Mechanisms of innate immunity in invertebrates (hemocytes)

Provides an overview of the mechanism of innate immunity in invertebrates, particularly insects. Types of hemocytes present in insects and their roles in innate immunity.

1. Mechanisms of Innate Immunity in invertebrates (hemocytes) Semester-4 Life Science Paper-1 Unit-2

2. Introduction Multicellular organisms are constantly exposed to different microorganisms, many of which can be potentially harmful. To protect themselves from these microorganisms, multicellular organisms have evolved cellular and molecular defense mechanisms against infection. These defense mechanisms are known as immunity. At the beginning of an infection from microbes , early mechanisms such as expression of antimicrobial products, recognition by pattern ‐ recognition receptors (PRRs), and activation of phagocytic cells get involved for eliminating the microbes. These mechanisms are collectively known as innate immune systems . Insects have a powerful innate immune system for fighting infections. The innate immune system of insects consists of physical barriers, humoral responses, and cellular responses

3. Introduction contd. Physical barriers include the integument and the peritrophic membrane. Humoral immune responses include production of antimicrobial peptides, activation of P rophenoloxidase ( proPO ), and production of reactive oxygen species. Cellular immune responses include nodulation, encapsulation, and phagocytosis

4. Introduction contd. Hemolymph, the liquid that fills the hemocele , transports nutrients throughout the insect body and contains several types of free ‐ moving cells or hemocytes. There are several types of hemocytes including granulocytes, plasmatocytes , spherulocytes , and oenocytoids All these types of hemocytes are not found in all the insects.

5. Hemocytes in different stages of life cycle Hemocytes  can be divided into two categories: Embryonic L arval. Embryonic hemocytes are derived from head mesoderm. Larval hemocytes are released during the pupa stage

6. Types of Hemocytes

7. Types of Hemocytes contd … Secretory cells are not released into the hemolymph. They send out signaling molecules responsible for cell differentiation. Plasmatocytes are the hemocytes responsible for P hagocytosis, represent about 95% of circulating hemocytes. They are around 10 μm in diameter, but extend large lamellipodial protrusions and form dynamic filopodia Crystal cells are large cells, only found in the larval stage, and they are involved in melanization. Microbes are engulfed by a gel and destroyed) Lamellocytes are flat cells also found in larval stage and can encapsulate invading pathogens.

8. Phagocytosis Phagocytosis is the process by which cells recognize, bind, and ingest relatively large particles Phagocytic hemocytes are mainly P lasmatocytes and granular hemocytes Recognition of target particles for phagocytosis can be direct by specific cell ‐ surface receptors, or indirectly by O psonins . Cytokines capable of activating hemocyte functions

9. Nodulation When the initial phagocytic immune response is not sufficient, hemocytes activate other mechanisms to control infections. To deal with large bacterial infection, hemocytes form nodules to control the infections. Nodulation involves the formation of multicellular hemocyte aggregates that entrap large numbers of bacteria

10. Encapsulation H emocytes respond by forming a capsule around the foreign organism. Lamellocytes are the effector cells of encapsulation. Lamellocytes bind to the target in multiple cell layers until they form a capsule around the invader. The capsule is normally melanized at the end by degranulation of crystal cells. Inside the capsule the invading organism is killed by reactive cytotoxic products or by asphyxia

11. Melanization Melanization is the process of melanin formation. It is activated during wound healing and in nodule and capsule formation against large pathogens or parasites The enzyme phenoloxidase (PO) is a key in this process

12. Types of Hemocytes and their role.

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