Cells involved in Adaptive Immunity - Lymphocytes and Antigen Presenting Cells

Cells involved in adaptive immunity or acquired immunity or specific defence:

The third line of defence of immune system has strategize to eliminate or control the pathogen either by employing T cells which is called as cell mediated immunity or employing B cells which will produce antibodies to eliminate pathogen and is called as humoral immunity. Both these strategies are mounted against the pathogen that initiated the immune response. Specificity is the key feature in adaptive immunity.

Cells involved Immune system

2 major groups of cells are involved

1) Lymphocytes: B lymphocytes or B cells and T lymphocytes or T cells are the major players in adaptive immune response. In adults, these cells are derived from hematopoietic stem cells in the bone marrow. T cells mediate cell mediated immunity whereas B cells are behind antibody mediated or humoral immunity. They possess antigen binding cell surface receptors responsible for specificity, diversity memory and self/non-self recognition by the immune system. During embryonic development, these cells develop from the stem cells of liver and yolk sac. The ratio of T cells to B cells is ~ 3:1.

Lymphocytes

A) T lymphocytes or T cells:

T cells like B-cells originate in the bone marrow but maturation takes place in the thymus, therefore called as T-cells. T cells once activated by an antigen produces clone of cells.

a)Helper T cells (CD4 positive cells): These cells are responsible for overall regulation of specific immune response (regulatory cells). Major functions include activating B cells for antibody production, secreting lymphokines that activate cytotoxic T cells and B cells, attracting neutrophils and enhance the ability of macrophages to engulf microbes.

b)Cytotoxic T cells CD8 positive cells: These cells directly attack and kill virus infected cells, tumor cells and antigens (effector cells). They employ two mechanisms for destruction either by release of perforins which destroy cell membranes, or induction of programmed cell death or apoptosis.

c)Suppressor T cells: prevents immune cells like cytotoxic T cells from attacking body’s own cells.

d)Memory T cells: are programmed to recognize once encountered pathogen for enhanced immune response later. These cells have long life span.

B)  B lymphocytes or B cells:

B cells originate and mature in bone marrow itself. B cells are responsible for antibody mediated humoral immunity. B cells are short lived and are replaced every few days from the bone marrow. Two major functions of B cells are

• they differentiate into plasma cells and produce antibodies
• they are antigen presenting cells

a) Plasma cells or Effector B cells: Antigen bound T cells stimulate specific B cells to multiply and differentiate to clone of specialized cells called plasma cells. Plasma cells secrete antibodies to the blood which interact with the pathogen.

b) Memory B cells: some of the activated B cells remain dormant and becomes memory B cells just as memory T cells.

2. Antigen Presenting Cells (APCs):

Any cell that can process and present antigenic peptides in association with class II MHC molecules and deliver a co-stimulatory signal necessary for T cell activation. These cells possess class II MHC molecules on the surface. APCs include macrophages (monocytes as blood macrophages and histiocytes as tissue macrophages), B cells and dendritic cells.