Immunology: An Overview

Introduction

Immunology is basically the study of immune system of an organism which protects the organism from physical, chemical and biological invasions.

Immunity: Immunity is our body’s protective mechanism against pathogens which includes the disease inducing micro-organisms like bacteria, virus, fungi, protozoa etc. We do need immunity as our body constantly attacked by several pathogens in a day to day life.

How Does Our Immune System Works?

Immune system includes a complex set of tissues, cells and specialized molecules which protects our body from any foreign particles like microorganisms that enter our body.

For effective functioning of our immune system, it must carry out 2 tasks which includes −

• Immunological recognition which is the ability of our immune system to detect the presence of disease.

• Immune regulation which exhibits an immune response towards the antigen by limiting the damage to a host cell.

Defense Mechanisms: Our body have mainly has three lines of defense mechanisms to eliminate the pathogens.

1^st line of Defense – This include passive barriers like skin, mucous membrane and stomach acids (pH is about 2) which will inhibit majority of pathogens.

2^nd line of Defense – It involves our body’s innate immune system in which we have different cells and proteins that would be non-specific to different pathogens and kill them as many as possible. This includes complement response pathways, phagocytes, granulocytes like neutrophils, macrophages, NK cells etc. Granulocytes have secretary vesicles.

3^rd line of Defense – Here our body’s adaptive immune system plays a major role. This includes both B and T lymphocytes.

Innate and Adaptive Immunity

Immunity is categorized into innate and adaptive immunity.

Innate Immunity

• Any part of our immune system which is inherent in our genes that is we born with it and is passed to our next generations.

• It does not change over a life time.

• This immunity involves the physical barriers like skin etc. and the 2nd line defense cells. So anything which exposes to the outside environments must have innate defense mechanisms.

• Proteins and chemicals are also used to fight infections. These include cell enzymes, complement proteins and acute phase reactants (made by liver in acute inflammation).

• This innate immunity has limited specificity but is fast acting immune response towards the invaded pathogen.

• If these cells encounter any uncommon pathogens and infection gets out of control this innate immune system alerts the adaptive immune system where the more complex process occurs to eliminate pathogen.

• Any immune response exhibited by an innate immune system does not contribute to the immunological memory.

Innate Immune Cells

These cells kills the pathogen in a primitive way which is called phagocytosis. It involves the eating and digesting the pathogens. Pseudopods which are the arm extensions of immune cells engulf pathogen forming a phagosome. This phagosome then fuses with the lysosome having catalytic enzymes and leads to its break down.

Examples of phagocytes are: Macrophages, Neutrophils, Dendritic cells.

Adaptive Immunity

• Immunity which is acquired and not part of the inherited immunity is called as adaptive immunity.

• This immunity gets better with the time.

• This type of immunity involves both T lymphocytes and B lymphocytes. Here the major role of B cells is to produce antibodies specific to the invaded antigen or foreign pathogen.

• This immunity takes long duration to exhibit immune response when compared to innate immunity. But it shows highly specific and powerful immune responses.

• This immunity results in the immunological memory for T and B cells. These memory cells responds more quickly when same or similar pathogens attacks the immune system.

Cells of Adaptive Immunity

As innate system is limited in detecting the pathogens like viruses which lives in the host cells and replicate themselves, adaptive immune system is most helpful in such situations and other cases of large infections on a greater scale. The immune response here is divided into 2 phases.

• Cell-mediated immunity – which is handled by T cells (Tc and TH cells)

• Humoral immunity – Mediated by the B cells. Here the word humoral refers to the plasma fluid. B cells which are differentiated into these plasma cells are mainly involved in antibody or immunoglobulins production.

Stem Cells: Origin of Immune Cells

Discussion of immunology involves always the discussion of immune cells and their interactions.

All the cells of an immune system mainly derived from the stem cells which are in undifferentiated form. Once these cells differentiate, they mature and become specific cell by its function.

Multiple hematopoietic stem cells which are present in the red bone marrow of all of our bones during childhood give rise to all the blood cell lines in our body and are CD34 positive. Whereas in adults the skull bones, vertebrae, sternum, ribs, scapula, proximal humerus and femus are blood producing sites. And all other bones are converted into fat which is called as yellow marrow.

Stem cells can be differentiated into 2 lineages depends on the chemical messenger they will release. If they release cytokine IL7 they will go towards the common lymphoid progenitor in which we will find agranulocytes like T and B lymphocytes and large granular lymphocyte called natural killer cells. And if they release IL3 then they will go towards the myeloid progenitor cells where we have most of our blood cells like megakaryocytes which produce platelets from their membrane, erythrocytes, myeloblasts form granulocytes like basophils, neutrophils and eosinophils, agranulocytes like monocytes which later forms macrophages.

T cells once mature get differentiated into cytotoxic T cell and helper T cells whereas B cells gets differentiated into antibody producing plasma cells.

Lymphoid Organs

Our body also consists of different lymphoid organs which are the sites for the production of blood cells and maturation of some white blood cells. These lymphoid organs are classified into 2 types.

Primary Lymphoid Organs

It includes bone marrow and the thymus. Thymus is the maturation site of the T cells (thymocytes). Thymus once the puberty attains is converted into sternal fat tissue and its role is taken over by secondary lymphoid organs.

Secondary Lymphoid Organs

These includes lymph nodes, tonsils and adenoids, appendix, spleen and Peyer’s patches. These organs are the binding sites for different pathogens and these expose these antigens to the lymphocytes for their later function in the adaptive immune system.

Conclusion

Immunology is the study of structure and functioning of the immune system within an organism. It actually deals with how the host responds when a foreign pathogen attacks the tissues and how it eliminates the pathogen by protecting itself from further infections.

Immune system which comprises of molecules, cells, organs, tissues provides both non-specific and specific protection against the several organisms and this is mostly categorized into 2 types. They are innate immunity (inherited) and adaptive immunity (acquired).

Innate immune system provides an immune response by engulfing the invaded pathogen through the process phagocytosis whereas the adaptive immunity provides a specific immune response either by cell-mediated immunity or by antibody mediated immunity.