What are the cells of the immune system of birds?

The immune system of birds consists of specialized cells, tissues, and organs that work together to protect the bird from infection and disease. Birds have an innate immune system, which provides immediate, nonspecific defense against pathogens. They also have an adaptive immune system, which generates a pathogen-specific response and immunological memory. The key immune system cells in birds include phagocytes, natural killer cells, B lymphocytes, and T lymphocytes. These cells circulate in the blood and lymph and are also found in immune organs such as the thymus, spleen, and bone marrow.

Innate Immune System Cells

The innate immune system is the bird’s first line of defense against pathogens. The main innate immune cells are phagocytes, natural killer cells, and heterophils:

Phagocytes

Phagocytes are cells that engulf and digest pathogens, cell debris, and foreign particles. The main phagocytic cells are heterophils, monocytes, and macrophages.

– Heterophils are the avian equivalent of neutrophils in mammals. They are granulocytes with lobed nuclei and cytoplasmic granules containing antimicrobial substances. When a pathogen invades, heterophils are quickly recruited to the site of infection, where they phagocytose and kill the invaders.

– Monocytes are mononuclear phagocytes that circulate in the bloodstream. When monocytes migrate into tissues, they differentiate into macrophages.

– Macrophages reside in tissues and are longer-lived than heterophils and monocytes. Macrophages phagocytose pathogens and present antigen fragments on their cell surface to activate the adaptive immune response. Some tissue macrophages are specialized, including Kupffer cells in the liver and microglia in the central nervous system.

Natural Killer Cells

Natural killer (NK) cells are cytotoxic lymphocytes that can spontaneously recognize and kill virus-infected cells and cancer cells. NK cells are especially important in containing viral infections until the adaptive immune response activates. Birds lack lymph nodes, so NK cell activity occurs primarily in the spleen and mucosal tissues.

Heterophils

As mentioned above, heterophils are the avian version of neutrophils. They phagocytose and kill bacteria, fungi, and other pathogens. Heterophils contain granules with antimicrobial proteins, oxidants, and proteolytic enzymes that help kill ingested microbes. These cells are usually the first leukocytes to arrive at a site of infection.

Adaptive Immune System Cells

The adaptive immune system mounts a specific response to pathogens and generates immunological memory. The key cells involved are B and T lymphocytes:

B Lymphocytes

B lymphocytes, or B cells, develop in the bursa of Fabricius (a lymphoid organ unique to birds). After maturation, they circulate in the bloodstream and lymphatic system. B cells have antigen receptors on their surface that can bind to specific foreign antigens. When a B cell encounters its matching antigen, it proliferates and differentiates into plasma cells that secrete antibodies. These antibodies bind to the antigen and neutralize it or mark it for destruction. Some B cells become long-lived memory cells that confer long-term immunity.

T Lymphocytes

T lymphocytes, or T cells, arise from stem cells in the bone marrow and mature in the thymus gland. Mature T cells circulate between the bloodstream, lymph, and tissues. T cells have T cell receptors that recognize fragments of antigen displayed on cell surfaces. There are several types of T cells:

– Helper T cells (CD4+ cells) activate B cells, cytotoxic T cells, and macrophages after recognizing antigen. This promotes the adaptive immune response.

– Cytotoxic T cells (CD8+ cells) directly kill infected cells and cancer cells that display foreign antigen fragments.

– Regulatory T cells suppress immune responses by other lymphocytes to prevent excessive inflammation and autoimmunity.

– Memory T cells persist after an infection and enable a faster immune reaction if the same pathogen is encountered again.

Immune System Organs and Tissues

In addition to circulating immune cells, birds have lymphoid organs and tissues that are involved in immune cell development and immune responses:

Bursa of Fabricius

The bursa of Fabricius is a lymphoid organ unique to birds, located near the cloaca. This is where B lymphocytes mature and diversify to create a diverse antibody repertoire. The bursa atrophies after sexual maturity but still contains B cells.

Thymus

The thymus is a bi-lobed lymphoid organ above the heart, where T cells mature. The thymus is largest early in life when the T cell repertoire develops, then starts to atrophy after sexual maturity.

Spleen

The spleen filters blood and contains areas called white pulp that are enriched in lymphocytes. It removes old erythrocytes and is a site of immune cell activation and antibody production.

Bone Marrow

Avian bone marrow produces the stem cells that develop into B cells and heterophils. It is also a reservoir for monocytes. In severe infections, extra marrow can activate inside bones to ramp up leukocyte production.

Mucosal Associated Lymphoid Tissue

Mucosa-associated lymphoid tissue (MALT) lines respiratory, digestive, and urogenital tracts. MALT contains lymphocytes that provide immune protection at mucus membranes, the bird’s first line of defense against inhaled or ingested pathogens.

Harderian Gland

The Harderian gland, located behind the eye, transports antigens to nearby lymphoid follicles. This promotes a local antibody response when pathogens enter through the conjunctiva.

Conclusion

Birds have an intricate immune system comprised of innate and adaptive components. Key innate immune cells are phagocytes like heterophils, monocytes, and macrophages that ingest extracellular pathogens. Natural killer cells provide early response by killing infected cells. The adaptive immune system centers around B lymphocytes that produce antibodies and T lymphocytes that activate other immune cells or directly kill infected cells. These cells circulate through lymphoid tissues and organs like the bursa of Fabricius, thymus, spleen, mucosa, and bone marrow, which support immune cell development, activation, and memory. Further study of avian immunology will provide greater insight into how birds resist infection and disease. Understanding the avian immune system can also inform poultry health and vaccine development to protect commercial and wild bird populations.