Leukocytes

Memory cells

These lymphocytes are long-lived. They wait in the lymph nodes to activate when they bind to the antigen they recognize, initiating a secondary immune response.

Neutrophils

Engulf and digest bacteria and fungi; first responders to microbial infection. Also degranulate releasing cytoxic compounds and enzymes.

Cytotoxic T cells

Bind to the antigen they recognize and MHC I. This activates the leukocyte causing it release cytotoxic molecules such as granzymes and perforin. Granzymes signal the cell (likely infected with a pathogen such as a virus) to undergo apoptosis. Perforins lyse the cell by creating holes in its membrane.

Basophils

Degranulation releases histamine and other mediators involved in inflammation; participate in allergic responses.

Monocytes

Differentiate into macrophages and dendritic cells in tissues; involved in phagocytosis and antigen presentation.

Plasma cells

Produce large amounts of antibodies that neutralize pathogens and mark them for destruction.

Dendritic Cells

Capture antigens and present them to lymphocytes (often in the lymph nodes), initiating adaptive immune responses.

Plasma cells

Produce large amounts of antibodies that neutralize pathogens and mark them for destruction.

Eosinophils

Combat multicellular parasites by releasing toxin compounds and degradative enzymes from their granules. Also participate in allergic reactions.

Helper T Cells

Bind to the antigen they recognize and MHC. This activates the cell which releases cytokines to regulate the other leukocytes. For example, depending on the cytokines released they could activate B cells or macrophages.

Macrophages

Engulf and digest pathogens, dead cells, and debris. They have Toll-like receptors that detect potential pathogens and the cells then release cytokines in response as signals for other parts of the immune response.

Eosinophils

Combat multicellular parasites by releasing toxin compounds and degradative enzymes from their granules. Also participate in allergic reactions.

Regulatory T Cells

Suppress and regulate immune responses to maintain tolerance to self-antigens and prevent autoimmune disease.

Natural Killer (NK) cells

Destroy virus-infected cells and tumor cells by releasing cytotoxic granules that induce apoptosis. These cells are part of the innate immune system and thus don't directly recognize viral proteins. They can recognize these virally infected cells using antibody depending cellular cytotoxicity.

Mast cells

Similar to basophils, however they are found in the tissues. They degranulate to cause inflammation and are often invovled in allergies.