DEVELOPMENT OF CELL-MEDIATED IMMUNITY

DEVELOPMENT OF CELL-MEDIATED IMMUNITY

INTRODUCTION

Cell-mediated immunity is defined as the immunity developed by cell-mediated response. It is also called cellular immunity or T cell immunity. It involves several types of cells such as T lymphocytes, macrophages and natural killer cells and hence the name cell mediated immunity. Cell-mediated immunity does not involve antibodies.

Cellular immunity is the major defense mechanism against infections by viruses, fungi and few bacteria like tubercle bacillus. It is also responsible for delayed allergic reactions and the rejection of transplanted tissues. Cell-mediated immunity is offered by T lymphocytes and it starts developing when T cells come in contact with the antigens. Usually, the invading microbial or non-microbial organisms carry the antigenic materials. These antigenic materials are released from invading organisms and are presented to the helper T cells by antigen-presenting cells.

ANTIGEN-PRESENTING CELLS

Antigen-presenting cells are the special type of cells in the body, which induce the release of antigenic

materials from invading organisms and later present these materials to the helper T cells.

Types of Antigen-Presenting Cells

Antigen-presenting cells are of three types:

1. Macrophages

2. Dendritic cells

3. B lymphocytes.

Among these cells, macrophages are the major antigen- presenting cells.

1. Macrophages

Macrophages are the large phagocytic cells, which digest the invading organisms to release the antigen.

The macrophages are present along with lymphocytes in almost all the lymphoid tissues.

2. Dendritic Cells

Dendritic cells are nonphagocytic in nature. Based on the location, dendritic cells are classified into three

categories:

i. Dendritic cells of spleen, which trap the antigen in blood.

ii. Follicular dendritic cells in lymph nodes, which trap the antigen in the lymph.

iii. Langerhans dendritic cells in skin, which trap the organisms coming in contact with body surface.

3. B Lymphocytes

Recently, it is found that B lymphocytes also act as antigen-presenting cells. Thus, the B cells function asboth antigen-presenting cells and antigen receiving cells. However, B cells are the least efficient antigenpresenting cells and need to be activated by helper T cells.

Role of Antigen-presenting Cells

Invading foreign organisms are either engulfed by macrophages through phagocytosis or trapped by

dendritic cells. Later, the antigen from these organisms is digested into small peptide products. These antigenic peptide products move towards the surface of the antigen-presenting cells and bind with human leukocyte anti gen (HLA). HLA is a genetic matter present in the molecule of class II major histocompatiblility complex (MHC), which is situated on the surface of the antigenpresenting cells.

B-cells ingest the foreign bodies by means of pinocytosis. Role of B cells as antigen-presenting cells

in the body is not fully understood.

MHC and HLA

Major histocompatibility complex (MHC) is a large molecule present in the short arm of chromosome 6. It is made up of a group of genes which are involved in immune system. It has more than 200 genes including HLA genes. HLA is made up of genes with small molecules. It encodes antigen-presenting proteins on the cell surface. Though MHC molecules and HLA genes are distinct terms, both are used interchangeably. Particularly in human, the MHC molecules are often referred as HLA into two types:

1. Class I MHC molecule: It is found on every cell in human body. It is specifically responsible for

presentation of endogenous antigens (antigens produced intracellularly such as viral proteins and

tumor antigens) to cytotoxic T cells.

2. Class II MHC molecule: It is found on B cells, macrophages and other antigen-presenting cells. It

is responsible for presenting the exogenous antigens (antigens of bacteria or viruses which are engulfed

by antigen-presenting cells) to helper T cells.

Presentation of Antigen

Antigen-presenting cells present their class II MHC molecules together with antigen-bound HLA to the helper T cells. This activates the helper T cells through series of

events .

Sequence of Events during Activation of Helper T cells

1. Helper T cell recognizes the antigen displayed on the surface of the antigen-presenting cell with the

help of its own surface receptor protein called T cell receptor.

2. Recognition of the antigen by the helper T cell initiates a complex interaction between the helper T

cell receptor and the antigen. This reaction activates helper T cells.

3. At the same time, macrophages (the antigen-presenting cells) release interleukin-1, which facilitates the activation and proliferation of helper T cells.

4. Activated helper T cells proliferate and the proliferated cells enter the circulation for further actions.

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