What are the Four Types of Adaptive Immunity?

Understanding the 4 Types of Acquired Immunity

Adaptive immunity is the second line of defence against pathogens. It occurs if the innate immune system fails to fight off the disease-causing microbe. It depends on cellular development and helps to identify and respond to non-self pathogens.

It is also referred to as ‘acquired immunity’, as it is the resistance mechanism an individual acquires throughout their lifetime. To understand its types, it is crucial to learn about the primary mechanism of adaptive immunity. Keep reading for a detailed insight.

What are the primary mechanisms of the adaptive immune system?

The adaptive immune system provides specific resistance against disease-causing microbes. These mechanisms rely on developing T (T-lymphocytes) and B (B-lymphocytes) cells. Here’s a detailed breakdown of the two types of immune mechanisms provided by the adaptive immune system:

• Humoral Immunity
  
  In this mechanism, the helper T cells assist the B cells to divide into plasma B cells, which can generate antibodies against a particular antigen.

The antibodies produced by the B cells help bind to the antigens of the freely floating pathogens to neutralise them. This process is also known as lysis or phagocytosis, which is the destruction of cells by a lysin.
 

• Cell-mediated Immunity
   

This mechanism occurs inside the infected cells and is assisted by T lymphocytes. The pathogen's antigens are present on the cell surface.

The helper T cells (CD4+ T cells) release cytokines that help activate cytotoxic T cells (CD8+ T cells) that recognize the MHC-antigen complex on the infected cells. This activation causes the cytotoxic T cells to proliferate (multiply) and become effector cells that destroy the infected cell. 

What are the types of adaptive immunity?

Adaptive immunity is categorised into 2 types: active immunity and passive immunity. Both of these can be obtained either naturally or artificially. Hence, there are 4 kinds of adaptive immunity:

• Naturally Acquired Active Immunity
   

When an individual is exposed naturally to antigens, falls ill, and recovers, they develop active resistance to the antigenic stimulus. This is possible through immunological memory.

Once exposed to antigens from a pathogenic disease, the memory B cells remember the antigen to fight back if it attacks again. For instance, if an individual suffers from chickenpox, they usually develop lifelong immunity against it.

Naturally acquired active immunity is long-lasting and is naturally induced by the host immune system through clinical infections and everyday life experiences of a human being.
 

• Artificially Acquired Active Immunity
   

When an individual is intentionally exposed to foreign particles through clinical means, such as vaccination, they will develop resistance against the weakened antigens introduced through the vaccine.

Vaccines introduce specific weakened microbes into the host cell, allowing the individual’s immune system to fight them off easily and gain immunological memory against the particular pathogen.

For instance, the flu, MMR (measles, mumps, and rubella) vaccines, and COVID-19 vaccines.
 

• Naturally Acquired Passive Immunity
   

Passive immunity is resistance acquired through the transfer of antibodies from the mother to her child. Antibodies are transferred from the placenta to the foetus or through breast milk due to the secretions of colostrum.

For instance, in the first few months of life, an infant develops resistance to certain diseases due to maternal antibodies.
 

• Artificially Acquired Passive Immunity
   

This process involves the transfer of already formed antibodies or lymphocytes from a host organism that has developed resistance against the disease. The readymade antibodies present in the organism's immune serum are transferred into the host organism through injection.

For instance, if an individual is exposed to hepatitis A or rabies, antibodies like immunoglobulin are administered to provide immediate but temporary protection. This treatment does not stimulate the patient's own immune system to create memory cells, so it does not result in lifelong immunity.

The major function of the adaptive immune system is to identify and neutralise non-self pathogens. If it successfully fights off a certain microbe, it will build immunological memory against it. However, active immunity is more sustainable than passive immunity, which is acquired passively through vaccines or immune globulin therapy.

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