What is the Adaptive Response to Disease?

Adaptive Immune System Explained: How Your Body Builds Resistance

Our body comes into contact with disease-causing organisms, called ‘pathogens’, almost every day. Therefore, our immune system has a multi-layered structure that enables it to fight off these pathogens. This defence mechanism has two types of responses – the innate or non-specific immune response and the adaptive or specific immune response.

Adaptive immunity, also known as acquired immunity, is only present in vertebrates. It is a distinct response to the disease-causing organisms and is generally acquired over an individual’s lifetime via exposure to vaccines or pathogens.

Read ahead to learn more!

What are the components of the adaptive immune system?

The adaptive immune response is the second line of defence against non-self pathogens. It utilises immunological memory to recall past threats and becomes activated when exposed to specific pathogens. It responds slowly but has long-lasting and sustained effects in fighting pathogens.

This system depends on lymphocytes that are produced from stem cells called multipotent haematopoietic stem cells, which are located in the bone marrow. Here is a comprehensive explanation of the process:

1.  B Cells

• These cells originate in the bone marrow and then migrate into the lymphatic system to circulate throughout the body.
• These are known as naive B cells, which start to mature when they encounter an antigen. They will subsequently display antibodies on their cell surface, also referred to as membrane-bound antibodies.
• When this naive B cell encounters an antigen that matches its membrane-bound antibody, it rapidly divides and forms either a memory B cell or an effector B cell, also known as a plasma cell.
• Memory B cells express the same membrane-bound antibody as inherited from their parent B cell. Whereas plasma B cells express the same antibody as their parent B cell, but these are not membrane-bound.
• Plasma B cells can also secrete antibodies that help identify and neutralise free pathogens circulating in the body.

2.  T Cells

• Firstly, the T progenitor cells are formed in the bone marrow, and then they migrate to the thymus to mature and differentiate into T cells.
• These developing T cells possess T cell receptors and other receptors known as CD4 and CD8. A single T cell has the ability to express either T cell receptors or CD4 or CD8 receptors, but not both.
• The T cell receptors help identify antigens bound to receptor molecules known as Major Histocompatibility Complex class 1 (MHC I) and class 2 (MHC II).
• CD4 and CD8 receptors help identify and activate T cells by binding to either MHC I or MHC II.
• The T cell receptors undergo a process of rearrangement to generate a wide range of binding capabilities. However, this diversity may also lead to accidental damage to the self-cells, as the rearrangement configurations may mistakenly imitate the person’s self-molecules and proteins.
• For an appropriate immune response, mature T cells must identify only foreign antigens combined with self-major histocompatibility complex (MHC) molecules.

To protect the body’s own cells and tissues from immune attack, the mature T cells undergo two types of selection processes. Without these two processes, autoimmune diseases may occur. These selection processes are:

• Positive Selection:  In this process, cells are tested for their ability to distinguish between self and non-self proteins. To pass this selection process, they must bind with self-MHC molecules. If they bind with non-self MHC molecules, they are destroyed by apoptosis.
• Negative Selection : In this process, cells are tested for their self-tolerance. They will pass the test if the T cell binds to the self-MHC molecule expressing a foreign antigen. T cells that bind with high affinity to self-MHC molecules presenting self-antigens are destroyed by apoptosis. This ensures self-tolerance.

After passing these tests, three types of mature T cells are formed: helper T cells, cytotoxic T cells, and T regulatory cells.

• Helper T cells are essential in initiating the activation of T cells, B cells, and other immune cells.
• Cytotoxic T cells express CD8 receptors and help in eliminating pathogens and infected host cells from the body.
• T regulatory cells express both CD4 receptors and another receptor called CD25. These cells help in the distinction of self and non-self molecules, thereby reducing the risk of autoimmune diseases.

What are the two important mechanisms of the adaptive immune system?

The adaptive immune system has two primary mechanisms that provide immunity against pathogens. These mechanisms rely on the functions of B and T cells. Here’s a detailed overview of these mechanisms:

• 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 in binding 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 release cytokines that help activate and proliferate CD8+ T cells (cytotoxic T cell precursors) that recognize the MHC-antigen complex on infected cells. These activated CD8+ T cells then differentiate into effector cytotoxic T lymphocytes (CTLs) that destroy the infected cell. 

The adaptive immune system depends on antigens and is assisted by T and B lymphocytes, which take longer to respond. Through rapid division of these lymphocytes, immunological memory is built. It helps in displaying a faster response if repeatedly exposed to the same antigen.

Also Read:

→ 3 Most Common Sites for an Avulsion Fracture

→ What Diseases Cause Nerve Entrapment

→ What is Adipose Tissue

→ 7 types of Fractures

→ Different Types of Avulsion