What are Components of the Immune system – Types and Functions

Components of the Immune System

The immune system is vast enough as any country’s defense system. At various points checks posts have been built –up.

Main weapons of this troop or components of the immune system:

  • WBC (White Blood cell)
  • Antibodies
  • The Complement system
  • The lymphatic system
  • Spleen
  • Thymus
  • Bone Marrow

To understand immunity and its components first of all we should know about four main types of pathogenic microbes. These are bacterias, pathogenic fungi, parasites, and viruses. Against specific pathogen adaptive immune response produces antibodies.

An adaptive immune response is an eternal modification as an adaptation to infection with pathogenic microbes. Phagocytic cells e.g., macrophages digest these pathogenic microbes.

These macrophages are instantly accessible to fight against these pathogens and are the main components of the innate immune system. Production of antibodies only possible after infection and specific antibodies are produced against a wide scale of antigens these antigens trigger the production of antibodies.

Components of the immune system

Components of the Immune System
Components of the Immune System

Responses of both the adaptive immune response and innate immunity depend upon the activities of WBC. Key components of innate immunity are granulocytes and macrophages.

The main components of the Immune system are Adaptive immune system are lymphocytes that provide lifetime immunity. Adaptive and Innate Immune system both in collaboration builds up a powerful defense system. Due to these strong immune systems, in this world full of infectious microbes we become ill infrequently.

Numerous inflammations can be overcome by innate immune systems and no disease occurs while other infections which can’t be determined by the innate immune system will stimulate adaptive immunity and then easily cured by remaining immunological memory. Leukocytes arbitrate immune response and these leukocytes are derived from progenitors or precursors in the bone marrow.

Hematopoietic Stem Cells

It also Components of the Immune System These stem cells induce the production of lymphocytes and myeloid these lymphocytes are accountable for adaptive immunity and myeloid lineages have a role in both innate and adaptive immunity.


These include neutrophils, eosinophils, and basophils and they are circulated in the blood act as effector cells at sites of inflammation. Granulocytes are Components of the Immune System.

Macrophages and mast cells are involved in the differentiation of tissues and work as effector cells and stimulate inflammation.

Macrophages engulf bacteria and Switch on other phagocytic cells which are neutrophils. Mast cells are responsible to arrange defense mechanisms against pathogenic parasites and stimulate allergic soreness and they further manage eosinophils and basophils which are exocytic.

Immature phagocytes e.g., dendritic cells penetrate in tissues and start engulfing antigens. Then these dendritic cells with antigens go towards lymphoid tissues.

Two types of lymphocytes

  • B lymphocytes: These become matured in the bone marrow.
  • T lymphocytes: Their maturation spot is present in the thymus.

Bone marrow and thymus are primary lymphoid organs while mature lymphocytes recirculate in blood by secondary lymphoid organs through lymphatic vessels. When a recirculating T cell recognizes a specific antigen on the surface of a dendritic cell it will stimulate the adaptive immune system.

Secondary lymphoid tissues are of three types as spleen this spleen capture antigen from the bloodstream then lymph nodes caught antigen from the spot of infection in the tissues and mucosal-associated lymphoid tissues (MALT) which collect antigen from the epithelial surface of the body.

Adaptive immune responses begin in these secondary lymphoid tissues in which T cells that meet with antigen then these T cells will multiply and change into antigen-specific effector cells and B cells will grow, multiply and transform into antibody-secreting cells.

Now come back to the main discussion which is components of the immune system. It is clear to some extent that cells of the immune system mainly produced in the bone marrow where some become matured and then move from here to protect peripheral tissues by circulating in the bloodstream and through the lymphatic system

The main weapons that are white blood originated from precursors in the bone marrow.  Red blood cells are responsible for the transport of oxygen.

Platelets that are involved in blood clotting at a damaged portion in the body and WBCs are all obtained by hematopoietic cells or pluripotent hematopoietic stem cells which are precursor cells in the bone marrow. These stem cells produce different types of blood cells.

Myeloid Progenitor

Myeloid Progenitor is Components of the Immune System The precursor of the granulocytes, macrophages, dendritic cells, and mast cells of the immune system is the myeloid progenitor.


Macrophages play an important role in innate immunity. They are the mature shape of monocytes, these monocytes circulate in the blood and covert continuously into macrophages upon transportation into the tissues.

Dendritic cells

These cells catch an antigen and exposed it for identification by lymphatic cells. Migration of immature dendritic cells from blood to stay in tissues and then phagocytic and macro-pinocytosis start digesting a massive amount of fluid which is outside from the cells.

When these cells came across a pathogen they quickly become mature and move towards lymph nodes. When these dendritic cells are attacked by pathogens these become mature enough and move towards lymph nodes.

Mast Cells

Precursors of mast cells are not well defined, however, these mast cells also differentiate in the tissues they stay near the blood vessels and after activation, these release their substances which affect vascular permeability. Mat cells have a role to plan and arrange allergic responses and protect mucosal surfaces against antigens.


Granulocytes or polymorphonuclear leukocytes have granules in their cytoplasm.

Three types of granulocytes are:

Neutrophils: Most important component of the immune system, their deficiency leads to increases in bacterial infections.

Eosinophil:  Eosinophils have a role against parasitic infection.

Basophils: Its function is the same as eosinophils and mast cells.

Lymphoid precursor

This progenitor gives rise to the lymphocytes.

The Two types of lymphocytes

B lymphocytes: When these are activated these are differentiated into plasma cells that secrete antibodies.

T lymphocytes: There are two main classes of T cells, one class converts into cytotoxic T cells on activation which kill virally infected cells, while another class activates other cells like B cells and macrophages. These all are the type of immune system.

Maturation of lymphocytes in bone marrow or Thymus

Lymphoid organs are well-characterized tissues having plenty of lymphocytes with non-lymphoid cells. In lymphoid organs, there is an interaction between lymphocytes with non-lymphoid cells for development, initiation of adaptive immune responses, and long-lasting lymphocytes.

Two main types of lymphoid organs

Central or primary lymphoid organs include bone marrow and thymus organs in the upper chest where lymphocytes are produced.

Both B and T lymphocytes are generating in the bone marrow while just B lymphocytes become mature in the bone marrow. For the maturation of T cells, these T cells go towards the Thymus.

B cells are known as B cells because these are derived from Bone marrow and T cells are derived from Thymus so their name is T cell because their origin is Thymus. 

After maturation of both types of lymphocytes both launched into the blood circulation and through the bloodstream both move towards secondary or peripheral lymphoid organs.

The next step of the defense system is that the peripheral or secondary lymphoid organs are ready to capture antigen and permit the inauguration of adaptive immune responses and provide signals to support recirculating lymphocytes.

The antigen can move into the body in many ways and causes infection or inflammation anywhere but suddenly attacked by secondary or peripheral lymphoid organs i.e., the spleen, lymph nodes, and mucosal lymphoid tissues.

Lymphocytes are also recirculating in these tissues (spleen, lymph nodes, and mucosal lymphoid tissues) where the antigen is brought from the spot of infection. These antigens are initially present within macrophages and dendritic cells. After the maturation of dendritic cells then these matured dendritic cells will expose antigen to lymphocytes.

Lymph Nodes

These are well-organized structures and their location is at the merging points of blood vessels or the lymphatic system which is a system of vessels for collection of extracellular fluid from tissues and again return it to the bloodstream.


This extracellular fluid which is produced consistently by filtration from the blood is known as lymph. And the vessels are called lymphatic vessels.

Location of spleen

The spleen is a palm-sized organ behind the stomach that gathers antigens from the blood. It also accumulates and destroys old red blood cells. It’s the main spot of disposal of old red blood cells.

Despite the different appearance of the lymph nodes, spleen, and mucosal-associated lymphoid tissues all work on the same rule i.e., capturing or trapping antigen from the point of infection and stimulating an adaptive immune response.

The peripheral or secondary lymphoid tissues provide signals to the lymphocytes that do not attack by their particular antigen, so these lymphocytes continue to live and recirculate until they come across their specific antigen.  

This process is necessary to sustain the correct numbers of circulating T and B lymphocytes and make sure that only potential lymphocytes for an antigenic response should survive. Lymphocytes circulate between blood and lymph.

Small B and T lymphocytes that completed their maturation in the bone marrow and thymus but have not still attacked by antigen are known as naïve or natural lymphocytes.

These cells circulate constantly from the blood into the peripheral or secondary lymphoid tissues, where they enter by compression between the cells of capillary walls.

They are then returned to the blood through the lymphatic vessels or, in the case of the spleen, return directly to the blood.

When any infection appears lymphocytes identify the infectious factor and then this lymphocyte is pick up by lymphoid tissues where it multiplies and convert into effector cells which are necessary to cope up with the infection.

When a huge amount of antigens are absorbed by dendritic cells which move towards lymph nodes via afferent lymphatic vessels. In these lymph nodes, these dendritic cells expose antigen to T lymphocytes and these T lymphocytes will activate B cells which will capture antigens.

After these antigen-specific lymphocytes will multiply and differentiated making lymph nodes effector cells and peripheral lymphoid tissues will initiate an adaptive immune response.

Diffused mucosal lymphoid tissues will emerge in response to infection and then vanished. And then they will appear as more organized during infection e.g., B cell follicles of the lymph nodes enlarged as B lymphocytes, multiply and the entire lymph node will be expanded this phenomenon is known as swollen glands.

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Principles of innate and adaptive immunity

Front Line soldiers

Macrophages and neutrophils of the innate immune system provide defense against many microbes and are important for the elimination of various common bacterial infections. However, they cannot always remove infectious agents and cannot identify some pathogenic agents.

Second Line troop

To cover up these front-line soldier’s lymphocytes of the adaptive immune system jump into the battle and ensures more strong defense system and protect against reinfection with the identical and same pathogen. It takes 4-7 days to initiate an adaptive immune response so in this duration, the innate immune system has a crucial role.

Function of antibodies

Antibodies deal with pathogens and their toxic products. These are the foremost products of adaptive immune response found in fluid elements of blood, plasma, and extracellular fluids. Immunity stimulated by antibodies is known as humoral immunity.

Antibodies bind to the pathogen and restrict their toxic products by obstructing their approach towards body cells. This process is known as neutralization which is important for protection against pathogenic toxins.

Just binding antibodies with pathogens is not enough to stop the replication of bacteria that proliferates from outside of the cells. In this way antibody allows a phagocytic cell to engulf and dispose of bacteria.

This is necessary for many bacteria that are resistant to direct recognition by phagocytes instead the phagocytes identify the constant region of antibodies coating the bacterium. The covering of pathogens and foreign particles in this way is known as opsonization.

The compliment System

The system of plasma protein is known as a complement system that is activated by antibodies or without the help of antibodies on many microbial surfaces which is helpful to innate as well as adaptive immunity.

Pore formation by activation of complement substances directly smash bacteria and this process is necessary for some bacterial infections.

However, the main purpose of the complement system is to coat antigens with antibodies and make engulfing possible by phagocytes to demolish bacteria’s which may remain unrecognized if not assisted with the complement system.

Complement also increases the germicidal performance of phagocytes in other words it “Complements” the activities of antibodies. It’s all about the components of the immune system.


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