantation refers to the implantation of a cell, tissue or organ from one site to another site in an individual or from one individual to another. It involves implanting of organs or grafting of tissues to repair defects and to stimulate regeneration.

A tumor or neoplasm is an abnormal growth of the cells to form a lump or mass of transformed cells. This growth occurs due to loss of regulatory control of cell division. The majority of cells and organs have a balance between cell renewal and cell death. 

The wellbeing of a person depends on his immune system. Any deficiency in any component of the immune system is called immunodeficiency and it may impair the normal functioning of his body and makes the person more susceptible to infections.

One of the characteristics of the immune system is to recognize and discriminate self from the non-self antigens. But, sometimes, there is a failure to discriminate between self and non-self antigens in the body and an autoimmune response occurs. Body starts producing antibodies against its own cells/antigens. The self antigens against which the immune system responds are called autoantigens and the antibodies produced against self antigens are called autoantibodies.

In the study of immunology, several techniques are used. These techniques are called immunotechniques. These techniques are used not only in immunology but are the methods for research and diagnostic purpose in many fields such as microbiology, biochemistry, endocrinology, hematology and in pathological laboratories.

Acquired immunodeficiency syn[1]drome(AIDS) 316 Activation of complement 126 Active immunity 70 Acute phase proteins 59