Introduction
The immune system is a defensive system in a host consisting of widely distributed cells, tissues and organs that
recognize foreign substances and microorganisms and acts to neutralize or destroy them. The cells responsible for
both nonspecific and specific immunity are the leukocytes or white blood cells.
All leukocytes arise from a type of
cell called the hematopoietic stem cell. A hematopoietic stem cell is multipotent cell. During hematopoiesis,
hematopoietic stem cell differentiates along one of two pathways, giving rise to either a common lymphoid
progenitor cell or a common myeloid progenitor cell.
Common lymphoid progenitor cells give rise to B-cells,
T-cells and natural killer cells and some dendritic cells. The common myeloid progenitor cells give rise to red blood
cells (erythrocytes), white blood cells (neutrophils, eosinophils, basophils, monocytes, mast cells, dendritic cells)
and platelets.
Lymphoid progenitor
Lymphocytes are mononuclear leukocytes which constitute 20 to 40%
of total white blood cells (or leukocytes). They occur in large numbers in the blood and lymph and in lymphoid
organs such as the thymus, lymph nodes, spleen and appendix.
Lymphocytes are of three main types:
- B-lymphocytes or B-cells
- T-lymphocytes or T-cells
- Natural killer (NK) cells
B-lymphocytes
- The B-lymphocyte matures in the bone marrow in many mammalian species, including humans (in birds it is Bursa of Fabricius) and expresses membrane-bound antibody.
- After interacting with antigen, it differentiates into antibody secreting plasma cells and memory cells.
- B-cells also serve as Antigen Presenting Cells (APCs).
Properties of B-cells
|
Origin |
Bone marrow |
|
Maturation |
Bone marrow
(Bursa of Fabricius in bird) |
|
Expression of
Ag receptor |
Bone marrow |
|
Differentiation |
In lymphoid
tissue |
|
Surface
immunoglobulin |
Present |
|
Immunity |
Humoral |
|
Distribution |
Spleen, Lymph
nodes, Bone marrow and other lymphoid tissue |
|
Secretory
product |
Antibodies
and cytokines |
|
Complement
receptors |
Present |
T-lymphocytes
- T-lymphocytes arise in the bone marrow.
- Unlike B-cells, which mature within the bone marrow, T-cells migrate to the thymus gland to mature.
- During its maturation within the thymus, the T-cell comes to express a unique antigen binding molecule, called the T-cell receptor, on the membrane.
- T-cells do not make antibodies but perform various effector functions when APC bring antigens into the secondary lymphoid organ.
- T-cells help in eliminating APCs, cancer cells, virus-infected cells or grafts which have altered self-cells.
Thymus
- Thymus is the site where T-cells mature.
- Progenitor cells from the bone marrow migrate into the thymus gland, where they differentiate into T-cells.
- It is a flat, bilobed organ situated above the heart. Each lobe is surrounded by a capsule and is divided into lobules, which are separated from each other by strands of connective tissue called trabeculae.
- Each lobule is organized into two compartments: the outer compartment, or cortex, and the inner compartment, or medulla.
- T-lymphocytes mature in the cortex and migrate to the medulla, where they encounter macrophages and dendritic cells.
- Here, they undergo thymic selection, which results in the development of mature, functional T-cells, which then leave to enter the peripheral blood circulation, through which they are transported to the secondary lymphoid organs.
- It is in these secondary lymphoid organs where the T-cells encounter and respond to foreign antigens.
Dendritic cells
- Professional Antigen-Presenting Cells (APCs)
- Bridge innate immunity → adaptive immunity
- Only cells capable of activating naïve T lymphocytes
- Derived from bone marrow hematopoietic stem cells
- Immature DCs are found in: Skin, Mucosa (respiratory, gut), Blood & peripheral tissues.
- They stay immature until they encounter danger.
- They recognize pathogens using Pattern Recognition Receptors (PRRs): Toll-like receptors (TLRs), C-type lectins, NOD-like receptors.
- Dendritic cells present antigen to T cells using: MHC II → CD4⁺ Helper T Cells and MHC I → CD8⁺ Cytotoxic T Cells.