Question

how do antigen presenting cells exactly work? Are they infected cells that must be destroyed or what is their deal?

Answer 1

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 anti- gen-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 as both antigen-presenting cells and antigen receiving cells. However, B cells are the least efficient antigen- presenting 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 antigen (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 antigen- presenting 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 mole- cule 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. molecules. MHC molecules in human beings are divided 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 .