How many types of cell death are there?

There are three main types of cell death – apoptosis, autophagy, and necrosis. 

Apoptosis 

Also known as type I cell death, apoptosis is a type of programmed cell death that is characterized by chromatin condensation, membrane blebbing, and shrinking of the cell. It is the fastest form of cell death and is accompanied by the activation of caspase proteases. There are two major signaling pathways that trigger apoptosis - the death receptor or extrinsic pathway, and the mitochondrial or intrinsic pathway. Both pathways result in the activation of caspase proteases, which cleaves the intracellular proteins and leads ultimately to the death of the cell. 

Autophagy

Autophagy, also known as autophagic cell death or type II cell death, is characterized by the appearance of large intracellular vesicles. This is primarily a catabolic process induced by some sort of metabolic stress such as nutrient deprivation, oxidative stress, high glucose, hypoxia, or genotoxic stress. It is also responsible for eliminating damaged protein aggregates, dysfunctional organelles, and infecting organisms. 

Necrosis

Necrosis, also known as necrotic cell death or type III cell death, is characterized by swelling of cells, ruptured plasma membrane, and lack of organelle structure without chromatin condensation. Passive necrosis may be induced in case of extensive damage such as mechanical stress and high temperatures that damage the integrity of the cell. Secondary necrosis is typically triggered during the late stages of apoptotic or autophagic cell death when phagocytosis fails to eliminate the dead cells.