Eosinophils: Characteristics and Functions

Cluster of Differentiation Molecules

• Cluster of differentiation (CD) molecules are found on the surfaces of many cell types and serve as indicators of the functional capacities of leukocytes and other cells.
• Over 350 CD molecules have been identified.

CD Molecules in Adaptive Immunity

• Three major CD molecules are important in adaptive immunity: CD3, CD4, and CD8.
• CD3 supports the T-cell receptor (TCR) and transduces trans-membrane signaling when the TCR is engaged.

Lymphocytes

• Lymphocytes are the major cells of the adaptive immune system.
• They can be classified according to three general methods: type of receptors involved, place of basic training, and type of expression of membrane molecules (CD).
• Lymphocytes range in size from small (4-7 µm) to large (11-15 µm).

B Lymphocytes (B Cells)

• B lymphocytes derive their name from bone marrow, their major site of maturation.
• They can be distinguished by their expression of membrane molecules known as cluster of differentiation (CD).

Eosinophils

• Eosinophils have "eosin-living" granules and are bilobed granulocytes with cytoplasmic granules that contain basic proteins.
• They are an active component in both innate and adaptive immune responses to parasitic infections.

Major Histocompatibility Molecules (MHC)

• MHC molecules are also known as the human leukocyte antigen (HLA) complex.
• They are a segment of chromosome 6 containing several genes that encode enzymes and structural molecules needed for the activation and function of B and T cells.
• MHC molecules play a major role in considering a transplant tissue as foreign or not (histocompatibility).

HLA Molecules

• HLA class I molecules are expressed on the surface of almost all nucleated cells.
• They consist of a large polypeptide (α-polypeptide) and a small polypeptide (β2-microglobulin).
• HLA class I molecules can bind to short antigenic peptides in their groove.
• HLA class II molecules are expressed on the surface of antigen-presenting cells (macrophages, dendritic cells, and B cells).
• They consist of two large polypeptides (α and β polypeptides).

Distinguishing between Self and Non-Self

• The immune system coexists with every cell in the body, which are identified as 'self' and marked with protein markers coded by the major histocompatibility complex (MHC) genes.
• Any cell that does not display the specific MHC marker is treated as an antigen.

MHC Proteins

• MHC Class I proteins are found on all body cells that have a nucleus.
• MHC Class II proteins are found only on B and T lymphocytes and macrophages.
• Both markers contribute to the specific identity of the cell.

Receptors in the Immune System

• Receptors are important for the function of the immune system.
• Engagement of receptors provides a wide range of responses depending on the type of cell or molecule associated with the receptor and the type of receptor and ligand.