Parasites are organisms which thrive at the cost of a living host and can possibly damage the latter. In medical practice, the term is restricted to mean worms and protozoa. They are the subject of parasitology.

Passive immunization is the injection of antibodies or an immune serum into a recipient. Active immunization, on the other hand, involves the triggering of an immune response by injecting antigens.

Pathogenic microorganisms, or pathogens, are infectious microorganisms which cause an illness in their hosts.

Pathology is the study of disease. The term is also used to describe tissue damage.

Peripheral lymphatic organs: The peripheral lymphatic organs include the lymph nodes, the spleen and lymphatic tissue associated with the mucous membranes in which immune responses are initiated. The central lymphatic organs, on the other hand, are where the lymphocytes develop.

Peyer's Plaques: Lymphatic tissue in the intestine. It contains about 80% of the cells producing antibodies and therefore plays a key role in immune defence, for many pathogens enter the body along with food.

Phagocytes are large, single-nucleus phagocyting cells which play a role in acquired immunity and in early phases of the immune response. They can function as antigen-displaying cells as well as effector cells in humoral and cellular immune reactions. These cells are derived from precursor cells in the bone marrow and can be found in most of the tissue types of the body. They are of immense significance for the defence against foreign bodies and pathogens. Macrophages do not attack intracellular bacteria until they have been activated by a T cell. This activation is very important for the control of an infection. However, it causes damage to neighbouring tissue as well.

Phagocytosis is the ingestion of particles by cells. Phagocytes are generally macrophages or neutrophile cells, while the particles are bacteria which are ingested and destroyed. The ingested material is first found in a vesicle, a so-called phagosome, which is then merged with one or more lysosomes to make a phagolysosome. The lysosomal enzymes play an important role in the destruction of pathogens and their being broken down into small molecules.

Plasma is the liquid components of the blood. It consists of water, electrolytes and the plasma proteins.

Plasma cells are differentiated B lymphocytes. The are the most important antibody-forming cells of the body. They are found in the medulla of the lymph nodes, in the red pulp of the spleen and in the bone marrow.

Plasmodia: One-cell blood and tissue parasites: The best-known of them is Plasmodium falciparum, which can cause malaria.

Polio: The polio virus causes infantile paralysis or poliomyelitis.

Polygenesis: The main histocompatibility complex is both polygenetic (it contains various loci which code proteins with identical function) and polymorphous (it possesses several allele for each locus: polymorphism).

Polymerase chain reaction: The polymerase chain reaction (PCR) is a method for the reproduction of certain DNA sequences. The desired DNA fragment is selectively reproduced over the course of several synthesis cycles with the help of two primers which bind to the two strands of the double helix at each end of the goal sequence.

Polymorph nucleus leukocytes are white blood cells with strongly lapped nuclei and cytoplasmic granulomas (so also called granulocytes). There are three types of polymorphonuclear leukocytes: the granulomas of the neutrophile leukocytes can be dyed with neutral dyes, the eosinophile ones with eosin and the basophile ones with basic dyes.

Poliomyelitis: The polio virus causes infantile paralysis.

Precipitin reaction: The precipitin reaction was the first method for quantitative measurement of antibody production. The antibody quantity is determined on the basis of the quantity of precipitate which is received with a certain quantity of antigens. The precipitin reaction can also be used to determine the valence of an antigen and to find out about surpluses of one or more components in a mixture of antibodies and antigens.

Primary follicle: The primary follicles of the lymphatic tissue consist of follicular dendritic cells and quiescent B lymphocytes. When activated B cells enter, seed centres form in the primary follicles which then become secondary follicles.

Primary immune response: The primary immune response is the acquired immune reaction as a consequence of first contact with an antigen. The primary immunization, often called priming, triggers this primary immune reaction and leads to the formation of an immunological memory.

Professional antigen-displaying cells normally trigger the reaction of naïve T cells to antigens. This ability has been proven only for dendritic cells, macrophages and B cells. A professional antigen-displaying cells must be able to display peptide fragments of antigens on suitable MHC molecules and, in addition, carry co-stimulating molecules on its surface.

Programmed cell death, also known as apoptosis, is triggered by the cell's own mechanisms. Programmed cell death is of great importance because it keeps the number of lymphocytes down to a reasonable range.

Protective immunity is the resistance to certain infections as a result of previous infections by the same pathogen or a vaccination.