Immunization is an important primary health care service that aims to prevent the emergence of vaccine-preventable diseases by vaccinating infants, children or adults before the period when the risk of infection is highest, and thus to prevent deaths and disabilities caused by these diseases.

What is immunization? 

Immunization is an important primary health care service that aims to prevent the emergence of vaccine-preventable diseases by vaccinating infants, children or adults before the period when the risk of infection is highest, and thus to prevent deaths and disabilities caused by these diseases.

What is a vaccine? 

Vaccines are biological products that provide protection against disease by stimulating the immune system, causing an immune response similar to a natural infection. The effect of the vaccine occurs when the immune system responds to the weakened or killed microorganism or some parts of it. Thus, when the person encounters the disease agent, the immune system will remember the agent and develop a response quickly and the disease will be passed before it occurs or with a mild picture.

In order to understand the mechanisms of action of vaccines, it is necessary to have knowledge about the human immune system and response mechanisms.

What is the immune system? 

The immune system is the system that distinguishes pathogens, foreign substances, tumor cells from healthy body cells and tissues and tries to neutralize them.

What are the organs of the immune (immune) system? 

The immune system consists of primary and secondary (peripheral) lymphoid organs. These work in interaction with each other. These bodies are:

Primary lymphoid organs :

  • Thymus,
  • Bone marrow,

Secondary lymphoid organs :

  • Lymph nodes,
  • Tonsil,
  • Lymph nodes,
  • Liver,
  • Spleen,

What cells are involved in the immune system?

Many cells take part in the immune system. Although each of them has separate duties, they work in interaction with each other. These cells and their functions are:

    • lymphocytes
      • T lymphocytes : It constitutes 60-70% of body lymphocytes. Bone marrow originates from stem cell cells and matures in the thymus. It has its own receptors. Thanks to CD4+ signaling proteins, it supports immunity by activating interleukin, interferon, macrophage and natural killer cells. It kills virus-infected cells and tumor cells by secreting cytokines with CD8+ signaling proteins.
      • B lymphocytes: Originate from the bone marrow and are found in the bone marrow, lymph nodes, spleen, and gastrointestinal tract. It transforms into plasma cells and creates antibodies (IgG, IgA, IgM, IgD and IgE).
    • Natural killer cells: An important part of the innate immune response to various attacks. It destroys various tumor cells, virus cells and some normal cells without previous sensitization.
    • Macrophages : In cellular immunity with dendritic cells, in cases where T lymphocytes cannot be triggered by free antigen, they play a major role in the adhesion of antigen to CD4+ T lymphocytes. It also plays a role in late type hypersensitivity thanks to its rich cytokine content. On the other hand, it kills pathogens in humoral immunity.
    • Dendritic cells : While some of them act as receptors for inexperienced T lymphocytes in the lymph tissue, heart and lung, some of them capture the antibody-bound antigen in the spleen and lymph node and maintain the immune memory.
    • Eosinophils: Play a role in histamine release, especially in allergies and parasitic diseases.
    • Basophils: It acts in interaction with neutrophils such as eosinophils. Their phagocytosis capabilities are limited. When it reaches the tissues, it becomes a mast cell.
    • Mast cells: Kill pathogens by phagocytosis. It generally has an important place in allergic reactions and anaphylaxis. They also play a role in tissue repair, coagulation, fibrinolysis and parasitic diseases.

What are the main features of the immune response? 

The immune response created by the immune system includes some unique basic features. Thanks to these features, the body can create the perfect response to external dangers. In cases where this response is insufficient or one or more of these basic features are impaired, diseases and tumors occur.

The main features of the immune response are:

  • Antigen Specific: The immune system is always specific towards the structural parts of different antigens. Some specific parts of antigens (epitopes) are recognized by lymphocytes. Such promoter molecules of antigens are recognized by the receptors on the membrane of T and B lymphocytes and are attached to their specific structures.

When the antigen first enters the organism, it selects the lymphocyte clone carrying its unique surface receptor and becomes activated, multiplies and differentiates as memory cells and memory cells (primary immune response). In encountering the same antigen again; As a result of the immunization in the previous encounter, the immune response develops earlier and stronger with the enlargement of antigen-specific lymphocytes (secondary immune response).

  • Differs: 109 different types of antigens have been found that can be identified in the mammalian immune system. This situation reveals that antigen-binding receptors on lymphocytes can be very different and variable.
  • Memorizes: When the immune system encounters an antigen a second time, it produces a faster and stronger response. Each exposure with an antigen causes an increase in cells specific to that antigen and immune memory. Memory cells formed by antigenic stimulation are long-lived. It can live for a long time even without antigen. Thus, it is always ready for antigens.
  • Self-limiting : The purpose of the immune response is to destroy the harmful one and the cell that carries it. It is self-limiting as the normal immune response will be exhausted following antigenic stimulation. When it reaches its goal, the immune response is completed, as the antigenic stimulus will have disappeared.
  • It distinguishes between self and non-self : One of the most important features of the immune system is to recognize self and non-self antigens. Generally, lymphocytes recognize non-self antigens and respond to them. However, they cannot give an adequate immune response against the antigenic substances formed in the individual himself. This is called immune tolerance. Lymphocytes acquire this ability to distinguish in the thymus and bone marrow at the developmental stage. In some cases where this feature is not fully acquired, lymphocytes respond to their own cells and cause autosomal diseases.

What is antigen? 

All pathogens, foreign substances, tumor cells that are not body cells or tissues are antigens for the immune system. The immune system primarily tries to prevent their entry in superficial barriers such as the skin, respiratory system and digestive system. For antigens that cross this barrier, defense cells come into play. If macrophages and phagocytes cannot destroy the antigen, the adaptive immune system is activated. While T lymphocytes and natural killer cells try to destroy the antigen, B lymphocytes begin to produce antibodies according to the characteristics of the antigen.

What is Natural Immunity? 

It is the immunity that protects the organism by preventing or eliminating the non-antigen-specific, foreign and harmful substances formed by the first encounter with the antigen. This response system is innate. The cell knows that a pathogen should not be there and attack.

The basic elements involved in innate immunity are: physical barriers of skin and mucous membranes, phagocytic cells in blood and tissues (macrophages, neutrophils, eosinophils, natural killer cells), acute phase proteins, cytokines and the complement system.

It develops in the first 0-4 hours and does not provide a long-term immunity. 

They have no memory, and when they encounter a foreigner again, they respond with the same intensity, performing the task of promotion and warning to the immune system elements whose foreign antigens have been acquired. 

What is acquired (acquired, adaptive) immunity? 

Memory is formed in an active (through the disease) or passive (vaccination) form, and when it encounters the same antigen again, it recognizes and responds more strongly. The response given is pathogen and antigen specific. Response may take days, not effective in the first attack but provides long lasting immunity and prevents subsequent infections

The main elements involved in acquired immunity are T lymphocytes, B lymphocytes, antibodies and cytokines.

Acquired (Acquired) immunity is mainly examined as humoral and cellular immunity. In humoral immunity, antibodies formed mainly by B lymphocytes play a role. Antibodies prevent extracellular pathogens from attaching to the cell wall and infecting the host. Tlymphocytes play a major role in cellular immunity. It enables phagocytes to destroy pathogens within the cell through cytokines and chemokines.

Acquired immunity memory occurs in two ways:

1.Passive Immunity:

Passive immunity is protection by an antibody or antitoxin produced by a human or animal and transferred to another. Protection starts quickly, but within weeks or months, the antibody level drops rapidly.

IgG antibodies produced by the mother pass to the baby through the placenta and protect the baby against diseases such as measles, rubella, tetanus, whooping cough for a few months after birth.

Passive immunity can also be achieved by transfusion of blood products through washed and reconstituted red blood cells.

2.Active immunity:

Active immunity is a protection system produced by the person’s own immune system. The immune system is stimulated by an antigen to produce antibody-mediated and cell-mediated antibodies. Memory often lasts a lifetime. Once a disease is cured, long-term immunity is formed against that disease. B lymphocytes stay in the blood and bone marrow for a long time, when they encounter the antigen again, they multiply rapidly and start to produce antibodies again.

Another way to gain active immunity is vaccination. Thanks to the antigen they contain, vaccines stimulate the immune system as if they had a natural disease. In this way, antibodies are formed without being exposed to natural disease and its complications.