The system that consists of different cells and their combinations that protects the body from infection and the harmful effects of pathogens is called the immune system.

DIFFERENT CELLS OF THE IMMUNE SYSTEM
The cells of the body that are involved in the immune system are lymphocytes, nodule cells, mononuclear phagocytes, granular leukocytes, mast cells, dendritic cells. Mononuclear phagocytes are again two types of monocytes and macrophages.

TYPES OF IMMUNE SYSTEM ORGANS
There are two types of immunity. Primary lymphoid organs and secondary lymphoid organs. The primary lymphoid organs are the thymus gland, bone marrow, lymph, lymph nodes, and the secondary lymphoid organs are the spleen, lymph nodes, tonsils, etc.

ANTIGENS
Acquired immunity does not come if foreign germs or toxins do not enter the body. Each bacterium and toxin contains one or more specific chemicals, which play an important role in the acquisition of acquired immunity. These are usually proteins, large polysaccharides, or large lipoprotein compounds. These chemicals are called antigens.

CHARACTERISTICS
Characteristics of antigens: The specific place where antibodies are attached to the antigen is called epitope or antigenic detergents. Antigen detergent sites contain specific chemical groups where the antigen is attached to the para-top or antigen-binding site. Most antigens have many antigen locations, so they are called multivalent. Antigens have two special properties, namely: immunity and reactivity.

TYPES
Types of antigens: Antigens that are produced outside the animal body are called exogenous antigens. The antigens that are produced inside the animal body are called endogenous antigens. Homogeneous antigens that are found in the bodies of ethnically different species are called xenogenic antigens. In special cases, when an organ-forming substance acts as an antigen, it is called an autologous antigen. Antigens of two animals of the same species that are genetically regulated but separated from each other by antigenic detergents are called allogeneic antigens.

PROPERTIES
General properties of antigens: Antigens are mainly proteins. Some antigens are lipoproteins and polysaccharides. Antigens are usually extrinsic without special fields. The molecular mass of a functioning antigen is usually more than 10,000 Daltons. For a substance to be an antigen, it needs to have some detergent group. Organ-specific antigens are found in certain tissues or organs.

ANTIBODY
Antibodies are the proteins that appear in the body against toxins or bacterial-borne antigens and those that destroy toxins or toxins secreted by them.

TYPES
Types of antibodies: IgG: These types of antibodies are present in the blood, they can easily cross the placenta and are transferred from the mother’s blood to the fetal blood. IgA: This type of antibody is present as a monomer and dimer. This type of antibody is found in the colostrum secreted from the mammary gland. IgM: These are the largest antibodies. They live in the blood as pentamers. IgD: They are present in very small amounts in the blood. They act as antigen receptors for B-lymphocytes. IgE: They are also present in very small amounts in the blood. They are attached to the mast cell and basophil leukocyte membranes in the monomer state.