Lymphocyte: Any white blood cell that is responsible for the immune response. They detect infection, "sound the alarm", produce antibodies specific to that antigen, detect & kill infected body cells.

Phagocyte: Any cell that engulfs and devours microorganisms or other particles via endocytosis.

Antibody: Proteins produced by lymphocyte cells in response to a foreign substance and carried on the plasma membrane of the lymphocyte. Each antibody binds only to one antigen (based on its membrane ID markers).

Pathogen: Any organism or virus that causes a disease.

Antigen: Any foreign substance, usually a protein or polysaccharide that stimulates an immune response. In pathogenic cells, the antigen may be the ID marker on the plasma membrane.

Action of Phagocytic Cells:

Phagocytic cells recognize bacteria, viruses, and other pathogens by binding to the ID markers on the membranes of these invaders. The phagocytic cell recognizes the invader if the ID marker does not match its own ID marker. The phagocytic cell then ingests the invading cell via a type of endocytosis known as phagocytosis. Once the pathogen is contained within a vacuole in the cytoplasm, lysosomes fuse with this vacuole, dumping enzymes into the vacuole. The enzymes catalyze the destruction of the pathogen.

Phagocytic cells search for and attack pathogens both in the blood as well as by squeezing between cells of the body tissues.

Action of Antibodies: Antibodies are specialized proteins produced by lymphocytes that are produced in response to the presence of a specific antigen in the body. The diagram below is a simplified illustration of how this may occur. (Note: The blue lymphocyte shown below is known as a B-cell or B-lymphocyte!