What are the types of cell fixation?

There are three main types of cell fixation: 

1. Heat Fixation – In this process, the slide containing the sample is passed through the flame of a Bunsen burner several times. This helps to deactivate the proteolytic enzyme and prevent autolysis. Heat fixation preserves the overall morphology sufficiently enough for further analysis but does not preserve the internal structures. It is usually used for preparing bacterial and archaea samples. 
2. Perfusion – In this method, fixation is carried out via blood flow. The selected fixative is injected into the heart. From here it spreads through the entire body. The tissue remains alive and intact until it is fixed. This allows you to get a sample with intact, undamaged morphology. The advantage of this procedure is that it preserves morphology impeccably. The downside is that the subject dies during this process. Also, a larger volume of fixative is necessary for larger organisms, increasing the cost of the process considerably.  
3. Immersion – This process involves immersing the tissue sample in a fixative solution. The solution fixes the tissue by diffusing completely though the sample. Since complete diffusion is integral to this process, size and density of the tissue sample, as well as type of fixative used are major considerations. Immersing the sample in the appropriate fixative solution at a volume of minimum 20 times greater than the volume of the tissue to be fixed gives the best results.