The fusion mechanism is the mechanism by which cell fusion

Cell fusion

Cell fusion is an important cellular process that occurs during differentiation of muscle, bone and trophoblast cells, during embryogenesis, and during morphogenesis...

 takes place. Cell fusion is the formation of a hybrid cell from two different cells of different species. Cells from the same organism may fuse together as well. This is often observed during lytic viral infection, where alterations of the cell can be seen under a light microscope. These include changes in morphology, formation of vacuoles and fusion of cells to form syncytia

Syncytium

In biology, a syncytium is a large cell-like structure; filled with cytoplasm and containing many nuclei. Most cells in eukaryotic organisms have a single nucleus; syncytia are specialized forms used by various organisms.The term may also refer to cells that are connected by specialized membrane...

.

The term cell fusion mechanism refers to the receptors

Receptor (biochemistry)

In biochemistry, a receptor is a molecule found on the surface of a cell, which receives specific chemical signals from neighbouring cells or the wider environment within an organism...

 required for cells to fuse, whereas the term cell fusion refers to the action of formation of the new cell.

One example of the cellular fusion mechanism is that of HIV

HIV

Human immunodeficiency virus is a lentivirus that causes acquired immunodeficiency syndrome , a condition in humans in which progressive failure of the immune system allows life-threatening opportunistic infections and cancers to thrive...

 infecting the body. HIV infects by fusing with the membranes

Cell membrane

The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It basically protects the cell...

 of immune system cells

White blood cell

White blood cells, or leukocytes , are cells of the immune system involved in defending the body against both infectious disease and foreign materials. Five different and diverse types of leukocytes exist, but they are all produced and derived from a multipotent cell in the bone marrow known as a...

. In order for HIV to fuse with a cell, it must be able to bind to the receptors CD4

CD4

CD4 is a glycoprotein expressed on the surface of T helper cells, monocytes, macrophages, and dendritic cells. It was discovered in the late 1970s and was originally known as leu-3 and T4 before being named CD4 in 1984...

, CCR5

CCR5

C-C chemokine receptor type 5, also known as CCR5, is a protein that in humans is encoded by the CCR5 gene. CCR5 is a member of the beta chemokine receptors family of integral membrane proteins...

, and CXCR4

CXCR4

C-X-C chemokine receptor type 4 also known as fusin or CD184 is a protein that in humans is encoded by the CXCR4 gene.- Function :...

.