What is the classification of receptors?

The various types of cell receptors can be classified into two main categories: 

1. Cell Surface Receptors: These are present on the outer surface of the cell membrane. They bind to an external ligand molecule and are involved in converting extracellular signals into intracellular signals. There are three types of cell surface receptors: 
1. Ion Channel-linked Receptors: These receptors are directly linked to ion channels that allow select ions to pass through the plasma membrane. When a signaling molecule binds to the receptor, it causes the ion channel to open or close, leading to changes in the cell's membrane potential.
2. G Protein-coupled Receptors (GPCRs): GPCRs are the largest family of cell surface receptors. They interact with G proteins, which, when activated, can initiate intracellular signaling cascades, which modulate cellular function. 
3. Enzyme-linked Receptors: These cell surface receptors have an enzymatic domain that becomes activated on binding with a ligand. The enzymatic activity often initiates a series of intracellular events.
2. Intracellular or Internal Receptors: These are located in the cytoplasm of the cell and bind mainly to hydrophobic signaling molecules such as steroid hormones. When activated, they can influence gene transmission directly by binding to DNA and regulating transcription. There are 5 types of intracellular receptors: 
1. Growth Factor Receptors: Growth factor receptors play a crucial role in regulating the development, division, and differentiation of cells. 
2. Cytokine Receptors: Cytokine receptors respond to signals transmitted by other parts of the organism, thereby facilitating communication between cells and their extracellular environment. They are also involved in gene expression, cell cycle changes, and release of various mediators including cytokines.  
3. B Cell Receptors: B-cell receptors (BCRs) are membrane-bound immunoglobulin proteins found on the surface of B cells. BCRs recognize and bind to specific antigens, initiating a series of signaling events that lead to B cell activation and the production of specific antibodies that target the identified antigen. 
4. T Cell Receptors: T-cell receptors are protein complexes found on the surface of T cells or T lymphocytes. They play a central role as effectors and regulators in the immune system. 
5. Granulocyte Receptors: Granulocyte receptors are found on the surface of granulocytes, a type of immune cell that has small protein-containing granules in the cytoplasm. Granulocytes release enzymes to help fight off allergies and infections.